TECH-LINE

TERMS & DEFINITIONS

Racing & Engine Building have many terms and definitions. The following are some of the most common and often confusing terms used, which can make even a seasoned engine builder pause in thought. This page was originally intended to be only a summary of information explaining some of the terms we use in valve trains, which by itself can be quite extensive. But in adding this information I found that more questions came up which were associated to cylinder heads and overall engine building. Because valve train is an integral part of cylinder head design, and because we address so many problems of valve trains from missed information or incorrect assumptions about cylinder heads, I've expanded this section to include more depth on both of these categories. I've also included some basics on Turbos, and added comments and links on the major MPG products. Lastly, a few rudimentary terms and definitions have been included as an introductory courtesy to the novice engine builders just starting out, and is provided as a courtesy within the limitations of Copyright laws, for which we hold all rights to.

NOTE: This is an abbreviated tech page that's provided at no charge. The advanced tech and additional upgrading features, including a NEW "jMs Blog" is available only to registered owners of MPG, PVS and MEI rocker systems; or by subscription for $39.77 per year. Click HERE FOR MEMBER LEVEL TERMS & DEFINITIONS.

1/3 RULE: Refers to the traditional method engineers used for rocker arm design on an Over-Head Valve engine. See GEOMETRY, TRADITIONAL.

ABDC: Cam lingo, for AFTER BDC (Bottom Dead Center). Sometimes used on CAM CARDS.

ATDC: Cam lingo, for AFTER TDC (Top Dead Center). Sometimes used on CAM CARDS.

ACCELERATION DYNAMICS, VALVE: The "net" valve lift, acceleration rate and duration (time open); or what the valve actually sees as a result of the camshaft's instructions - as translated by the rocker arm design geometry and it's installed geometry. Since a cam profile is actually a blueprint for accelerating the valve train from closed, to open, then trying to slow it down on the closing side, acceleration "dynamics" is changed by the rocker arm's geometry on either end. So when we add this term to the valve, we are really talking about the effect that rocker ratio and geometry have upon the velocity imposed at the valve, from what the cam's acceleration initiated. See: VALVE LIFT DYNAMICS (VLD).

ACCELERATION RAMP: (Camshaft) Refers to the specific portion of a CAM LOBE that lies between the BASE CIRCLE and the CAM'S NOSE, which is precisely calculated to increase the TAPPET lift at a specific rate of acceleration, then slowing as the tappet rides across the cam's peak lift point, and begins to following the DECELERATION RAMP, where the compressed pressure, inertia and mass of the entire valve train are hopefully constrained within operating limits of the desired RPM's, to allow the valve to seat with minimum bounce.

ADJUSTING NUT (ADJUSTER): Traditionally named on STUD MOUNT rocker arms for securing the rocker arm upon the rocker's mounting stud. Incorporates a SET SCREW which locks using an Allen wrench, to secure more tightly than OEM factory interference fit adjusting nuts when setting valve lash on performance camshafts.

ADJUSTING SCREW: Self descriptive, usually 3/8" or 7/16" SAE (fine) thread, with either a 3/8" diameter "ball" or a 5/16" diameter "cup" on the mating end for pushrod contact; used to facilitate valve lash adjustment on fixed height, stand mounted rocker arms (also known as shaft systems), or non-stud type mounting.

AFTERCOOLER: (Turbocharger) Often mislabeled as an "Intercooler," even by manufacturers and distributors of Turbochargers and accessories, the Aftercooler is essentially a radiator, designed in one of two fashions, whereby it reduces the hot air temperature exiting the COMPRESSOR side of the Turbocharger, before being sent to the engine's induction system. The efficiency of Aftercoolers pertains to how much reduction of the Compressor Discharge Temperature (CDT) can be attained, and in most cases the best of them will be in the 60% to 65% range. Meaning: whatever the CDT temperature is over the ambient Compressor Inlet Temperature (CIT) , approximately 60% of that difference can be reduced. This post-Aftercooler Temperature is known as the Air Inlet Temperature (AIT), which is measured within the manifold runner between the Aftercooler and the throttle body of the engine, but usually as close to the throttle body inlet as possible.  (Abbreviated)

AIR INLET TEMPERATURE (AIT): (Turbochargers) Sometimes confused as the temperature of air entering the Turbocharger, it is really the air entering the engine at the throttlebody. It is particularly the result of cooling the much warmer CDT (Compressor Discharge Temperature) air exiting the Aftercooler, on Turbocharged engines so equipped.

 ALPHA BAR : The cool looking horizontal bar of "alphabetical" letters at the TOP  ^  of THIS LIST, that YOU should be using to navigate this page. Choose the FIRST LETTER of the word GROUP that you're looking for, the scroll down until you hopefully find it. You can E-MAIL us on any words you'd like to see here.

NOTE: HIGHLIGHTED, off-color words in any term will take you directly to that definition automatically. All UNDERLINED RED words are active Hyperlinks that take you to a new web page (except this example).

ANGLES: Is the fundamental division of a circle or any derivative thereof, whereby the measurement stems from a common axis or tangent point. A slice of pie's axis is the point where two edges come together. These measurements are referred to in DEGREES, MINUTES and SECONDS.  (Abbreviated)

ANGULAR ACCELERATION: Refers to the rate of change in the velocity of a device rotating around an axis, as measured from e specific perspective. In our sector (engines) this would pertain to the "perspective" of the valve centerline and the device would of course be the rocker arm.  (Abbreviated)

AREA-UNDER-THE-CURVE: is a term which describes what a graph of the valve lift cycle would look like if charted on paper horizontally across TIME, in crank degrees of rotation, and vertically in thousands of an inch of LIFT. The quicker the valve opens, and "hangs" there, the greater the space beneath the curve from opening to closing.  (Abbreviated)

ASPECT RATIO: This is a term used in a variety of forums, from engineering and physics to art. It simply means the divided sum of a dimension by an associated second dimension.  (Abbreviated)

ASSEMBLY LENGTHS: Another big factor in determining your pushrod length involves any changes made to the head deck, valve seat depth, valve stem length, the block deck, and the cam's base circle. Different lifter heights must also be considered; for instance, you can NOT use the same pushrods for a roller cam, as that which was used for a mechanical or hydraulic cam - since the vast majority of roller lifters are much higher than typical lifters.  ^

ASYMMETRICAL: The two sides of a concept or design which do NOT mirror each other; each having a unique profile, however still dependent upon each other for completing a task. The opposite of symmetrical. See: SYMMETRICAL. Asymmetrical and Symmetrical are two terms often used in CAMSHAFT terminology, referring to the simpler, older cam profiles which had even open and closing ramps, or the more current and most frequently used since the mid-1970's, asymmetrical profiles which lift the tappet quickly to open, and use slower deceleration ramps to set the tappet down in closing. See: CAM RAMPS.

ATTACK ANGLE: Refers to inclines toward or away from operating linear components, like the pushrod's incline towards or away from the valve centerline, as it operates on an assembled engine. We also refer to this for the mounting stud or rocker pad's incline with the valve, which highly influences the range of valve lengths to be used, or rocker lengths, as measured from the axis of rotation to the roller's centerline. In CYLINDER HEADS we use this term to define the PORT WINDOWS created with various port entrance and exit angles to and from the COMBUSTION CHAMBER. See: ROCKER MOUNTING & VALVE TIP HEIGHT for illustrations that depict port silhouettes within the head, relative to valves. (Abbreviated)

AXIS: The true center of any shaft or pin device in which a another device (usually bearing or roller) rotates about.

BDC: BOTTOM DEAD CENTER: Refers to the position of the PISTON (and crankshaft) in relation to the stroke of the crank. It is the ultimate bottom of the crankshaft's stroke, before initiating its travel back up.

BBDC: Cam lingo, for BEFORE BDC (Bottom Dead Center). Sometimes used on CAM CARDS.

BTDC: Cam lingo, for BEFORE TDC (Top Dead Center). Sometimes used on CAM CARDS.

BACK CUT: (Cylinder Heads) Refers to an angle that is precision ground on the valve and behind the VALVE FACE angle, which is always a lesser value, and which is specifically intended to do two things: First, to adjust and reduce the width of the Valve Face angle to a specific width, usually equal (or close) to the VALVE SEAT width; and second, to blend the valve seat's angle to the valve head's even more shallow angle, so airflow across it at low (and high) lifts is less disturbed cross the Valve Seat.

BALL FUCLRUM: Refers to a simple "ball" principle of rocker arm mounting upon a Stud Mount principle to hold the rocker body to the cylinder head. The ball, which is actually a half sphere, operates directly upon a matching inside radius formed into the bottom of the rocker body to provide the rocker's central location to pivot on during its reciprocating operation. Such a design provides broad strength across more contact area for long life under OEM applications, where this design gets its roots many decades ago, providing it gets continuous flooding of cooling oil, usually from the push-rod. But its "friction bearing" principle was replaced long ago with various rocker arm designs graduating to some form of needle bearing.  (Abbreviated)

BALANCE TUBE: (Exhaust Systems) This term refers to the practice of connecting both sides of an engine's exhaust coming from each head through a common tube that crosses from one bank of exhaust to the other. Also used on engines having only two cylinders, such as motorcycles and other smaller powered vehicles. The main principle behind doing this is to capture negative pressure waves coming from one side to enhance a reduced pressure for succeeding waves that follow from the other side by more or less making them act as one. The diameter and position down from the head is important. (See: COLLECTOR) (Abbreviated)

BOLT STRETCH: Is the very specific and precise method of measuring the true value of a fastener's specific resistance in creating a locking force. It is a TENSILE measurement resultant of actually stretching a bolt to a predetermined length from its static (non-load) dimension, through the torque used to secure it.  ^

BOSS 429: Trade name of Ford Motor Company.  It also created the most complex valve train issues, which are at the root of MID-LIFT technology's evolution, and has served as the model of precedents still used to this day for most rocker arm design solutions by Miller Engineering Inc.

BRAKE SPECIFIC FUEL CONSUMPTION (BSFC): Refers to the efficiency of fuel consumption, per pound, per horsepower produced. The values need to include WEIGHT and TIME per Horsepower. It is acknowledged that 0.42 lbs//Hr. is a standard reference point per Horsepower. At 6.0 lbs per gallon of fuel, a given engine will burn 0.07 GALLONS of fuel per horsepower, per hour. At 400 Horsepower, that would be: 400 x 0.42= 168 lbs / 6 lbs (per gallon)= 28 Gallons/hr. Air density, humidity and temperature outside of standard conditions will vary this mean value.  ^

BYPASS VALVE: (Turbocharger) Sometimes referred to as a "Blow-Off Valve" (BOV), this is a check valve with a predetermined pressure limit designed to release excessive amounts of "boost" from the Turbocharger's Compressor feeding the engine, to prevent over-boost and engine damage.

CAM: An eccentric shaped circle that has a base diameter which is less than 360 degrees that rotates about an axis which increases its diameter from the axis as it rotates through a 360 degree rotation. Generally egg shaped on designs that work with a direct contact running surface "cam follower," or having more squared radius` for designs where the cam follower incorporates a roller ended running surface. See: CAM FOLLOWER.

CHOOSING A CAM: The following is some basic fundamentals relevant to selecting a cam. It is by no means a cut in stone blueprint where some of these variables can't be stretched. But this should keep your feet on the ground to avoid losing perspective of what is important, over what someone is hyping to make a sale, or worse, failing to give you what you really need.  (Abbreviated)

CAM BASE CIRCLE: (a.k.a. The "HEEL") The lowest point of measurement to the axis of rotation, this is the constant radius, NON-eccentric running surface of the camshaft that has NO effect on lift. It is also the mathematical foundation, as measured in degrees of rotation, for the starting and stopping points of where the lash ramps and acceleration ramps begin and end.  (Abbreviated)

CAM CARD: The printed record of cam specifications provided with a camshaft to give fundamental design information necessary for selection, use and installation within an engine. This information should include CAM LIFT, CAM DURATION (often shown in two values, but most important is the .050" Tappet Lift), CAM LOBE SEPARATION, CAM PHASING (installed timing with crankshaft) and VALVE LASH (if mechanical). Additional information often included is "Theoretical" VALVE LIFT (based on specific rocker ratio, often not accurate to reality) and "Theoretical" DURATION (often taken at irrelevant lifter values close to the closed valve point, such as .012", .015" or .020" Tappet Lift; and usually of most value to hydraulic grinds, but often not considered of much value to experienced engine builders).

CAM CENTERLINE: Actually has two meanings. The usual meaning, refers to the cam's designed INTAKE centerline position, in CRANK degrees, as measured with the CRANKSHAFT position AFTER TOP DEAD CENTER (ATDC) of the piston. It is also known as the specification which cam "PHASING" is used. Again, the advancing or retarding of the camshaft with the crank's timing, as per the cam manufacturer's design specifications. (See: CAM LOBE SEPARATION) The second meaning, pertains to the more obvious implications of its name, meaning the actual centerline running through the base circle's axis, up through the cam's nose, thus dividing the opening and closing events equally. Even this though, can be misleading, since many asymmetrical cams don't have nose specifications that are evenly divided. None the less, there is a theoretical centerline to the computer's data which calculated and assembled the CAM RAMP dynamics. (Abbreviated)

CAM DURATION: This the period of "time" in which the CAM LOBE is active beneath the CAM FOLLOWER, as measured in one of two ways: degrees of rotation at the cam, or degrees of rotation at the crankshaft, the latter being the more preferred standard with regard to engine builder terminology. Cam Duration historically has been measured from various, non-conforming reference points of tappet lift for its "gross duration," which technically is the total duration measured from the instant tangent point of cam ramp departure from, and returning back to the BASE CIRCLE. To get some perspective on selecting cam duration for your engine, see: CHOOSING A CAM.  (Abbreviated)

CAM FOLLOWER: (TAPPET or LIFTER) A cylinder shaped component that rides upon the cam lobe of an OHV engine, and accepts a PUSHROD on the opposite end which provides the reaching link to the rocker arm atop the cylinder head. Cam followers are either hydraulic "roller," hydraulic "mechanical," or mechanical roller or straight mechanical, aka "SOLIDS." The ultimate performing cam follower is a SOLID ROLLER (no hydraulic valve lash preload); often simply referred to as a "roller."  (Abbreviated)  ^

CAM HEEL: See: CAM BASE CIRCLE.

CAM LIFT: This the actual measurement of the cam lobe's total rise from the BASE CIRCLE of the cam which is imposed on the CAM FOLLOWER. Cam lift on OHV engines is always a lower value of net VALVE LIFT because ROCKER RATIO multiplies this by 1.5:1 or more in most cases.

CAM LOBE: The individual eccentric of a cam SHAFT that drives each valve through other components or directly.  ^

CAM LOBE SEPARATION: Pertains to the physical separation of cam lobe centerlines, in CAM degrees. This is also the reverse perspective of "OVERLAP." Both terms deal with the physical timing between the intake and exhaust lobes of a camshaft for the given cylinder in question. Traditionally, OVERLAP was the term used to deal with cam tuning to cylinder heads and cubic inch needs on a given cylinder head. In certain circles this still is. But LOBE SEPARATION became prevalent in the late 1970's as lower compression ratios became the norm from the Detroit manufacturers, adapting to more stringent EPA regulations. Increasing LOBE SEPARATION (which decreases overlap) is a way of building higher cylinder pressure on lower compression engines. See Net Effective Cylinder Pressure (NECP). To get some perspective on selecting cam lobe separation for your engine, see: CHOOSING A CAM.

CAM NOSE: The highest portion of the cam lobe eccentric where valve lift is at its greatest.

CAM OVERLAP: Refers to the time when both, the intake and exhaust valves of a given cylinder are open at the same time, measured in CAM DEGREES. Specifically, it is when the INTAKE valve begins to open, as the EXHAUST valve is just closing. The piston is still rising UP, chasing the exhaust valve closed, and the intake valve is opening to meet it. Increased overlap is needed on higher RPMs to provide additional time for the cylinder to "purge" itself of leftover gases from the prior combustion process. Physically, as explained in LOBE SEPARATION, the overlap cycle is the opposite perspective of the same consequence to cam design. They effect the engine's "running" COMPRESSION RATIO. You are merely saying the same thing from opposite perspectives. (Abbreviated)

CAM RAMPS: Opening and Closing, as they are known, are the connecting portions of the cam's running surface that lies between the BASE CIRCLE and the NOSE. Sometimes referred to as the cam's FLANKS. The cam's LASH RAMP is also included here, and are designed to make the transition between the base circle and the more aggressive (quicker velocity rise) "ACCELERATION RAMPS."

CAMSHAFT: The multi-lobe shaft consisting of the required number of eccentric cam lobes needed to operate the valve train, as it is timed by a chain or belt with the crankshaft. It is made of a variety of materials, with 8620 steel being the most common on high performance roller cam engines. Although numerous high tech coatings and other manufacturing methods are changing this norm on certain applications. NASCAR breaks the rules on cost and technology in this regard. CAMSHAFTS of today's high performance OHV (Over-Head-Valve) engine technology are the "heart" of a FIVE (5) component system that makes up the valve-train, which is arguably an antiquated principle of design, dating back nearly a century. Aside from the last component in our list: the "VALVE," these other three components are respectively known as the LIFTER (or tappet, or cam follower), the PUSH-ROD and the ROCKER ARM.

CAM VELOCITY: Is an often used term by cam companies to describe, in a general way, the cam lobe's rate of lift over other profiles. It of course has a very specific engineering value to cam designers, which is given a value of thousandths of an inch per degree of either cam rotation, or crank rotation (double the cam value). "RATE of ACCELERATION" is another general term used for the same meaning, usually pertaining to cams; but we often use the term for valve acceleration.

CANTED VALVE: This term relates to engines which use a 3 plane valve centerline (not to be confused with valve FACE angles of valve jobs). The Chevrolet big block and 351 Cleveland and 460 Ford are just three examples which have their valves leaning on three separate planes, as compared to the cylinder bores of the engine block, for instance. Small block Chevrolets and 302 Fords, 340 Chryslers, etc., are on a "2 plane" valve array; meaning the valve's inclination to the cylinder bores can be described on 2 dimensions - with the 3rd dimension being zero - since they are all in-line with each other when viewed from an end perspective of the cylinder head.  ^

CENTERLINE: Self descriptive, with rocker arms we are referring to the true center of trunnions, shafts, roller pins (and roller), bearing, valve and/or pushrod.

CENTERLINE, CAM: See: CAM CENTERLINE.

CFM: Usually cylinder head lingo; it's an acronym for CUBIC FEET per MINUTE. Most relative to AIR FLOW, measured through a passage, whether it is a cylinder head port, intake manifold, carburetor, fuel injection metering body, or whatever. It requires a specific pressure value for measuring, known as FLOW DEPRESSION. Without knowing the flow depression used to measure anything that has a CFM rating to it, such as cylinder heads, the value is meaningless for comparison. Too often, cylinder heads are promoted with a CFM rating acquired from flow testing, and no flow depression value is mentioned. So comparing to another head is useless and misleading.

CLOSING THE VALVE: Granted, there is no single way to do anything; and there may be tests which show an improvement in horsepower by such rocker arm juggling -- but if so -- it can only be due to a deficiency of what the cam design needs to be at a given lift value. Using the rocker arm to induce additional valve acceleration through juggled geometry makes properly controlling the test data impossible. In all cases for doing this, there is LOST efficiency and horsepower from the excessive motion the rocker and valve train are now involved in. (Abbreviated) (See: OVER-ARCING)  ^

COIL BIND: (Cylinder head lingo) Refers to the collapsed dimension of VALVE SPRINGS. COIL BIND CLEARANCE is a necessary dimension that must be treated with exacting care and attention to cylinder head preparation. This minimum clearance varies with engine builders and some cam companies. Often taking the shallow side of safety. The type of valve springs chosen (single coil, double coil, triple coil) have specific cautions to what this minimum clearance is. Collectively, when all coils are assembled with the actual retainer to be used (because the steps for the inner springs can vary in height), this minimum dimension is acknowledged to be .050". Trust me: do yourself a favor and make it .100". For more specifics, see: VALVE SPRINGS & PRESSURE.

COLLECTOR: (Exhaust Systems) This refers to the larger diameter tube which is used to "collect" the PRIMARY TUBES of an exhaust system's HEADER. Primary tubes usually range in size on V6 and V8 engines from 1.375" to 2.250", or more on radical and large displacement applications. While the "collector" will usually range from 3.00" to 4.50" (or more). The collector's main benefit is to give the high pressure waves exiting each primary tube a common chamber that captures and uses the low (negative) pressure following each wave to enhance the scavenging of succeeding cylinders before these exhaust pulses reach the higher pressure outside (ambient) atmosphere. (See: VOLUMETRIC EFFICIENCY) The collector is often where (or close to where) a BALANCE TUBE is used.

COMBUSTION CHAMBER: (Cylinder Heads) The cast or machined area of a cylinder head which (1) seals the top of the engine block's cylinder, (2) contains the valves (on OHV and OHC engines) and spark plug (or plugs) on non-diesel fuel engines, and (3) most importantly provides a specific shape that compliments the necessary blending of angles to and from the valves, as well as (4) controls efficient flame propagation of the fuel ignition process. These shapes are varied, but follow two basic concepts dictated through the internal combustion engine's history that have been influenced by the type of valve and port angle configurations. These two basic shapes are known as WEDGE and HEMI (short for hemispherical). More advanced designs use more emphasis on cross flow dynamics that occur during the overlap phase of valve timing, where exhaust port flow depressions are used with specifically shaped combustion chambers having various figure "8" appearances follow SWIRL concepts, rather than random or previously ignored flow turbulent flow of simpler designs.  ^  (Abbreviated)

COMPOUND ANGLES: (Cylinder head lingo) Almost self describing, compound angles are the combination of three planes of dimensions (X, Y, and a straddled dimension between them ["Z" is the valve travel direction]. Most prevalent with canted valve cylinder heads, these include big block Chevrolets, 351 Cleveland and 460 Ford engines, among others. Compound angles are frequently used on pushrods, although the corresponding head may only be a 2 plane design; case in point: Chrysler Hemi. Here the valves are dimensioned in 2 planes, but the "offsets" needed for the pushrod clearance around the ports lean into a compound angle (compared to their respective lifter bores). (See: Compound Geometry)

COMPRESSION: Any force applied to or resultant from a collapsing pressure. The opposite of TENSILE.

COMPRESSION RATIO: Is a static (fixed) measurement of cubic centimeter (cc's) volume (or Cubic Inch) displacement between the Bottom Dead Center value of a given cylinder's size (piston down), plus the volume of the combustion chamber and head gasket; in comparison to the compressed volume of the Top Dead Center value (piston up) of the combustion chamber (and head gasket) by themselves. This is ONLY a reference value for building and analyzing engine specifications with. It incorrectly presumes (to the novice) that valve timing to let air into the engine also opens and closes with the perfect top dead center and bottom dead center positions of the pistons. It of course does not. Because of delayed reactions from weight of air, mass, inertia, reflection and other dynamics, the valves open before top dead center and close after bottom dead center. So this theoretical perfect TDC and BDC formula is meaningless in heavy data acquisition of cylinder head efficiency and cam timing. The REAL compression ratio an engine runs at is more like 2/3 of whatever the STATIC definition here is. Known more accurately, as the Net Effective Cylinder Pressure (NECP).  ^

COMPRESSION HEIGHT: (Pistons) Used to describe the height of the piston's top deck from the wrist pin centerline. The piston's "deck" is the main flat TOP of piston that meets the piston's side where the rings are. On some pistons, such as hemispherical, there may usually be a protruding piston dome extending up from this tangent point, leaving no specific "flat" to be referenced. But the corner where this design reference resides is still the same, and compression height is a term all engine builders should know who work with piston manufacturers on custom designs, especially when calculating connecting rod lengths and crankshaft stroke combinations for engine blocks of a known DECK HEIGHT.

COMPRESSOR: (Turbochargers) The cool section housing of a Turbocharger where the exhaust driven Turbine drives the compressor wheel (defined below) within the compressor, which then accelerates compressed ambient intake side air from an inlet side of the cool section through to the outlet side and into the engine's induction system.

COMPRESSOR INLET TEMPERATURE (CIT): (Turbochargers) This is the actual temperature as measured just before the Turbocharger's Cool Section inlet, coming usually from the air cleaner, which should be the same as ambient (normal outside temperature of the moment), unless influenced by a supplement system and/or heat radiated by adjacent components.

COMPRESSOR DISCHARGE TEMPERATURE (CDT): (Turbochargers) This is the actual temperature as measured directly after the Turbocharger's Cool Section outlet that is then fed to the engine's induction system, usually after passing through an Aftercooler designed to bring these temperatures back down by 50% to 60% of difference with the CIT. This increased temperature is the natural result of the Turbocharger's compressing of ambient air to a higher "boost" pressure.

COMPRESSOR WHEEL: (Turbochargers) The carefully designed impeller of the Turbocharger's cool section side, known as the compressor. The compressor wheel has two distinctly separate designed stages to its blade geometry, whereby the inlet side of the compressor housing feeds inlet blades known as the INDUCER, which takes the incoming air, compresses it and directs it through the integral exit blades, known as the EXDUCER.

COOL SECTION: (Turbocharger) This is the Aluminum housing half of the Turbocharger assembly, which contains the Compressor Wheel which accommodates the pressurizing of normal outside air into the higher pressure boosted air fed to the engine. It is called the Cool Section because although it is attached to the other half of the Turbocharger in an integral, but separable way, by comparison to the often glowing red 1,500° plus temperatures of the Hot Section, it is relatively sane, at only a fraction of that.  See: TURBOCHARGER.  ^

CROSS-SECTIONAL-AREA (CSA): Head lingo, that refers to the actual cubic inch (usually) measurement of area for a port in a head, or intake manifold passage (or any other device flowing air, gasses or fluids). Cubic Centimeters (CC's) is the other value of measurement, but rarely used in U.S. development, EXCEPT in the measurement of cylinder head combustion chambers, piston domes and cylinder's inch conversions to CC's with calculating NET static COMPRESSION RATIO.

CUBIC CENTIMETERS ("CCs", metric): A value we use in combustion chamber volume, piston dome volume, and deck volume during the process of determining static compression ratio. The CUBIC CENTIMETER to CUBIC INCH conversion formula is: 0.06102; while the CUBIC INCH to CUBIC CENTIMETER formula is: 16.3871. Unfortunately, CC's are also used to describe "port volume" which in the context they are used by many manufacturers and engine builders, is a useless value, as it is usually attached to the promotion or comparison of one head over another as if to correlate to "flow values," and this is incorrect and misleading. See: PORT VOLUME, for more perspective on this subject.

CUBIC INCHES ("CI"): Normally used to determine engine size. The formula for this is as follows: BORE x BORE x STROKE x .7854 x [number of cylinders]. After you calculate through to the .7854 value, you'll have the cubic inches of ONE cylinder; now just choose 2, 4, 6, 8, or however many cylinders the engine is to get the total value for the engine.

CYLINDER HEAD: The essence of what an "internal combustion engine" is all about; the cylinder heads ARE THE ENGINE. This is of course one of the terms added to this section that created the opening note about including "rudimentary terms" to this section, and I hope if you're reading this web site, you've already got a good idea of what a cylinder head is; but I've added this definition mainly to emphasize that all decisions about valve trains, induction systems and even cubic inches, as well as the end performance application for the engine... ALL come from the decisions made about the cylinder heads. (Abbreviated)

DECK: Usually refers to one of either, the CYLINDER HEAD or the ENGINE BLOCK (cylinder block). It is the flat, gasket sealed mating surface which adjoins the HEAD and BLOCK together; and in many engine designs is a critical datum plane (measuring reference) for corresponding dimensions to either or both.

DECK HEIGHT: (Engine Building) Refers to the actual height of the block's deck from the crankshaft centerline. But the term is also used to distinguish the "piston's" deck height beneath the block's deck when the piston is at TDC.

DEGREE-IN: aka: Cam degree-in, or degree-in the cam. See: PHASING.  ^

DEGREE WHEEL: A circular plate made of any number of materials, being of anywhere from 4" and over in diameter, and precisely marked in equal divisions of 360 degrees along its circumference for providing exact measurement of cam timing when mounted upon the end of the crankshaft with a pointer that is synchronized to perfect TDC of the #1 Piston position. A necessary tool for all serious engine builders.

DETONATION: (Cylinder Heads) The more extreme symptom and next stage of PRE-IGNITION, and also the most destructive form of uncontrolled, nearly spontaneous ignition within the cylinder, which is not initiated by the spark plug. Main destruction is to pistons and rings, where the explosive detonation and cylinder pressures occur against the piston's rising motion. See: FLAME PROPAGATION.

DESIGN GEOMETRY: See: GEOMETRY, DESIGN.

DEGREES of ROTATION: A reference to rocker arms, whereby the rocker arm's reciprocating motion (typically referred to in linear terms of measurement) is referred to in terms of its angle of operating motion. Useful to calculating its critical MID-LIFT point from different perspectives of setting its Installed Geometry.

DURATION: See: CAM DURATION.

DOHC: (Cylinder head, engine and cam lingo) Meaning DUAL OVER-HEAD CAM. See: SOHC.  ^

EXHAUST GAS TEMPERATURE (EGT): (Naturally Aspirated & Turbocharged) This is a critical temperature for analysis of cylinder air/fuel mixtures and tuning, per cylinder on any engine. Depending on the type of engine design, typical EGT temperatures with optimum air/fuel ratios and BSFC will hover around 1,450° Fahrenheit, give or take.  (Abbreviated)

EXDUCER: (Turbochargers) See: COMPRESSOR WHEEL.

FATIGUE: Is a term used to describe a real value to metallurgy and alloy properties, although it is not quantified as such. This is because it is the result of microscopic stress that are relative only to each component design, application and environmental operating characteristics. Specific designs of a product are subjected to cyclical tests, where the operating characteristics desired are exceeded in excessive endurance (time of use), operational stresses imposed, or both. Temperature is also a key component to the accuracy required to quantify the results. In summary: fatigue is the result of stress and time acting upon the molecular properties, where flex and temperature fatally disrupt the alloy's molecular integrity. It is especially susceptible and aggravating where machining flaws and STRESS RISERS can be found.

FLAME PROPAGATION: (Cylinder Heads) Refers to the character of the burning process of compressed air and fuel in the combustion chamber following ignition by the spark plug (or plugs); but specifically: its progression from the point of ignition, across the piston and within the cylinder head's combustion chamber in a "controlled burn." Although the process is measured in milliseconds, it is a "controlled burn," and not an explosion, as some presume. When the burning process reaches a point of random ignition, where the incoming fuel is ignited by something other than the spark plug, either through other "hot spots" within the combustion chamber, or improper (and/or inadequate) fuel mixture, or some other unplanned form of PRE-IGNITION, the malady soon graduates to an explosion which is then called DETONATION. Once pre-ignition graduates to detonation, it becomes critical and destructive.

FLANK: (Cam Lingo) See: CAM RAMPS.

FLOW DEPRESSION: Head lingo, used in relation to flow bench testing of cylinder heads, intake manifolds and passage ways. Flow depression is most often equated with "vacuum," and although this is usually the type of testing sought, it actually refers to either positive or negative pressure (vacuum) as measured in a relative of "differential pressure" from ambient (the pressure we stand and breath within and around us). (Abbreviated)

FULCRUM: A one piece device used predominantly with the O.E.M. design rocker arms, consisting of a hardened, radius bottom ball or ball and stand combination (then termed a PEDESTAL FULCRUM). Having a hole through its center for a mounting stud that affixes the rocker arm to the head, the FULCRUM provides the rotational friction surface base in which the rocker arm pivots upon. Not desirable on high spring loads and high RPM operation, where its friction characteristics limit such use, as well as rocker arm design - since it is primarily a "stamped steel" rocker component. This design requires excessive amounts of oil flooding to adequately carry the heat of any performance use of such a design away from the fulcrum. Never use oil restrictors to the valve train on performance engines using this design.

FULL COMPLIMENT: Bearing lingo, used in describing NEEDLE BEARING designs which have side-by-side needles (rollers). The other design reference opposite this, is NON-Full Compliment. This latter design incorporates a retaining spacer ring within the shell that acts like a cradle to keep the rollers separated from each other, with spacing that varies from design to design; but usually is about half as many rollers as FULL compliment. FULL COMPLIMENT is the most expensive, and is the higher standard of the two options, because of its inherent higher unit loading capabilities. Rocker arms having non-full compliment have made a compromising decision based on costs, usually not proportionate in real savings compared to the loss of operational integrity.  ^

G-Plane™: A Pending Trademark of MILLER ENGINEERING INC (MEI). Pertaining to the MEI PRO-SHAFT®, it is one or more machined faces of the rocker body which incorporate a specific angle to other fixed angles of the cylinder head in question, so that a precise reference measurement can be taken and used by the engine builder to facilitate accurate placement of the rocker arm's pivot height with the valve tip, otherwise known as the INSTALLED GEOMETRY.

G-TooL™: The Patent Pending MEI geometry tool developed for finding the exact length push-rod requirement to set a rocker arm's Installed Geometry, through three steps and two methods, including a means to confirm the accuracy of the cylinder head's mounting stud. G-TooL INSTRUCTIONS (Printed)  Illustrated G-TooL Instructions  (Web)

GAUGE HEIGHT/POINT: Is an engineering term for a specific point of reference to the design of something, often used for tangents to or from angles that need to be measured from a linear reference point. It is often used for many other things beyond engineering to reference procedures for inspection, manufacturing and assembly; especially where these other categories meet or overlap each other. We use reference to it here in describing the difference in measuring points needed for VALVE LENGTH, in discerning between measurements for designing your own length from the valve seat to the valve tip, versus the manufacturers usual term of overall length (OAL).

GEOMETRY, DESIGN: This is the geometrical parameters of which a specific rocker arm is designed, i.e., the dimensional distances between its connecting components - but specifically the "angles" in which they are placed within the rocker arm body; which are influenced by the operational angles of the pushrod to the valve on the engine in question. This correlation directly contributes to or from the translation of the camshaft's information to the valve. Because the tangent point locations and angle associated with the push-rod side of the rocker arm determine the degrees of motion the rocker arm rotates from whatever the cam lift is, we also refer to the Design Geometry as "push-rod geometry."

GEOMETRY, INSTALLED: This is the relationship of the rocker arm to the valve stem tip, as determined by the engine builder; or more specifically, the operating angle the rocker arm works as measured to the angle of valve. Although the rocker arm is in a "fixed" position during operation, which pivots about an axis that is stationary, the "perspective" of how it operates with the valve is determined by its pivoting relationship to the valve stem tip, or its shaft's height to the valve stem tip. Raising or lower this pivoting relationship directly causes the roller to move across the valve stem tip in varying amounts, known as the WEAR PATTERN, depending on where its "starting" point is at. If the rocker arms shaft sits too low to the valve stem tip, it spends more time rolling across the valve before it is pushing it "directly" down; therefore it is slower getting the valve off the seat. See comments for other consequences. NOTE: See VALVE TIP ADJUSTMENT.  ^

GEOMETRY, TRADITIONAL: The 1/3 RULE. Always based on two things: a SHOE TIP (scuff pad) contact surface for depressing upon the valve, and second: measuring the angles of motion from the contact surface of the shoe's contact pad. Measuring the pivot point's angular relationship with the valve through this contact point when the valve is approximately 1/3 of its total lift is how measuring the rocker arm's length and pivot position has been done since roughly when the Wright brothers went to Kitty Hawk. Incorrectly so, especially for roller tip rocker arms.

GROSS DURATION: See: CAM DURATION (above).

GUIDE PLATES: Referring to both valve train and cylinder heads; these simple devices have been used for many decades to keep rocker arm alignment with the valve by restraining the push-rod's side-to-side motion. This concept of rocker arm alignment has often been simple "slots" cut (or cast) into a cylinder head, rather than the separate components that traditionally mounted beneath the rocker mounting studs, stamped from "plate" steel, and heat treated. They DO REQUIRE HEAT TREATED push-rods to operate without wear. We highly recommend this concept of rocker arm alignment over the more recent, OEM cheap way out of using "self aligning" rocker arms. Avoid using self aligning rocker arms whenever possible, regardless who's brands of stud mounted rocker arms you use. SEE: Self Aligning.

HARMONICS: Like music, harmonics denotes a self description of "frequency" and "vibration." Both are true in engines, because all matter has a natural harmonic frequency in which it resonates, all based from fundamental physics of mass and density, as well as material composition. For instance, aluminum has a different harmonic frequency than steel; and titanium has a different frequency than either of these. (Abbreviated)

HEAD: (Slang) See: CYLINDER HEAD.

HEADER: (Exhaust Systems) This refers to the main manifold system attaching directly to the cylinder head's exhaust ports for the purpose of routing exhaust gases from the engine. But "good" headers are designed so that the leading tubes leaving each cylinder, known as "primary tubes," are of an equal length and specific diameter to accommodate a specific tuning of these high pressure, high temperature gases so that the energy created by their exiting the cylinder can be isolated from following cycles of the same cylinder, as well as used to enhance the scavenging of gases from adjacent cylinders, where each of these tubes come together in what is known as a "COLLECTOR." However, the term "header" is sometimes used with exhaust that have no collector, where only primary tubes exit from the cylinder head individually, as depicted in early designs on aircraft engines, boats, etc. Such designs still exist on supercharged engines, often used in drag racing, but are more often referred to in slang as "zoomies," because of their up-swept and short length that zooms backward with the flow of the wind. In supercharged applications the science of tuned lengths to cancel out REVERSION waves and match naturally aspirated flow depressions for high volumetric efficiency doesn't exist, because the forced air induction of the supercharger pretty much overrides all these laws of ambient physics.

HEEL: (Cam Lingo) See: CAM BASE CIRCLE.

HORSEPOWER: In its purest definition is: 1 HP= 33,000 Foot-Pounds of WORK Per Minute.  In measurement, it is TORQUE multiplied by RPM divided by 5252. The essence of how horsepower is derived is based upon an understanding of the difference between WORK and TORQUE. FORCE is the common denominator of the two. WORK is DIRECT FORCE operating across a distance; while TORQUE is RADIAL FORCE operating around an axis. The formula of these is: POWER = FORCE x DISTANCE ÷ TIME. To calculate power, both DISTANCE and TIME must be derived, using FORCE across both of these. Where engines are involved, the rotational values of the crankshaft by its diameter is needed. To generate a formula for the TORQUE, the DISTANCE aspect is measured in RPM (revolutions per minute), times the RADIUS, times Pi (3.1416), times 2 (or 6.2832.). TORQUE is measured as Pounds of Force Times Distance. POWER is measured as FORCE times Distance per Minute. The 5252 value in our equation comes from 33,000 divided by 2x Pi (6.2832). Therefore: HP = Torque x RPM ÷ 5252. Something worth noting, is that at 5252 RPM, both TORQUE and HORSEPOWER will be equal. Beneath this value, Torque will be numerically greater, while above it Horsepower will be greater. Just buy a Dyno!  ^

HOT SECTION: (Turbocharger) This is the heavy, cast iron side of the Turbocharger, which is mounted directly to or integral with the engine's exhaust system, often at the common collector for the engine's primary exhaust tubes. The Hot Section is carefully designed in its internal round shape and cross sectional area to feed the hot, high pressure, high temperature gases from the engine into the TURBINE WHEEL, at high speed, often from 40,000 to around 200,000 RPMs, which drives the COMPRESSOR WHEEL through a common shaft. It is not uncommon for the Turbocharger's Hot Section to get cherry red in many installations, and care to location and insulation (when possible) is needed in controlling this very high radiated heat. See: and TURBOCHARGER.

INCLINED STUD: Most engines have a stud that is inclined to the valve's centerline. With the most popular engines, this dimension varies around the 10 or 12 degree range. That is, the rocker arm stud leans into the valve this amount. The engineers arrive at these figures through several considerations revolving around each engine's particular needs - but the over-riding consideration is to average (or split) the difference of the inclination of the pushrod into the valve. With the small block Chevrolet, the stud leans 11ş 20' (11-1/3 degrees). The more you raise the valve stem tip, the higher the rocker arm must be; and the higher you raise the rocker arm - the closer the stud's centerline (and the rocker's pivot length) comes to the valve; making the juggling act of "centering" the roller to the valve nearly impossible from one engine to another. This is one major place (and theory) that most engine builders and manufacturers get into trouble. The logic of "centering" the roller on the valve is indicated as a good method for setting correct geometry, when obviously it's not.  ^

INDUCER: (Turbochargers) Pertains to the COMPRESSOR WHEEL of the Turbocharger's Cool Section, but specifically it is the smaller diameter, and the first stage of two for the complex blade geometry of the impeller. This is the specific part of the compressor wheel which takes in outside air (usually from the air cleaner and connecting tube). After compressing with the corresponding inlet shape of the surrounding Turbocharger's cool side housing, it then forces it outward across larger diameter, integral blades of the same component which are known as the EXDUCER, whereby the air is forced into higher pressure and corresponding temperature (not a virtue) into the engine's induction system, usually after passing through an AFTERCOOLER.

INSTALLED GEOMETRY: See: GEOMETRY, INSTALLED.

KEEPERS: (Cylinder Heads) See: VALVE SPRING RETAINER.

LAMINAR FLOW: (Cylinder Heads) Refers to the quality of airflow through a passage or over a surface. In cylinder heads, it is the quality of airflow through the ports, specifically the INTAKE port. Specifically, Laminar flow is LAYERS of smooth or spiraling airflow that is controlled and consistent. The opposite of this, is TURBULENT airflow, whereby air is reflecting and tumbling in all directions within the port passage. Turbulent airflow on a flowbench can be heard loudly over laminar.

LASH: See: VALVE LASH.

LASH RAMP: (Camshaft) Is the area of the acceleration ramp where the cam lobe actually begins lifting the TAPPET from the BASE CIRCLE (or Heel) of the CAM. Although the lash ramp isn't really its own entity of the cam, merely the bridging area of the cam that has the slowest rate of acceleration to allow the increasing rate of tappet lift (in both height and speed) to continue to increase without too much shock that launches the tappet's contact away from the cam's surface.

LIFTER: Also "tappet." See: CAM FOLLOWER.

LINEAR MOTION: Meaning "in-line," linear is really self descriptive. Here, it pertains to the devices which are designed to follow an in-line motion (but often don't, because of improper rocker arm design and installed geometry). These two devices are of course the "valve" and the "pushrod," which work through the rocker's RADIAL MOTION.  ^

LOBE SEPARATION: See: CAM LOBE SEPARATION.

LOBBING the VALVE: (Slang, Cam lingo) This is a technique of both cam design and application by the engine builder whereby a specific cam grind known to have acceleration ramps purposely designed with an overly aggressive velocity that will accelerate the cam follower and VALVE over the nose of the cam, literally throwing it into space at a given, terminal velocity from a predictable, high RPM, approaching the threshold of uncontrolled valve float.

LONG SIDE RADIUS: Cylinder head lingo, that refers to the top side of the intake or exhaust port, where the bend to (or from) the valve pocket area is made to connect the main port length's passage way, or port "roof."

LONG SLOT ROCKERS: Long slot rockers is a term used to define a modified stamped steel rocker arm body where the adjusting screw slot has been increased to accommodate a longer arcing motion of the rocker arm for more valve lift. The irony here, is why the manufacturers had to make them "long" from where they were, because they were long enough to accommodate the valve lifts being sought. But the real problem is that the slot was IN THE WRONG PLACE to begin with! We use a more appropriate term to describe them: "Wrong Slot Rockers."   ^

MID-LIFT: A Registered ® Trademark, MID-LIFT (HALF-LIFT) pertains to the middle of VALVE LIFT, which is the most critical dimension that must be known to install the rocker arm geometry correctly. But we use this also for the camshaft, which is critical in understanding and timing with setting up precise rocker geometry. You must remember that the rocker arm is a "messenger" of the camshaft, but more importantly, it is a SYMMETRICAL messenger. Which means that it duplicates its motion identically, on both sides of its "rocking". No matter what the geometry of the rocker arm is, whether it is sitting too high and under-arcing, or sitting too low and over-arcing; when it comes time to close, the pattern, the acceleration rate and the angular acceleration will be the exact reverse of whatever it did to open, except as influenced by an asymmetrical cam flank, which is of minor consideration to the geometry point being made here. It is for this reason, that these two motions must be divided equally.

MID-LIFT (Wikipedia: Link to the Internet's Dictionary).

MID-LIFT PATENT: A U.S. Patent, covering the concepts of balanced geometry, issued to James M. Miller. It is the only such Patent ever issued for a concept of precision geometry for any rocker arm.  ^

MILLER RACE ENGINEERING (MRE): Original and first shop of Jim Miller; opened in 1977 and specializing in cylinder head and intake manifold development -- originally for the BOSS 429, but quickly grew into high tech unlimited R&D on small block and big block Chevrolets, Hemis, along with Jim's personal preference with the Fords. Drawing work from all over the U.S., thanks in part to Jim's early writing talents for numerous automotive magazines that covered both in-house programs and the latest tips from competing shops alike, Jim was one of SuperFlow's earliest customers in 1975, working closely with Jack Roush, Holman-Moody and many other notable teams. Jim turned down an offer to head up as Crew Chief from Ford Pro Stock Legend, Dyno Don Nicholson in 1975 to open his own shop shortly later.

MILLER ROCKERS: is a pending Service Mark of J.M. Miller, and the general term used for more than 35 years of Jim Miller's custom designed, special application rocker arms covering many forms of engine applications from Teledyne Continental Aircraft engines, to Harley-Davidson Motorcycles and most American engineered V4, V6, V8 and V10 engines.

MINUTES (Angular Terms), See: ANGLES.

MOHAWK: A North American Indian having a unique hair style.

MOMENT of INERTIA: Is the center of gravity equivalent to any mass which, in the case of rocker arms, refers to the starting and stopping effects of this mass in a reciprocating manner about an axis of rotation. The term is unfortunately misused in rocker arm promotion, as though it's being "lower" is some accurate accomplishment of rocker body design.

MOTION LINE: Is the imaginary line to be made, drawing through the rocker arm's shaft or trunnion axis, and the axis of the roller pin. It is this line which is placed at a 90ş angle to the valve centerline, at MID-LIFT.

MOTION TRIANGLE: Is a term we use based on the tangents located on the rocker arm, at the centerlines of either the pushrod side or the valve side which creates the THREE POINTS of critical geometry; TWO are from a connection between FULLY CLOSED to FULLY OPEN (cam or valve) and the THIRD is the AXIS POINT of rotation of the rocker arm. (See: MID-LIFT ARCING)

MUSHROOM TAPPET, See: CAM FOLLOWER.

NATURALLY ASPIRATED: (a.k.a. NORMALLY ASPIRATED) Meaning an engine which operates through conventional and traditional breathing created by the sequential cycles of vacuum induced by the pistons in timing with the valves, that allow ambient (natural) air pressure to rush in for compression and ignition. Non-assisted by supercharging, turbo-charging, Nitrous Oxide or any other external means of pressurizing of the cylinders.  ^

NECP: NET EFFECTIVE CYLINDER PRESSURE: This is the REAL running compression ratio of an engine. It is the by-product of combined STATIC compression ratio, and cam/valve timing. As explained in COMPRESSION RATIO, the valve timing in reality, opens and closes beyond the perfect top and bottom stroke positions of the crankshaft, therefore not utilizing the full compression of the static compression value; usually about 2/3  or less (i.e., 12:1 CR = 8:1 NECP).

NET VALVE LIFT: See VALVE LIFT.

NOSE: (Cam Lingo) See: CAM NOSE.

OE SERIES: Refers to the MEI designed "Original Equipment" style, 4340 chromoly steel, "shoe tip" Ball Fulcrum rocker arm, with numerous innovations not found on any other such rocker arm (sorry for the commercial). Developed in 1998 for Precision Valve Systems, Inc., following the introduction of the PF Series rocker arm, the OE Series was aimed at stock rebuild market, but soon became THE "stock type" rocker arm to have for serious Late Model and Stock class competitors who were allowed radical camshafts, but constrained to the principle of stock type rocker arms. The first feature that made this rocker a must have, was the 4340 Chromoly steel which no other shoe tip rocker had, because it was too costly on tooling to make. But another revolutionary concept of the design, broke all engineering rules in adopting the MID-LIFT roller tip rocker principles to the rocker body's tangent points of motion. Simply stated, the rocker arm had a shoe (pad) tip, but acted like it was a roller tip, especially when the instructions were followed to the letter. This quickly became a high demand rocker arm which unfortunately was only made in a single 40,000 piece run. Today they have increased in the used market value by more than four times the original $116 new set cost!

OAL: "Over-All Length" refers (as it sounds) to measurements of linear objects, taken usually from their outside dimensions of one end to the other.

OEM: "Original Equipment Manufacturer" refers generally to the original automotive manufacturer, such as Chevrolet, Chrysler, Ford and so on.

OFFSET: This term is usually used to describe the difference of the pushrod alignment with the valve, as the head and rocker arm are looked at from an "over the fender" perspective (head's exhaust ports facing you). As intake ports are widened, there needs to be an offset of the pushrod away from the original port location with some head designs, to provide clearance. An offset guide-plate is the usual method with stud design rocker arms. To this day, it is still common practice to use stud mounted rocker arms which are "twisted" on the stud's axis. TABOO! (See: Twisted Rockers) When this is done, especially on the small block Chevrolets and Fords which have 2 plane valve layouts, you are introducing a 3rd plane angle that will, at some point throughout the lift cycle, LIFT the roller off to one side of the valve tip's edge. (See: COMPOUND GEOMETRY) The roller may lay flat in the closed valve position, but it will definitely leave this alignment during the valve lift cycle. You can see this on an engine that has operated as such, by a ring wear around one edge of the roller (indented ring in worse cases), and the rounding of the valve stem tip itself.  ^

OIL RESTRICTORS: Is the technique of limiting the volume of oil to the top end of the engine through one of several methods, depending on the engine. One method restricts the oil feeding the tappet's oil galleys with plugs in the back of the block, another method simply uses restricted oil hole sizes in the pushrods, but does nothing for limiting a surplus of oil bleeding off from the tappets, if they don't need it. Some restrict in the rocker arm itself, with orifices down to .032" to .043" in diameter. The thinking behind this is to reduce a surplus of oil that is not needed to the upper valve train, so that more is kept in the oil pan (or sump). A key method for doing this is in the main galleys which feed oil to the tappet bores. When Hydraulic tappets have been substituted for mechanical, the need for oil pressure is not as mandatory to keep the hydraulic valve within the tappet in a compressed state. This is reason number one. Reason number two, is the belief that excessive amounts of oil going to the top of an engine which is only used for short periods of time (such as drag racing), is a waste of resources from the crank's rod and main bearings. Reason number three, is "I got needle bearing rocker's and I heard I don't need that extra oil up there." In most cases, all three are wrong.

OPENING THE VALVE: Just as there are two sides of the rocker arm, there are two sides of the camshaft. Most valve train development from three to four decades ago, a pivotal era, had SYMMETRICAL cam ramps. That is, the rates of acceleration to the opening and closing sides of the cam were identical mirrors of either side of the cam lobe's center-line. In the early 70's, ASYMMETRICAL designs significantly increased acceleration of the OPENING ramps, while still setting the lifter down gently on the closing ramps, thus reducing valve bounce and impact shocks. (See: CAM BASICS)

OVER-ARCING and UNDER-ARCING: Is a condition of excessive radial motion across the contact surface of a linear component driving, or being driven by a radial device - as measured by the perpendicular relationship of the radial device's axis to the linear component. In this case, we are talking about the travel of the rocker arm (either side) in relation to the valve or pushrod. Over and Under are opposite consequences, but the net result is the same. In each case the specific angle of rotation from the rocker arm is reduced, compared to MID-LIFT geometry. In addition to this, the acceleration dynamics of the rocker arm upon the valve are influenced, with opposite consequences between the two. "Over-arcing," where the rocker arm is sitting too low in relation to the valve stem tip (the most common condition), will be slower to move the valve off the seat and begin to accelerate as it approaches full lift.  "Under-arcing," where the rocker arm is sitting too high in relation to the valve stem tip, accelerates quicker upon initial opening, then slows down as it reaches full lift, rolling "inward" and UNDER itself. Both of these conditions contribute excessive side loads on the valve guide and valve stem, with subsequent additional heat, additional drag coefficients, increased harmonics in more exaggerated examples and reduced horsepower in all examples. These "symptoms" of bad geometry have often caused engine builders to increase valve guide clearances, change valve specifications and suppliers, as well as other alterations in pursuit of fixing nothing more than "symptoms," instead of the cause (bad geometry). Additionally, dedicated cam testing will be tainted by the lost information from this wasted motion, forcing engine developers to use more aggressive cam profiles to make up for the wasted rocker arm motion; adding more work to the valve train with decreased efficiency and lost power. OVER-ARCING and UNDER-ARCING will impart LESS radial motion to the valve (less valve lift), in addition to accurately mapping the cam's acceleration profile, compared to equally dividing these arcs with MID-LIFT geometry. (See: MID-LIFT ARCING)  ^

OVER-ARCING & UNDER-ARCING (Wikipedia: Link to the Internet's Dictionary).

OVER-HEAD CAM (OHC): Refers to an engine design whereby the camshaft lays above the cylinder head's valve array, and either opens the valves through direct contact upon them with the cam lobes, or through a leveraged action created by first pushing upon a single or dual end rocker arm, which then opens the valve with direct contact from one end of the rocker arm. In dual end rocker arms with OHC designs, the rocker arm may be above the camshaft, while on single ended rocker arms the cam would mount above the rocker arm. Either single or multiple camshafts may reside upon each cylinder head, controlling one or more bank of valves along the X-Axis (length) of the cylinder head.

OVER-HEAD VALVE (OHV): Refers to the engine design that has the valves over the cylinder block, even though it refers to a "head." This is an ancient and misleading acronym. Acronyms for other valve-train designs, like OHC (over-head cam), or DOHC (Dual-Over-Head Cam) are more accurate in their description. The main point that OHV means, is that the engine design has PUSHRODS and ROCKER ARMS. If you're wondering why Over the "head" is distinguished, it is because the prior design (which predates this author) was the FLATHEAD engines, that had their valves in the block. When the small block Chevy came out in 1955, the term OHV was born; and the Flathead was on borrowed time!  ^

OVERLAP: See: CAM OVERLAP.

PA SERIES: Refers to the STUD MOUNT series of rocker arms that MEI designed for Precision Valve Systems, Inc., back in 1996. "PA" being designated for Precision Aluminum, these rocker arms were made available for Small Block Chevrolet and Ford OHV engines, and Big Block Chevrolet, which also fit (sort of) the Big Block Ford. Taking much of their design innovation from the MEI PRO -STUD rockers, like MID-LIFT Geometry (of course), and a "Precision" (Patent Pending) rocker body silhouette that had its top surface designed to match the mounting stud angle for a "precision" reference of measurement for the engine builder to use in setting "precise" INSTALLED GEOMETRY by using the exact, precise length push-rod. A task made especially simple when used with MEI's Patent Pending G-TooL™. Made from 7075-T6 Aircraft Strength extruded aluminum, the PA Series also incorporates the Rifle Drilled, dual flat 8620 trunnion. As can be seen, the word "Precision" in "PA" does NOT stand for the Aluminum, but the precision engineering of the rocker arm design and means to precisely install it on the engine. PA Series INSTRUCTIONS

PF SERIES: Refers to the MEI designed 4340 chromoly steel, "roller tip" Ball Fulcrum rocker arm, with numerous innovations. Developed in 1996 for Precision Valve Systems, Inc., and provided the unique stamping procedure developed by Jim Miller to allow making a stamped steel chromoly rocker arm, previously unheard of because of the high toll of tooling that chromoly imparted, making them very costly. This led to  the introduction of the OE Series rocker arm, which was aimed at stock rebuild market, but soon became THE "stock type" rocker arm to have, since none other would sustain use in high velocity valve trains (see: OE Series). The first feature that made this rocker a must have for a high strength roller tip design, was the 4340 Chromoly steel which no other stamped roller tip rocker had. The other concept of the novel design to such a rocker was the MID-LIFT roller tip rocker principles. The rocker arm also incorporates the precisely ground flat top surface, which facilitates a reference plane for measuring with the rocker stud to get exact Installed Geometry, especially when used with MEI's Patent Pending G-TooL™.  The result, like any MEI MID-LIFT geometry rocker arm, is that the area-under-the-curve was at its greatest, giving the most valve lift, velocity and duration for each degree of crank motion, throughout the entire valve lift cycle; while showing the same, minimal back-and-forth sweep atop the valve tip. The in-and-out pushrod motion was also nearly non-existent. PF Series INSTRUCTIONS

PS SERIES: Refers to the PRO-STAND™ "shaft" style rocker system developed by MEI in 2003 for the most popular brand and model aftermarket and OEM high performance OHV cylinder heads. It was developed to combine our novel manufacturing technique previously limited to NASCAR style rockers from Billet aluminum, with unified rocker offsets as needed per application so that a new standard of Professional, high strength, MID-LIFT geometry billet rocker arm and stand could be sold for a near "sportsman" level price. This approach of manufacturing places the molecular grain of the alloy "in--line" with the rocker body loads, rather than cross-grain as with the "extruded silhouette" designs used by all other "name brand" rocker manufacturers. We also have infinite flexibility in dimensional changes and material use so the very lightest while being the very strongest design can be assured for the amount of material used. Custom designs are also possible from concept to finished product in 24 hours! The instructions for the PS Series are unlike any other illustrations shown here, in that they were purposely designed and published so the ZOOM feature on your Adobe browser will allow you to zoom in for a high resolution, to see the detail of minimal motion results on top of the valve, in matching results of your installation.  PS Series INSTRUCTIONS  For current Product Line see:  PS Series APPLICATIONS

PEAK FLOW (Head Lingo): Refers to air flow through a cylinder head's intake or exhaust port. Specifically it is the point of valve lift where, as measured on a flow bench, the proportion of increase in air flow for the proportional increase of valve lift, as measured in "area," converge and go negative. RULE OF THUMB: When the increased value of air flow per 0.050" increase in valve lift reduces to less than 10% increase in CFM, the port is near death; and increasing valve lift beyond THIS POINT (NOT the "peak" where it loses air flow), is unnecessary forces on the valve train, and thus lost horsepower.

PEDESTAL: This refers to the rocker arm mounting, usually of OEM design which incorporate a fulcrum that has a base to it which provides a solid mount directly to the head, whereas it uses a bolt instead of the older stud and nut design. The bolt is mounted directly through the pedestal/fulcrum and tightened until bottoming out. These are usually "fixed," non-adjustable valve trains where all valve stem length dimensions, cylinder head deck heights, etc., must be set to factory specs. The hydraulic lifter is the only variable for minor assembly length variations of the valve train; and this variable is typically only .050".

PHASING: Is a term used in engine building which pertains to the synchronizing of the camshaft with the crankshaft; more specifically, the cam's INTAKE LOBE with the crankshaft's piston position within the cylinder, measured in crank degrees of rotation relative to the exact TDC (Top Dead Center) point of piston position. This measurement of the cam is often done in one of two ways, depending on the engine builder's preference. One, is by using the intake cam lobe's centerline for reference to the crank position; or two, the actual opening points on the cam ramps are checked with the cam card, dominantly chosen as the .050" lift position of the lifter. "Phasing" is a term limited to simply timing the cam with the crank. (Abbreviated)

PISTON to VALVE CLEARANCE (PV): See: VALVE CLEARANCE.  ^

PISTON VELOCITY: This is a relatively self descriptive term, but its real influence with engine performance is often not fully understood. Piston velocity, or "speed," is the result of two things: (1) Crankshaft stroke, and (2) RPMs (obviously). Piston speed is THE SOURCE for response characteristics of cylinder head air flow through the ports, in addition to the cubic inch volume each cylinder has with every stroke. These are two crucial elements to place in proper context: (a) Airflow VOLUME, and (b) airflow VELOCITY. Cylinder volume in conjunction with piston speed determines all of the tuning characteristics and specification needs within a given cylinder head's design, as well as the fundamental camshaft specs to meet these needs. These two elements can offset each other when choosing cylinder head packages and/or camshafts. In other words, if you have a large cubic inch small block engine, its increased stroke will achieve high torque at lower RPMs compared to a conventional size small block engine, but it can use larger port heads and greater camshaft specs that would more typically be chosen for a high winding engine of smaller cubic inches. (Abbreviated)

PIVOT HEIGHT: This is a term we refer to define the height of the roller pin axis above the trunnion/shaft centerline, when the valve is closed, which equals exactly half of the NET Valve Lift.

PIVOT POINT: The axis of the trunnion or shaft in defining the rocker body design. But also refers to the roller tip, when describing the height of the trunnion/shaft when pivoting the entire rocker body up or down from the roller tip, to establish INSTALLED GEOMETRY, as noted above in pivot height.

PLENUM CHAMBER: (Induction Systems) A component of the induction system which is designed with or added to the runners of an intake manifold. It resides on the entrance side of the intake ports, adjoining the runners into this common open chamber of volume, which is usually beneath the fuel delivery, whether it is a carburetor or fuel injection system, although some designs of fuel injection systems with injectors having direct cylinder head mounting have been used with plenum chambers atop their runners. Usually however, individual runner designs with a single throttle body (butterfly) or independent throttle valves per runner are chosen. Its main purpose is two-fold: (1) To provide a "buffer" between the REVERSION waves coming back from the reflected flow hitting the closing intake valves, and avoid their disturbance of the fuel metering of the carburetor or injector's metering valve (on non-direct port injection designs). (2) Provide a larger "bank account" of air volume directly above the operating port runner that demands it; drawing from the collective supply of air flow through multiple butterflies that usually feed the plenum chamber, which otherwise do not offer adequate volume individually for the instant, millisecond demands that each cylinder would make at high RPMs.

POINT-OF-INSTANT-CENTER: Refers to the tangent point where two intersecting lines meet, usually mentioned with angular devices which pivot and form linkage where this pivotal tangent point occurs. It is a phantom axis point. Pushrods that lean into the angle established by the valve centerline create a point of instant center several inches above the cylinder head which determines load force direction (induced by resistance to that force) upon the mounting locations of the rocker arm. A more appropriate use of the term applies to CHASSIS SUSPENSIONS, where 3-Link, 4-Link and 5-Link control arms, ladder bars and other similar devices mount in a way that run parallel to each other on one plane, with a positive attack angle on another plane that eventually creates a tangent point in space beyond the lengths of the component linkage itself, and having the effect as if they are this total length. A four-link suspension system on a drag car is adjusted so that if a line was projected out from both the upper and lower links, where they meet will have the same force of lift as if a more traditional ladder bar was used and its forward mounting point was at the same tangent point.  ^

POLY LOCK: See: ADJUSTING NUT. (Nick name or trade name traditionally used in slang for ADJUSTER.)

PORT VOLUME: Cylinder head lingo. This is usually referred to in cc's (cubic centimeters) of space. This is something that is nearly ridiculous to real cylinder head evaluation for port flow quality. It is a value, only relative to tuning or cylinder head and manifold design aimed at specific cubic inch engine sizes, where plenum volume in the intake manifold, rpm's being tuned for and engine sizes are all taken into consideration; and usually requiring computer analysis. Yet, this is often used to SELL cylinder heads, or EVALUATE cylinder heads, and this is NOT a good barometer for either. The only exception to this, is when the cc port volume of one given cylinder head combination is used against another by an engine builder who's quantified the "package" under a specific engine combination. (Abbreviated)

PORT VELOCITY: (Cylinder Heads) Refers to the speed of air flow traveling through any given cylinder head's intake and exhaust ports; usually with more emphasis on the intake ports. This is due to the entirely different relationship between relatively cool intake port airflow  versus extremely hot exhaust port airflow, which is really beyond using "air flow" as a term to describe violent high pressure gases escaping from the cylinder. With regard to the intake port, Port Velocity is the result of the Piston Velocity's influence on the port's cross sectional area. Even though its final value is determined by several other factors, such as cam timing, port length, reversion pressures and vacuums, on a naturally aspirated engine it is still ultimately limited to these two, fundamental components: CROSS SECTIONAL AREA and PISTON VELOCITY.

PORT WINDOW: Used in Cylinder Head lingo, refers to the view one has down the intake or exhaust ports of a given cylinder head, where maximum view of the valve head is seen. See: ROCKER MOUNTING & VALVE TIP HEIGHT for additional illustrations that depict port silhouettes within the head, relative to valves.

PRE-IGNITION: (Cylinder Heads) A term used to describe the untimely ignition of fuel in the cylinder before the timed ignition by the spark plug; usually a result of white hot residual particles left over from the previous cycle, or an excessively hot valve, resulting from too lean or inadequate fuel mixture, and/or improper timing which increased operating temperatures. See: FLAME PROPAGATION.

PRECISION GEOMETRY™: (aka "Design Geometry") Coined by Jim Miller refers to the precision duplication of attack angles between the pushrod and valve as defined for an assembled OHV engine, in the design of any rocker arm, to provide a 90 degree relationship on both sides. Traditionally, evidence and results have shown that top American rocker arm companies have only taken the cylinder design into consideration, and designed from a "closed valve" perspective. to this day, the top name companies still request "only" a cylinder for new designs, giving no regard to the more critical influence of the cam side of the rocker.

PRECISION STAND™: A term was dropped by MEI when summer 2005 development on the PRO-STAND systems was consolidated into a new level of high end rocker systems at affordable prices.

PRIVATE LABEL: A term used within the automotive aftermarket trade that refers to the sale of name brand products under a generic name, usually to a peer company or distributor who doesn't want the trade name used. Many companies buy products from name brand manufacturers, then repackage them, or have them packaged with their brand name on either the product and/or packaging.

PRO-SHAFT®: Is a Registered Trademark ® of MILLER ENGINEERING INC (MEI), Pompano Beach, FL., and the original, premium level STAND mounted rocker group for MEI, which originated from the BOSS 429, on through NASCAR designs of the mid to late 1990s.

PRO-STAND™: A Trademark Pending name for MEI, it is a newly released product group pertaining to STAND mounted rocker arms which has overtaken two design groups for two markets. PRO-STAND systems represents the ultimate, high end rocker arm for price conscious engine builders, that is also designed for distribution through select, top WD suppliers. Without a doubt, the ultimate "bang for the buck." PS Series INSTRUCTIONS   ^

PRO-STUD™: Is a pending trademark and a product group rocker arm of MILLER ENGINEERING INC, designed for maximum performance STUD mounted valve train requiring minimum weight, ultimate strength in an aluminum body rocker arm.

PUSH-ROD: The link between the CAM FOLLOWER and the ROCKER ARM; usually made of a single piece of steel tubing between 5/16" diameter on through to 7/16", they have many variations of length and end designs made to accommodate either a male or female radius end connection on both ends to provide freedom of misaligned movement while operating in a linear path. They have been made in different and exotic alloys, as well as shapes and end design combinations; but the net result and purpose is still the same: push the rocker arm up with the least amount of flexing.

PUSHROD CUP: The heat treated, pressed-in steel component of the aluminum rocker arm which mates with the ball-tip of the pushrod.  ^

PUSHROD GEOMETRY: This is a term we've used to be specific in explaining that "rocker geometry" is based on "push-rod" angles with the valve. They are one in the same. See: GEOMETRY, DESIGN.

PUSH-ROD HARMONICS: This is the vibratory motion that occurs at different frequencies and RPM'S from the side-to-side motion, the rebound of compressed forces, influences from heat, friction and worse: high performance operational linear loads on a non-linear axis. In simpler terms: the pushrod misaligning with the tappet's centerline, and excessive in-and-out motion following a pushrod cup (or adjusting screw) around the rocker's axis more than it needs to. Like a worn tire on a car that shakes at certain speeds, but smoothes out at others, "harmonics" also occur in waves before reaching their terminal point. Inaccurate and over-arcing rocker arm geometry magnifies both their symptoms and effects, which we believe also dominos into early valve spring harmonics that then tend to feed each other. See: HARMONICS.

PUSH-ROD LENGTH: This dimension is critical. Accurate pushrod length is also very under estimated and misunderstood as to just how critical it really is. The fact that pushrods are sold by nearly everyone in .050" increments is testimony to how casually the valve train industry treats this; when a fraction of this amount can influence area-under-the-curve and lost cam duration by several degrees. This is the essence of INSTALLED GEOMETRY; especially on the stud mounted rocker arms. Pushrods are the LAST components that should be bought for an engine, since their length cannot easily be established until the full valve train has been mocked up with the cam, lifters and finished cylinder heads in the final machined engine block. On stud mounted engines the pushrod length provides the sitting height of the trunnion below the roller tip's axis when the valve is closed. This "sitting height" is measured in angles to the valve stem centerline by a line that runs between the trunnion axis and the roller pin's axis. Shortening the pushrod drops the tail of the rocker arm, increasing this angle and lengthening the pushrod raises the trunnion on the stud and decreases this angle. This angle is always going to be HALF of the total angular motion that the rocker arm will rotate for its given valve lift. (See: FORMULA) Very few engine builders will need to know, or bother with converting dimensions to know these angles, especially to set their installed geometry accurately, since we instruct and provide easier alternatives. This is simply done by measuring the height of the trunnion's centerline below the roller pin's centerline, and adjusting the rocker's height to be half of the net valve lift. (See: INSTALLED GEOMETRY for illustration.) Because the dimension which influences this angle comes from the trunnion, but our adjustment comes from the pushrod, a required changed in trunnion height of .050" will require changing the pushrod by approximately whatever the rocker ratio is; which would be .075" for a 1.5:1 rocker arm.  ^

QUICK LIFT: With regard to rocker arm geometry, it is a term recently applied by a well known cam company to try explaining some logic to over-arcing rocker arms, which is not explainable (but that's just our opinion). See: VARIABLE RATIO.

RADIAL ARC: As we define and use with rocker arms, technically, is defined as "developing symmetrically about a central point," which is about as accurate a description of the MID-LIFT principle as it can get. Because it is always (and only) measured perpendicular to the axis of rotation, its lateral motion (in-and-out sweep) is the ultimate minimum.

RADIAL MOTION: Meaning "radius" (or circular motion), is the general measurement of any circular motion from a specific radius, but NOT relative to any specific lateral effect (sweep) or angle of incidence (attack angle). It's a generic term used in a general way to convey the difference from, and relative to LINEAR MOTION.

RAM TUNING: Induction system lingo. This refers to the design and tuning of an engine's induction system, and exhaust system, to achieve a flow depression across the piston that exceeds the standard vacuum it creates through natural operation. It is achieved with a concerted effort of exhaust manifold lengths, collector design and often cross-over tubes for the "initiating" aspect of creating this cylinder depression. But the cam and intake manifold are developed to compliment and complete the concept so a precise volume of air is waiting to fill the preceding and magnified flow depression in the cylinder that was created by timing the exhaust's supersonic reversion waves with: (1) the exact moment of peak piston depression and (2) cam initiation for the subsequent cycle. This is designed for a specific RPM for all this to occur. In simple terms: the vacuum waves created by the exhaust system are synchronized with the FLOW DEPRESSION created by both the piston and the reversion waves of the intake tract, so that inertia of the following volume of air (measured by port volume) can over-fill the cylinder. Sort of a naturally aspirated super-charging. See: Volumetric Efficiency.

RATIO: Referring to rocker arms, is the difference in lengths between the rocker's axis of rotation and its two respective opposite ends, divided into each other. This is broken into TWO specific divisions of terms: MECHANICAL RATIO and NET RATIO. The mechanical ratio is the "paper" and actual measured ratio taken between these three points; while the NET ratio is the actual, multiplied "running" ratio imposed on the valve by the rocker arm AFTER FLEX, MISALIGNMENT, GEOMETRY MOTION and other real world factors have been included. All MEI engineered rocker arms are designed for NET RATIO at ZERO LASH, whereby on each engine we have already calculated a compensation factor into the MECHANICAL ratio to allow for these variables. Each engine is unique to these required compensation factors, and any deviations from MID-LIFT geometry will also change the NET ratio.

RATIO DIFFERENCES: From time to time I am asked why we don't offer exactly the same ratios for some applications, like the 1.73 Ford ratios, and instead we only offer the 1.70. The short answer is the market doesn't demand it. But, even though it overlaps some of the points made above, my long answer is below:

RATIO, INCREASED ROCKER RATIO benefits is a question we are often asked. "Why do different engines use different ratios?" "What is the benefit of more ratio?" "What is the consequence of increased ratio?" Here is a few points to understand and consider. In the beginning, as with many engineering ideas pouring in from all over the world, there was no standard, simple answer. There was no need for one. Rocker ratio's are an easy way to increase the needed movement at the valve with less movement at the cam. (Abbreviated)

REVERSION: (Cylinder head lingo) Is a phenomena of reflected airflow which occurs in both the intake track and exhaust passages of an engine, predominantly in or near the cylinder head, but with the induction system, often within the intake manifold (or injection ports), and up through the carburetor or metering valve. It is the by-product of closing valves on the intake side, and subsonic and supersonic displacement of mass within the exhaust track. (Abbreviated)

RETAINER: (Cylinder Heads) See: VALVE SPRING RETAINER.

ROCKER ARM: Mounted atop the cylinder head in a variety of different ways, but always with one end atop the valve stem tip; the rocker arm is a "radial" instrument, designed to convey and transfer the "linear" movements of the pushrod and the valve, respectively. There are TWO (2) sides to the rocker arm, of which both sides "rock" back and forth about a common axis, usually pivoting on a "shaft," a "trunnion," or a "fulcrum." Yet, arguably, for more than 34 years of aftermarket engineering, it has been designed as though the valve is the only component to move, with well known companies teaching very bad and inaccurate philosophies to unsuspecting engine builders. One such fallacy, is to "use longer pushrods" to "place the roller in the middle of the valve stem" tip. Other trinkets have appeared over the years, supposedly to check this necessary pushrod length and set rocker geometry for you. But as you read a little further, you will notice there are many variables to setting a rocker in its correct location.

ROCKER DESIGNS: Every rocker manufacturer has their own reasons for any particular design of a rocker arm - but the truth of the matter is, most copy an already well used design; or worse, take what they think are good ideas from two or more designs and put them together. Without getting into mud throwing, which is not the intention of this information, let me simply state: there are so many variations between one company's rocker arms and another - for the same engine application, that using the logic of placing the roller on the center of the valve, will - more often than not - place the running geometry in a different state for each brand. I've even seen the SAME BRAND of rocker arm for the SAME ENGINE application have different lengths between the trunnion and the roller as a way to change the ratio.

ROCKER GEOMETRY: There are TWO kinds of rocker arm geometry: (1) DESIGN geometry and (2) INSTALLED geometry. You can't do too much about "design geometry" because that is determined by the manufacturer of the rocker arm. But the "installed geometry" is the result of how you installed the rocker arm on the engine, which you do have control over. But be forewarned, all of the roller rocker arm manufacturers use different design geometry, because they never designed from any established standard; consequently, the adjusting screw or pushrod cup is in different angles and heights to the roller tip's location. When you adjust the installed geometry of the rocker arm on the engine, by using different length pushrods or stand heights, you will have different height pushrod locations that will move excessively in one direction or the other - depending on the manufacturer. The lesser of evils is still to have a 90 degree relationship on the valve side of the rocker arm, so that the valve and guide will suffer the least amount of side loading and friction. See: Geometry, Design and Geometry, Installed.  ^

ROCKER HEIGHT: Is the dimension used to set accurate Installed Geometry, by measuring the rocker axis centerline (whether it is a shaft or trunnion), in relation to the roller axis (roller pin) in the CLOSED VALVE position. Whatever the NET VALVE LIFT will be as determined by the rocker's NET RATIO multiplied by CAM LIFT, the ROCKER HEIGHT should be HALF of this NET VALVE LIFT when the valve is CLOSED. As explained in the Installed Geometry page, all measurements must be in reference to a perpendicular angle with the valve centerline, and the valve spring retainer is the best reference plane for this. The first measurement, we've called STACK HEIGHT, accounts for roller tip above the retainer, while the second measurement finds the rocker shaft/trunnion's centerline in relation to this same plane, then added or subtracted as needed in meeting with whatever the NET Valve Lift dictates the MID-LIFT point to be.

ROD RATIO: (a.k.a. ROD ANGLE) Refers to the "connecting rods" between the piston and the crankshaft; but specifically, the mathematical division of a crankshaft's STROKE by the connecting rods LENGTH. Ranging anywhere from 1.3:1 (not good) to more than 2.0:1, the effects of rod ratio on an engine's performance and specifically its torque curve are significant. Rod ratio directly affects how the piston speed accelerates from its extreme positions of TDC and BDC (as well as slows down in approaching these points). The PISTON VELOCITY still reaches its same peak value, as dictated by the crankshaft's stroke; but this acceleration toward this is determined by rod ratio, with the smaller values of rod ratio making these quicker, and the higher values slower, respectively. (Abbreviated)

ROLLER: With roller tip rocker arms, we are referring to the roller which resides atop the valve stem tip. Mythically perceived to actually roll in operation, it does NOT. It merely provides a pivoting point for the direction of operational load imposed by the rocker body, much the same way as a roller lifter provides over a conventional "flat tappet" cam follower. With CAM lingo, we are referring to a ROLLER TAPPET. See: CAM FOLLOWER.

ROLLER PIN: Sometimes referred to as the "roller axle," it is the device which the roller itself pivots around. Various manufacturing methods to attach this axle to the rocker body are used, from pressed-in (interference fit) to "rivet" or "nail head" designs which use a "C-clip" on the other end.  ^

SAE: Standard Automotive Engineers. An automotive standard of engineering used for many aspects of manufacturing dimensions and tolerances beyond the automotive sector. Fine threads follow SAE specifications.

SECONDS (Angular Terms): See: ANGLES.

SELF ALIGNING: Referring to rocker arms, is a rocker arm whereby the alignment of its tip with the valve is assured by protruding rails (on shoe tip designs) or washers (on roller tip designs) which straddle either side of the roller, and have an outside diameter greater than the roller for which they are aligning. The washer (or rails) purpose is to hang to the side of the valve tip and prevent the rocker arm from misaligning during operation. Standard roller tip rocker arms by numerous manufacturers jumped onto this OEM concept by simply juggling the roller width, or slot widths of their original "non-aligning" rocker arm designs to allow for these washers, which usually only extend anywhere from .030", to .050" or .060" along side the valve tip.

SHAFT: Self descriptive, we refer to it here as the roller rocker arm's mounting type, rather than "trunnion," as used in "stud" mounted designs. Shaft mounting consists of one or more rocker arms mounted upon each shaft which is usually secured to an individual stand by two or more mounting bolts, or studs, on either end of the shaft. Shaft systems principle virtue, when designed accordingly, is tying the rocker arms together in forming a more rigid assembly, thus reducing harmonics to the rocker body and valve springs; their principle foe to longevity.

SHAFT SYSTEM: (Rocker lingo) Refers to STAND MOUNTED rocker arms, whereby the first person to coin this phrase was trying to make a distinction between rocker arms that use a removable shaft, from a rocker arm that has a fixed shaft, more aptly referred to as a "trunnion." In all reality, a trunnion (as pertaining to rockers) is a "shaft" and a shaft is a "trunnion." But this slang distinction stuck, and has been repeated for decades. We prefer "STAND SYSTEM" since it is a more accurate definition.  ^

SHIMS: A variety of different strips of metal formed in one of several ways, and of many thicknesses, often custom made from sheet steel or aluminum. They've been around since the beginning of time for adjusting many things on an engine, during engine development; but with regard to rocker arms and valve train, they have been used to change the height of rocker stands on shaft mount (what we call "stand mount") rocker systems. Because every rocker arm manufacturer we know of has always used stands that were too short for MID-LIFT geometry, they have always used this means to elevate their stands up to what each engine builder's requirements were. We consider their use to be band-aid engineering.

SHOE TIP (slang): Refers to the fixed, radius contact pad of the rocker arm which lays upon the valve stem's tip, and directly pushes against it during the operational process. Primarily used on OEM applications for their simplicity.

SHORT SIDE RADIUS: Cylinder head lingo, that refers to the bottom side of the intake or exhaust port, where the bend to (or from) the valve pocket area is made to connect the main port length's passage way, or port "floor."

SHOT PEENING: A manufacturing secondary process whereby select, predetermined sizes of differing alloy "shot" is literally blasted by high pressure across selected surfaces of any ferrous or non-ferrous component, to microscopically hammer down and work harden the surface for increased resistance to surface flaws, fatigue and STRESS RISERS. Although there is no doubt that this process can enhance the surface integrity of some applications; its merits are arguably minor in most cases where adequate materials, manufacturing processes and engineering are properly applied.

SOLIDS: (Slang, cam lingo) Refers to mechanical cam followers, also known as (aka) Solid Tappets or Solid Lifters. See: CAM FOLLOWER.

SOHC: (Head, cam and engine lingo) SINGLE OVER-HEAD CAM. Refers to engines having a single camshaft above the cylinder head, as opposed to traditional engine designs whereby the cam resides in the engine block and uses linkage between it and the head (or heads) for opening the valves. DOHC is DUAL OVER-HEAD CAM and is the same concept except for an engine design whereby the head (or heads) have two camshafts atop the cylinder head, usually dedicated to opening the intake and exhaust valve arrays separately.  ^

SPLAYED VALVE: Cylinder head lingo, referring to valve arrangement with the cylinder head's combustion chamber, as measured in angles along the cylinder head's axis`. Specifically, "splayed" valve array is the leaning of the valves along the "X-Axis" of the cylinder (either end), making the valve stem centerline less than perpendicular with the cam's rotational axis. Rocker stud or pad mounting with this valve arrangement always requires a dedicated, compound angle to achieve accurate arcing trace in line with the valve's motion.

SPRINGS: (Slang) See: VALVE SPRINGS & PRESSURE.

STACK HEIGHT: A term we coined to define the height of the roller pin axis above the valve spring retainer. Used in setting Installed Geometry, it is simply taken by measuring the height of valve tip above (hopefully) or below the top of the retainer, then adding HALF of the rocker's roller diameter in finding the axis. This measurement is then calculated with the second measurement taken from the valve spring retainer top to find the center of the rocker shaft (or trunnion) adding it or subtracting as needed. The final sum of the effort is known as the ROCKER HEIGHT, and this should equal HALF of the NET VALVE LIFT.

STAGGERED VALVE: Cylinder head lingo, referring to the valve arrangement with the cylinder head's combustion chamber, where the intake and exhaust are shifted from each other along the Y-Axis (width) of the head. There is no specific criteria for this definition including or precluding the valve angles to head's deck, but it does mainly apply to valve stem angles which are perpendicular to the cam's centerline. However, compound geometry valve arrays, such as Big Block Chevrolet (where the valve's are "splayed" to the X-Axis) are often included in this term.  ^

STAND: In rocker arms, it is the foundation of what the rocker "shaft" lays upon. In some OEM applications, it is cast into the head and it is more aptly referred to here as a Pedestal. Our description of other companies` "shaft" systems.

STAND OFF: (Induction Systems) The condition of fuel and air that has been pushed back through the carburetor inlets due to REVERSION and inadequate manifold design, to the point where a real and visible fog of this collective symptom lingers atop or at the opening of the induction tract or carburetors. See: PLENUM CHAMBER.

STRESS RISER: A term used to describe the microscopic flaws that develop, usually in a corner, where a concentration of stress and flex will focus, and typically leads to a fracture. (Abbreviated)

STUD: This refers to any single bolt type fastening device, usually made of steel in varying grades, sometimes titanium, whereby both ends are threaded. However, in VALVE TRAIN terminology it is the rocker arm mounting technique used with many engine designs, which we've always referred to as "STUD MOUNT." (Abbreviated)

STUD-BRIDGE™: An innovative Patented device, made of a single bar of aluminum using precision machining to accept specially machined 1/2" Dia. bolts that have half-moon cutouts which overlap extended length, precision ground adjusting nuts for securing the rocker arm mounting and adjustment to an stud mounted, OHV engine design. Follows the same intent of what is known in the trade as a "stud-girdle." (Only better.) STUD-BRIDGE™ is also a pending Trademark (™).

STUD GIRDLE: Is a device made usually of two or more bars of aluminum having half moon cutouts in each bar to provide an elliptical grip upon extended length adjusting nuts when these two bars are tightened together by cross drilled and mounted bolts. Its purpose is to increase rigidity to rocker arm mounting studs on stud mount valve trains, which would otherwise be free floating and flex under high spring pressures and RPMs. It dates back to the 1960's.

SWEEP: A term used in describing the "roll" (or scuffing) across the valve stem tip, or its "sweep across the valve."  ^

SWIRL: Cylinder head lingo, refers to COMBUSTION CHAMBER design for cross flow that induces a preset flow pattern into the combustion chamber which is hopefully controlled into a rotating, high speed "swirl" that fills the chamber in a more complete but aggressive way, which does two things: first, follows the more violent patterns of turbulence created in a wedge combustion chamber, that has better flame propagation tendencies; and second: sweeps across the piston dome during overlap to scavenge the cylinder better. The piston dome, is viewed under these philosophies as the "floor" of the combustion chamber. (Abbreviated)

SYMMETRICAL (ASYMMETRICAL): With rocker arms (or cams) this refers to the equal division of motion, in mirrored fashion. Whatever the first half of the total motion is, is reverse duplicated on the second half; more simply: "balanced," "proportional" or "even." "ASYMMETRICAL" is the opposite of symmetrical; having no balance or symmetry. With Asymmetrical, a term often used in cam designs, the opening cam lobe's acceleration profile is different than its closing side profile. This is done to get what is known as "area under the curve" during the greatest points of breathing capability, and then closing the valve where the most optimum point of ending this breathing is believed to be. Since the balance of these two goals is not symmetrical in proportion to the cylinder head's breathing characteristics, neither is most cams used in today's racing.

TANGENT: Is the term used to describe the actual meeting place of any intersecting lines of shape, form or physics. In rocker arm terms, we use this often to describe the axis points created within the pushrod tip or cup and the imaginary line that stretches between this point and the rocker arm's pivoting trunnion or shaft. Same for the imaginary line between roller's axis and the trunnion. Where these lines intersect is their "tangent" points.  ^

TAPPET: Also "lifter." See: CAM FOLLOWER.

TENSILE: Any force applied to or resultant from loads placed in an opposing direction. (Stretch) Opposite of COMPRESSION. It is also the most reliable form of predictable tension application in securing any component where maximum strength is required. In its ultimate use with fasteners, the actual stretch effect of TORQUE being applied is actually measured in its stretched value, rather than the torque of the applying force, which is less accurate.

THROTTLE BODY: (Induction Systems) Refers to the controlling "butterfly" valve and housing which governs the air/fuel supplied to the engine's intake system tract. More specifically referring to Fuel Injection (FI) systems, where the timed fuel delivery is automatically controlled, in most cases by a computer and prescribed size injector orifices located in either the throttle body itself (not as efficient), the intake manifold or the cylinder head; which are synchronized with the degree of butterfly opening created within the throttle body that is linked to and controlled by the driver's "foot." In these latter examples of fuel delivery, the throttle body is merely controlling the air into the system. In all cases it is designed with an ultimate limited capacity based upon its inside diameter that equals the butterfly's diameter.

THRUST WASHERS: (a.k.a. Thrust Bearings) Usually made of precision bearing steel, silicon bronze (or another suitable alloy as the application may require), these come in various dimensions from only a few thousandths of an inch, to whatever may be required. They are installed on the end of shafts or sides of components operating around a shaft to absorb lateral loads; often found on the end of camshafts, crankshafts, or other bearing supported shafts. (Abbreviated)

TOP DEAD CENTER (TDC): Refers to the position of the PISTON (and crankshaft) in relation to the stroke of the crank. It is the ultimate top of the crankshaft's stroke, before initiating its travel back down.

TORSIONAL TWIST: Is the rotational flexing that occurs around an axis of any linear object where a force of torque has been imposed upon one end that is greater than the absorbed torque transferred through to the restrained or driven opposite end of said object. Camshafts experience this, so do crankshafts (to a lesser and different degree). With camshafts however, the effect is an actual retarding of timing between cam lobes at the opposite end of the cam from those near the driven end. (Abbreviated)

TORQUE: A force applied to or from a rotating axis; measured at a fixed distance from that axis. See: HORSEPOWER.

TRACKING VELOCITY: Is the accuracy of how well the rocker arm converts linear information into radial motion from the cam. The closer to "in-line" that tangents remain with the linear path of the pushrod, the less deviation occurs from the cam's acceleration dynamics. Tracking velocity can be altered by placing the 90 degree tangent either closer to open valve or closed valve, with reverse consequences to each; but doing either alters the cam's profile at the valve, diluting or accelerating the valve's response of the cam information.

TRUNNION: This refers to the singular steel "shaft" within the rocker arm of a "stud" mounted design, having 2 bearings, 1 on each end, with a hole in its middle to accommodate the cylinder head's rocker mounting stud. Because a trunnion is also a SHAFT, we choose to call the other rocker arm design "STAND SYSTEMS" rather than "shaft systems." (If you're going to try to be known for accuracy, might as well go all the way.)   ^

TUNGSTEN INERT GAS (TIG): A term for the process of welding, usually high alloy, non-ferrous or exotic metals, whereby the heat producing arc is electronically conveyed through an electrode made of tungsten, and the arc is protected from the oxygen atmosphere by an inert gas, such as Argon. Filler rod is usually required to be added to the parent metals being welded. A more production oriented version of this same welding principle is done with a welding machine that feeds the filler rod through the Inert Gas nozzle (often like a hand held gun). Known as MIG for Metal Inert Gas, the arc is conveyed by the power fed filler rod being consumed in the welding process, rather than an tungsten electrode, thus saving the more time consuming hand fed filler rod of the typical TIG process.

TURBINE INLET TEMPERATURE (TIT): (Turbocharger) The critical temperature to know on turbocharged engines seeking maximum performance with minimal risk of overheating the Turbocharger, particularly on engines whereby the fuel to air ratio is adjustable. Measured by a temperature probe usually mounted within (but not into) two inches of the turbocharger's Hot Section inlet, where the collective primary tubes of the engine come together, it is a commonly accepted operating formula for the "sustained" maximum temperature to never exceed 1,650° Fahrenheit, and in most conservative circles of use, this limit is held to: 1,550° Fahrenheit. It should noted that these are collective flame/gas temperatures within the exhaust system, not component temperatures of the system, which run several hundred degrees less. However, the cast iron hot section of turbochargers operating in these upper temperature limits will literally become a glowing orange/red, exerting immense radiant heat to all components around them, especially in confined engine bay areas, where ventilation, space and heat shielding have not been implemented as part of the installation. Another important, but less noted (and used) term refers to temperatures taken on the discharge side of the Turbine section, known as TOT (below).

TURBINE OUTLET TEMPERATURE (TOT): (Turbocharger) This is the exhaust gas temperature exiting the Turbocharger after driving the Turbine, which is then passed through a connecting exhaust pipe and muffler on engines so equipped. As with the TIT, the temperature probe is mounted within 2" (two inches) of the Turbocharger's Hot Section outlet. This probe location, which sees the Turbo's cumulative temperature from operation under all types of loads, is instrumental in determining cool down time before shutdown of the engine; which is often required with most applications that have been operating at a high load, in preventing premature wear and failure. This cool down time can be anywhere from one to five minutes (or more) of low or no load operation before killing engine. This brief time is often determined by various manufacturers of so equipped vehicles, and it is all time required to lower the TOT temperature (which is sometimes called the EGT, because it may be the only temperature probe on the engine's exhaust) by 250° to 350° Fahrenheit, making all the difference in turbo life when it has been taken to these higher extremes under load.

TURBINE WHEEL: (Turbocharger) The carefully designed impeller of the Turbo's Hot Section which is the driving first stage of the Turbocharger's operation. The Turbine Wheel, which is driven at super-high RPMs of up to and over 200,000, often operates from sustained exhaust gas temperatures ranging between 1,400° to 1,650° Fahrenheit, but rarely more without incurring imminent damage. This temperature is measured as TIT, for "Turbine Inlet Temperature," whereby a probe is usually mounted within two inches of the Turbocharger's hot section inlet.

TURBOCHARGER (aka: "Turbo"): In its simplest definition, turbo-charging is the process of forcing air into the engine's induction system through use of the engine's exhaust pressure. Turbochargers are essentially two carefully designed circular air chambers that work together through combining a common shaft which has two impellers, one on either end, but each with specific blade geometry for the two extremely different tasks they do. One half of the Turbocharger is a cast iron Hot Section that connects directly to the exhaust primary tube collector, or cylinder head's exhaust manifold, and receives high pressure, high temperature exhaust gas pulses from the engine's exhaust. These gases act upon a Turbine Wheel (above) to drive it at very high speeds, through a common shaft which then drives a Compressor Wheel made of impeller blades formed to a  very specific, complex geometry with two stages of operation. The first stage is dictated by a smaller (minor) diameter group of blades known as the Inducer, which grab and pull the incoming air drawn from the air cleaner (or an ambient intake source), which then "compresses" the air, which raises its temperature by 100° or more, and forces it out through a second stage of blades with a larger (major) diameter, known as the Exducer, that force the pressurized air through a precisely shaped, spiraling passage in the aluminum "compressor" housing, also referred to as the Cool Section. The compressed air is [usually] then sent through an aluminum "heat exchanger" known as an Aftercooler, (but often incorrectly referred to as an "intercooler") where the air temperature is reduced before being sent on to the engine's induction and throttle body. Both sections, the hot and the cool, have inlets and outlets; and in addition, both have a means to control the pressure development of both sections. On the Compressor side (Induction), a Bypass Valve is used, also known as an Over-boost Valve, which allows excessive pressure in the intake tract to escape when a predetermined value has been reached. On the Turbine side (Exhaust), a bypass section designed integrally with the cast iron housing, or separately connected to the discharge side of the hot section, known as a Wastegate, provides excessive pressure from the engine's exhaust system from running the Turbine faster than necessary for the loads which it is designed for. Literally, the inlet side of the Turbine is connected through this alternate passage to the outlet side, and thus "bypasses" the Turbine itself, dumping directly into the  final exhaust system. Some wastegates are adjustable, either manually, or automatically, to compensate for different temperature and/or atmospheric pressure conditions.  ^

TURBULENCE: Cylinder head lingo, referring to air flow patterns and dynamics. Specifically, it is the point at which LAMINAR airflow patterns leave controlled, smooth layer (or swirl) flow characteristics and collide in uncontrolled, random (and violent) shifting patterns, capable of motion in all directions, including "up-stream." Some poorly shaped ports inhibit these bad dynamics throughout the majority of their valve lift ranges. Two of the largest culprits to such ports are PORT WINDOWS that are too straight across the SHORT SIDE RADIUS, creating a "ski ramp" effect, where airflow leaves the port floor as it approaches the valve bowl area, effectively having no flow across the near side perimeter of the valve head because air is literally skipping over it. The second is too big of a transition from a smaller CROSS-SECTIONAL-AREA upstream; where the flow slows down too quickly as it enters this larger area; gets in its own way and tumbles.

TWISTED ROCKERS: This is a term that we use to describe an abnormal operating condition of a rocker arm being turned, or "twisted" on its mounting axis from the correct alignment required to maintain its reciprocating arc true and in-line with the valve's centerline. This is most prevalent on STUD mounted rocker arms where the pushrod has been offset and the stud location has been shifted to the same direction, an incorrect flaw done too often by cylinder head manufacturers, forcing the rocker arm to now be rotated out of alignment with the cam's centerline, as is needed on in-line valve array cylinder heads that use roller tip rocker arms, where this error most often occurs. (Abbreviated)

UNDER-ARCING: A term we use to describe the radial path a rocker arm makes on either end which reaches a 90° relationship to its axis with the valve centerline BEFORE reaching the MID-LIFT point of total sweep; causing it to cut beneath itself as it approaches peak lift. See: OVER-ARCING (above).

VALVE BOWL: (Cylinder Heads) Refers to the immediate material making up the inside diameter of the cylinder head port, just beneath the lowest angle of a valve seat, where the port is traditionally its smallest cross sectional area on a performance head; although many OEM heads, or stock type aftermarket replacement heads may in fact have even smaller ports further upstream (on the intake), to keep velocity high in near stock performance application engines. This area of space that is called the valve bowl is about 1/4" to 3/8" below the valve seat, depending on the head model. It is a very critical standard that works in harmony with the valve diameter to establish THE STANDARD for which a cylinder head's ultimate potential and efficiency are established; as well as the yard stick for port dimension limits that should not be exceeded in pursuit of high efficiency laminar flow ports, on any racing engine. The valve bowl and the valve are GROUND ZERO for all port dimensions and limits. See: CROSS-SECTIONAL-AREA for references.

VALVE CENTERLINE: In referring to rocker arm geometry, the valve's centerline is the most important angle to reference from. This is what must move with the least amount of side load, friction, or wasted motion. Additionally, it is the bearer of the valve spring. And when the various levels of motion, heat and impact are added together; the last thing that is needed - is to ask the valve to straighten itself against an over-arcing rocker arm that is throwing its side loads down at 80 cycles per second. The VALVE MUST move STRAIGHT up and down in its guide, and nothing else. The valve centerline is the basis for assuring the rocker arm operates accurately.  ^

VALVE CLEARANCE: Is a term that refers to PISTON to VALVE (PV). A critical dimension, that is varied in some regards due to the engine application being built for (i.e., Drag Racing, Circle Track, Street, etc.), as well as the RPMs, camshaft type (Roller Tappet, Flat Tappet, Hydraulic Tappet), and also on the engine builder's preference. Some engine builders like more clearance than others, some don't have many options for how much clearance they can build for, due to cylinder head, compression ratio, valve lift and a few other factors that are more important than a certain safety value that exceeds what they may be confident they can "get by with." But it is a dangerous game. The published "norm" for many years has been .080" for the Intake valve and .100" or more for the Exhaust valve. The BOSS 429 Ford Hemi engine requires even more, by as much as .020" on each; while some high RPM small block engines using Titanium valves and reasonably high, but not insane valve lifts can get by with as little as .060" and .080", respectively. If you have the luxury of dictating what you need, without losing compression ratio from over cutting the pistons to accommodate this, then by all means run the higher figures. If you accidentally over-rev the engine, then this extra margin will have paid for itself.

VALVE FACE: (Cylinder Heads) Refers to the precision ground surface of the valve that actually seals against the mating surface of a like angle ground into the cylinder head, known as the VALVE SEAT. Usually, there is a 1/2 degree "positive" angle of interference between these two mating surfaces, meaning that the valve's outer edge contacts the seat first, to allow for valve head flex and efficiency of the valve's perimeter breathing (maximum diameter being used).

VALVE HEAD: The actual major diameter of the valve which encompasses the valve face angles used to seal against the valve seat of the cylinder head.

VALVE LASH: Is the mechanical clearance of free play between the valve tip and the rocker arm, designed to provide adequate clearance in keeping the valve's closing un-interfered with from operational heat and dynamics. Traditionally applied in meaning to mechanical cams, such as solid roller and solid flat tappet designs, it can technically be applied to the hydraulic cam followers too, even though this lash is controlled by hydraulic valve compression within the tappet, through metered bleed-off of the engine's oil pressure. For purposes of continuity across cam applications, we always use ZERO LASH in calculating ROCKER RATIOS.

VALVE LENGTH: (Cylinder Heads) Is an easy to understand, self descriptive term with a couple of notes that should be kept in mind, especially for novice engine builders. First of all, in buying aftermarket valves, these are often referred to by an extended length over stock. So you'll often hear "hundred long," or " two hundred long" meaning .100" or .200" longer than stock, respectively. But be careful, because on some heads models of some manufacturers, these "stock lengths" changed over the years; so this can be a moving target if you're not careful. It's better to have a way of measuring what you really need, rather than just fitting pieces together because they are the most commonly used, or in stock. (Abbreviated)

VALVE LIFT: Is self descriptive, as the amount of lift the valve operates with. What should be noted though, is that this term is used (and unfortunately trusted as accurate) on CAM CARDS or in cam sales, when in fact the reality of it is that it is often NOT accurate. These are more appropriately termed "theoretical" Valve Lift, while an engine's true valve lift is called "NET" Valve Lift. Net Valve Lift is usually LESS than "advertised," but sometimes when improper rocker arms are chosen or incorrectly installed, can be MORE than expected (although rare). In either event, expensive errors in critical clearances, like Piston to Valve can result from "trusting" theoretical calculations. Always check. Valve lift is the result of a multiplied RATIO by the rocker arm of CAM LIFT. Never really standardized before MID-LIFT, "NET" valve lift at "ZERO LASH" is the best ground zero method to calculate net valve lift by, because it leaves the variables such as "lash" up to the engine builder in determining the engine's final valve lift specifications.  ^

NOTE: For CHOOSING VALVE LIFT, see: CYLINDER HEADS to get the whole perspective.

VALVE LIFT DYNAMICS (VLD): This is a term we use to describe the actual, net, valve lift events, in the same way they are analyzed at the cam: LIFT, DURATION and VELOCITY. Rate of acceleration and deceleration is the more accurate description of "velocity" as we use it here. What is so critical about this perspective, even more so than the cam's events, is this takes the entire valve train system into account, and gives us the magnified results of cam information as it is translated through the rocker arm. Any changes in rocker arm geometry, either design geometry or installed geometry, directly and significantly influence the VLD, and the TRACKING VELOCITY. It is through plotting the VLD on graph paper that AREA-UNDER-THE-CURVE is seen, quantified and analyzed for what is really happening to study HORSEPOWER data.

VALVE LOCKS: See: VALVE SPRING RETAINER.

VALVE MARGIN: Is the width of material on the valve head's major diameter, which lays between the valve's bottom face and the valve seat face. It is very critical to valve longevity, to disperse (or absorb) heat in conducting it to the cylinder head's valve seat. It is also critical to air flow, mostly at low lifts, and especially the exhaust, but also has an effect at mid-range lifts, depending on the port, combustion chamber and application. It is sometimes used in conjunction with a radius on the valve's bottom (chamber side) to induce flow across the exhaust, but most testing has shown this not to be beneficial, and in fact opposite.  (See: VALVE TIP LENGTH for illustration of margin's location.) (Abbreviated)

VALVE POCKET: (Cylinder Heads) Refers to the area just beneath the valve seat area of the cylinder head. It is usually 10% less area in size than the valve itself is, on HEMI style heads, and 12-15% less for WEDGE heads. It is also the transition portion of the port's short and long side radius` to the valve seat's bottom angles.

VALVE SEAT: (Cylinder Heads) Refers to the precision ground angle in the cylinder head that provides a seating surface for the valve's VALVE FACE to close upon. It is usually .040-.080" wide for INTAKES, and .080-.120" for EXHAUST valves. Different engine designs, applications, materials and "tricks" by various engine builders will vary these general dimensions in both directions.  ^

VALVE SEAT INSERT: (Cylinder Heads) Refers to the independent component which is separately installed into a cylinder head in providing the precision valve seating surface, necessary with all non-ferrous (aluminum or magnesium) cylinder heads, or any iron heads which required an alternate valve seating material of higher strength, higher operating temperatures or simply restoration from original valve seat damage. Valve seat inserts are made of a variety of materials, from low tech iron to high tech beryllium, Stellite, and other high temperature alloys. With titanium valves, these extra hard nickel and cobalt alloys can't be used without beating the valve faces to death. Emphasizing the importance of choosing the proper material in working with different fuels and various alloy valves.

VALVE SPRINGS & PRESSURE: (Cylinder Heads) Valve springs and their pressure are notoriously misused. There are two interesting things to keep in mind about valve springs. One: They are a torsional component; meaning...when they open what is really happening is that their individual wire is "twisting" on its own, coiled axis, as the spring is collapsing. The second point of interest, is that valve spring pressures, after a point are not really increased because of MASS, INERTIA and RPMs accelerating these things. Valve spring pressures are increased to overcome the extreme waste of valve spring pressure WHEN HARMONICS reach their critical state. See: COIL BIND.  ^

VALVE SPRING RETAINER: (Cylinder Heads) Nearly self descriptive, the valve spring retainer is a flat appearing round disc-like item that sits atop the valve spring and affixes itself to the valve stem by two, tapered, steel cylindrical halves, known as VALVE LOCKS or KEEPERS. As we all know, valve locks act as a door stop in that their two halves are compressed against the valve stem by a corresponding tapered hole in the center of the retainer, of which the upward pressure imparted by the valve spring forces the tapers to "grip" against a matching male/female groove between the keepers and a valve stem. Valve spring retainers from the OEM are made in steel, but aftermarket uses aluminum and titanium, and are substituted from OEM with any new valve spring package to fit larger springs and often to reduce weight. Two basic angles of LOCKS are most often used, 7° and 10°, the latter believed by many to better resist pulling through on radical cams and high spring pressures. With this rudimentary definition out of the way, we can now make the point of FIT needed in a stable valve spring package.

VALVE STEM: The longest diameter portion of the valve which rides within the cylinder head's valve guide and adjoins the valve tip with the valve head, also encompassing the keeper groove or grooves, to affix the valve spring and retainer by split locks. This is also a wearing surface through its continued operation, showing signs of this wear along its outside surface as "scuffing" or galling - in worse cases - from a variety of usage reasons. In valve train efficiency, the lack of proper rocker arm geometry becomes evident on the valve stems and valve tips, first, before most other symptoms are seen.

VALVE TIP: Upper end opposite the valve HEAD, where direct contact from the rocker arm is made to open and control closing of the valve on OHV engines. The VALVE TIP is everything; EVERYTHING. It is from here that all true consequences of the rocker arm's geometry occurs; yet, engine builders indiscriminately add longer and longer valves, thinking all they need to do is to use longer pushrods to compensate. (See: VALVE TIP LENGTH for illustration.)

VALVE TIP ADJUSTMENT: With higher end STAND MOUNTED ROCKERS ("shaft systems") using a common shaft for both the Intake and the Exhaust rockers of a common cylinder or an entire bank of rockers for one head, the VALVE TIP is the only way and the BEST way to accurately adjust the rocker arm's pivot point in relation to the valve tip. Since Exhaust valve lifts will differ from Intake valve lifts more often than not, there is no easy way to adjust the rocker's shaft height of one and not the other, since they are joined. Yet they NEED to be set accurately. (Abbreviated)

VALVE TIP LENGTH: refers to the dimension taken from the end of the valve (tip) to the nearest edge of the KEEPER groove. Usually either .250" or .310", this is critical for standardizing to the retainers and valve springs you choose.

VALVE TIP HEIGHT: This is a term we established long, long ago (in our cylinder head prepping days) to standardized the HEIGHT of the valve tip over the valve spring PAD. It is the best way we know of to keep valve length requirements consistent in designing rocker arms. This is necessary because all heads have either an incline TO or FROM the valve of the rocker arm mounting pad (or studs). Making any deviations in valve lengths impossible to establish an accurate ROCKER LENGTH that keeps the roller in the middle of the valve tip. (See: VALVE TIP LENGTH for illustration.)  ^

VALVE TRAIN: is the most temperamental and demanding category of an engine's design. Most engine failures are the result of something breaking in the valve train, usually a valve spring or rocker arm. But most improvements in engine performance come from valve train development. The most precisely designed and driving component of the valve train, which is aptly referred to as the "heart" of the engine, is precision ground to tolerances less than 2/10 thousandths of an inch (.0002"), and computer designed to tolerances 10 times greater. This precision component works in harmony with three other components of the valve train to open the valve, and determines all of the critical timing for the engine's ability to breathe the greatest amount of airflow, while venting wasted gases, all in an effort to produce maximum horsepower. It is known as the CAMSHAFT.

VARIABLE RATIO (aka: ACCELERATED RATIO): This is a confusing, cam company "hype" term which may be good for selling the virtues of over-arcing rocker arms to unsuspecting engine builders, but it has no bearing in fact; mainly because there is no virtue to a rocker arm exceeding its arc more than necessary. But this "variable ratio" and "quick lift" term is used specifically by some companies (one in particular) to promote a rocker arm which has a different (higher) ratio in the closed valve position, then decreasing in its ratio as it approaches full lift position.

VELOCITY: (Generally) Means the rate of speed of any vehicle, item, component, fluid (including air) as measured in some constant value of time calculated by distance; or distance calculated by time. Angular Velocity is another derivative, and term sometimes used in rocker arm analysis with comparisons of rocker arm lengths and their affect with valve lifts. Port Velocity, Cam Velocity and Piston Velocity are all common terms used with engine builders.

VOLUMETRIC EFFICIENCY (VE): Is a percentage measurement of NET cylinder filling and operational throughput of airflow through the engine in comparison to its displacement. More simply stated, a cylinder having a volume of 100 cubic inches should theoretically be able to only hold 100 cubic inches. In so doing, it has achieved 100% "volumetric efficiency." In reality though, engines utilizing RAM TUNING for specific RPM ranges can optimize their efficiency beyond 100%, going over 110% or more.(Abbreviated)  ^

WASTEGATE: (Turbocharger) The secondary passageway of a Turbocharger's exhaust driven hot section side, that provides either a fixed or adjustable relief of the engine's exhaust system pressure around the Turbocharger's Turbine, to limit unnecessary speed and Turbo pressure beyond a specifically designed amount for the engine in question.

WET FLOW: (Cylinder Heads) Refers to the relative air flow characteristics of a cylinder head's intake port in the more relevant "wet" state that an air/fuel mixture would have, in comparison to the "Dry Flow" characteristics that most flow bench study is done under, where only the dry, ambient air pulled through the port by the flow bench, and used to develop port shape and flow quality values. Although used for many years with specialized flow bench modifications or dedicated designs, it adds another dynamic that can not really be duplicated in a laboratory test stand condition, as it really acts in operation within a competitive engine. Violent inertia pulsations of the heavier liquid within the air flow jet stream from reversion waves; rapid temperature variations, port surface texture effect at these different speeds within the port, in most cases open up more questions to a comparison of dry flow analysis than what may get answered. Which is why most port development to this day is still constrained to dry flow testing, backed up by dyno testing, where more real world data acquisition can be taken and correlated to dry flow data.

WEAR PATTERN: (a.k.a. ROLL PATTERN) In rocker arms we use this term most often to describe and measure the actual wear pattern of the contact surface between the rocker arm's depressing roller or shoe tip upon the valve stem tip. Very specifically predictable under MID-LIFT geometry, it is actually a symptom of correct or incorrect geometry, NOT an accurate method for setting final geometry, because it's measuring the accuracy of a dimension that is at a right angle to its results. Unfortunately, some tools rely on this perspective to confirm correct geometry, lending to a false security.

WRONG GEOMETRY: One of the most confusing and most abused interpretations of setting rocker geometry relates to the "middle of the valve," which I have noted for many years has NOTHING to do with setting correct rocker geometry. Yet here we are, 2009, and after more than three years (2006) since I made the connection of this term to their web site link, we still have one of the most well known, most advertised "brand names" still throws high octane fuel on the fire of confusion.   ^