The
above Illustration shows UNDER-ARCING, whereby the roller tip
side of the rocker arm is moving inward and "UNDER" its path as it
pushes downward upon the valve. Note the lower "Motion
Line" angle of load at FULL LIFT (30° down), between the trunnion
and roller axis, and consider the effect to the valve guide. As with the
OVER-ARCING illustration, which is
purposely shown as the exact opposite perspective, both extremes
illustrate how the "symptom" of a NET valve lift can be the same, while
opposite circumstances prevail. The main thing to understand from this,
aside from the obvious increase in roller SWEEP over the
MID-LIFT-ARC example, is that the
valve lift speed is inverted between over-arcing and under-arcing. As
stated in the drawing, this example has the valve starting off the seat
fast, then slowing as the roller begins to leave the linear path pushing
down upon the valve. When it begins to follow under its axis, as shown
above, the speed of the valve's opening slows proportionately. This is
measured at the crankshaft as additional degrees of rotation to open the valve a specific amount, compared to MID-LIFT
geometry.
Remember, these
illustrations are shown only as a "snap shot" of full valve lift, but
the effect on additional crank rotation is throughout the lift cycle,
requiring the crank to turn further for each increment of valve lift.
But it is more detrimental than that, because it is wasting all THREE
Cam Dynamics: LIFT, DURATION and VELOCITY from what the cam is really
designed to implement. It's also worth noting, that like "ratio" (also
measured only as a snap shot taken at full lift), no one ever checks
these effects in between closed valve and full open valve lift; unless
you've got a Cam Doctor or go through the extra time to graph this
acceleration on paper in fine valve lift and crank degree increments. So
you never appreciate these losses.
