Walt,
I had to think about it for a few minutes, but you're right. I had not
considered the fact that, given the same initial valve lash (.010), the
higher ratio rocker will contact the valve stem earlier and release
later.
Touchi.
Several things though when considering the original post. Something to
do with a decrease of 30 PSI per the compression gauge reading.
As you pointed out the valve lash would only need to be altered by
.0004" to completely negate the effect on duration. 4 tenths of one
one-thousandth of an inch would be the difference in the amount of drag
felt as the feeler gauge was slipped between the rocker and valve stem.
I thought about using my CAD program to simulate the difference in dwell
degrees, but I'm certain that it would be negligible. Minutes, maybe
only seconds of a degree...I think. Engines are not precise enough to be
affected by a .0004" difference in valve lash, or the subsequent
difference in duration that may be caused.
Jim
-----Original Message-----
Jim,
I agree with your premise that you can't change the cam profile with
rocker
ratio, but I believe you can change the effective duration at the valve.
Since valve travel and acceleration are related to the cam profile by
the
rocker ratio, the valves will actuate more quickly with the higher ratio
rockers. If you have .010 of valve lash, a 1.5:1 rocker will require
.0067"
of lift at the cam to obtain "zero" lash. If a 1.6:1 rocker is used,
.0063"
of lift at the cam is required to get to "zero" lash. Since the lower
number
occurs earlier on the cam lobe, your duration has been slightly
increased.
I'm not sure how significant this would be, but it will effect overlap
and
duration to some degree. My guess is that the increase in valve lift and
acceleration with the increased ratio provide most of the performance
increase.
I agree that compression tests can't really be used to compare different
engine set-ups. There are too many other variables in the equation.
Walt
74 TR6
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