The
above is what's happening. When the valve's INSTALLED Geometry has been
set correctly, the maximum amount of angular translation is delivered to
the valve. Note the TRIANGLE
created by the two yellow "Motion Lines" and the "balanced" load in
both closed & full lift on the valve, and valve guide loads. In this case, it is .709" for 30 degrees rocker
rotation. To get to this illustration, you've hopefully seen the
OVER-ARCING and
UNDER-ARCING examples. In so doing,
you've noticed that the NET Valve Lift for each (although the same) was
significantly less, showing only .685". This was a "symptom." The
rocker's rotation was still 30 Degrees, but the net down motion effected
to the valve's linear path, was reduced by .024" Net Lift. As you may
have perceived, it is because the roller was leaving the linear path,
and following itself either over or under the axis of rotation, thus
losing net motion in-line with the valve.
If you
compare this .709" over .685" as percentage, you will see the
"efficiency" has increased 3.4%. If the DURATION of cam event also
increased 3.4%, a 270° cam would now yield 279°. Is that
increase worth MID-LIFT®
rocker arms? YOU decide.
NOTE "C", the
ROLLER SWEEP in our MID-LIFT example above. It is .047". As
you've seen on the preceding examples, they were .184" across the
valve. This too is a symptom. Granted, it is a "predictable" symptom for
those using a computer or draft table accurately. But it is NOT the best
way to set ROCKER Geometry, as one tool being sold does. I will admit
that it is better than plastic pushrod checkers, but if falls way short
of the accuracy needed to maximize cam efficiency at the valve.
The rocker height
Reference (E) is shown to be in the middle of what was needed,
from the OVER-ARCING and UNDER-ARCING illustrations, while the Rocker
Length of 1.370" remains the same. What is NOT shown on any of these,
though, is the effect of raising and lowering the rocker arm, or
extending your valve lengths in relation to the stand or stud mounting. (See:
ROCKER MOUNTING)
The NUMBER ONE QUESTION: "What
makes Miller MID-LIFT rocker arms better?"...
The
problem with "everyone else's" rocker arms, historically, is this
simple: EVERYONE designed in the "closed valve" perspective. This
created the problem of an over-arcing rocker arm that was longer than it
needed to be, because it was sitting too low in relation to the valve
tip. When they divided their length dimensions on paper to establish
"ratio" they always ended up with LESS NET valve lift than what the cam
lift said it should be. A 1.50 ratio rocker arm was easily 1.42, or
1.45. Everyone attributed this to "flex." Eventually, complaining engine
builders forced these companies to "fix" this. So they simply moved the
pushrod cup (or adjusting screw) closer to the trunnion, thus increasing
the ratio, and calling it a "correction factor." But
THAT WASN'T the real
problem.
Since rocker ratio is ONLY
measured at FULL LIFT, they didn't realize how much motion was lost at
the other valve lifts. They had no logic for measuring this because they
had NO STANDARD. Their rocker arm's geometry was not MID-LIFT, so their
designs were over-arcing and wasting the radial motion to the valve, as
you've seen in the preceding illustrations.
They were measuring and
correcting a design that wasted radial motion. The result, is the valve
was reacting slower to the rocker's rotation at all other valve lifts
before full lift is reached. The rate of valve lift at all stages
approaching this will always be less with over-arcing geometry. The area
under the curve is less, the crank turns more for the same amount of
valve lift, and the cam needs to be more violent to make the heads work.
There's more harmonics, more friction, more wasted motion, and LESS
efficiency. Less power. It's that simple. Period. The rocker arm is
"downstream" from the cam; it is the messenger of the cam; and you
cannot judge cam efficiency or performance without always having the
rocker arm set to the MID-LIFT standard. Good or bad, the value of
information from any cam tests is tainted, and the cam's performance can
never be optimized.
^
