Brian N. wrote:
<<Why don't the numbers work for my application? I am measuring 180 psi on a
9.6 to 1 motor, and a 150 on a 7.5 to one motor. There must be some other
factor we are not considering. Inertia? Chamber shape? Latitude? Left
Coast? >>
Doug Mitchel wrote:
<< I just talked to a friend of mine who works in engines, and he tells me
that you *cannot* calculate the compression ratio from the measured
compression. He further states that you *cannot* accurately predict
compression from the known (or idealized) compression ratio.
One of the variables affecting the compression is valve timing. As the
intake valve opens later and later, compression will drop. >>
But the "theoretical" (rough theory) figure that we're calculating is a
MAXIMUM, assuming a relatively slow cranking speed, and Doug's friend's
assertion applies perfectly. If we had higher cranking speeds, etc., then we
could perhaps explain a higher pressure, but at a slow speed, how could the
pressure get higher on a normally aspirated engine? Valve timing under these
circumstances could only cause a lower absolute pressure, not a higher one.
-Karl
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