Re: Compression checks

[Joe Elliott <jee@domain.elided>]

I think people are forgetting that the exhaust valves still open 
every other revolution, even when you're doing a compression check. 
It's not a matter of just increasing pressure until the combustion 
chamber leaks.  For example, a turbo car with a 7:1 CR will give you 
much lower readings than a Porsche with an 11:1 CR.

The affect of altitude on the gauge is that it uses the atmospheric 
pressure for its zero reference--e.g. when it reads 180psi at sea 
level, the absolute pressure in the cylinder is 194.7psi.  At high 
altitude it would read slightly higher for a given absolute pressure 
but since the absolute pressure in the cylinders is roughly 
[(v1/v2)^k]*P1 where (v1/v2) is the CR, P1 is atmospheric pressure, 
and k is a constant where 1 < k < 1.4, that pressure will also be 
reduced at higher altitudes.

Joe Elliott
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