Peter wrote:
> Ok, here is where I show my stupidity.
> Bought my first compression guage. It is calibrated in psi. How
do I
> determine the compression ratio? Ie. how do I convert x psi to
n:1?
No stupidity here . . . if you didn't know better, a compression
gauge _should_ read in compression ratios. shouldn't it?
Unfortunately, it's not that simple. The number that we know as
"compression ratio" is just the ratio between the "swept volume" of
the cylinder and the volume of the combustion chamber. It is pretty
much pure math and easy to compute when you know the bore, stroke and
chamber volume. It fails to consider the leakage past the valves,
the leakage past the rings and, very importantly, the losses of
compression due to "cam timing overlap" or periods when the intake
valve is open during the compression stroke. The gauge reading, in
pounds per square inch, will reflect all these things. The gauge
_will_ tell you important things about engine condition when compared
to the specifications for your engine. Remember to block the
throttle open when testing the compression in order to get an
"normal" amount of air into the chamber for "compressing".
Tony
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