Can you tell about what CR you have by using a compresson gauge? I know there is something called cam bleed and don't know if that would effect it. I also don't know what cam is in it, any way to tell that without tearing it down? Phil

This expression usually gives a realistic value for the cranking compression reading:

Cranking Compression = (RCR^1.25)*(14.7psi)-(14.7psi)

RCR is the residual compression ratio after the valve has closed at slow cranking speed. It is (what's left of swept volume+clearance volume)/(clearance volume). You can calculate RCR after a cranking compression test.

Measure intake valve close time off a tappet, then use an online calculator to get the RCR value by trial and error with an assumed CR (Compression Ratio). This will give you your CR based on cranking compression and intake valve close time measurements.

It ain't perfect, but CR will be close. Use CR and expected VE in the midrange to pick your required octane rating. Once the engine is in the midrange on up and ripping waves through the intake runner, the intake valve close time becomes unimportant compared to VE and CR.

Were talking the difference between static compression ratio defined as taking the swept volume of your cylinder and compressing it the amount of the stroke (without having to worry about valve timing), and dynamic compression encountered when valves are open part way through the compression stroke.

Dynamic is always less than static compression (the number in the piston book). A compression test is more useful to compare cylinders (they should all be about equal) than to guess your compression (When was the last time the gauge in your hand was calibrated at the bureau of weights and measures for accuracy?)


Larger Dave

If you can tell me your cranking cylinder pressure (what you read on the gauge) and your cam specs or grind number (I'm after the IVC number at .050" lift) I can usually get in the ball park.

Cranking compression (on the gauge) is basically Dynamic compression, isn't it?

If I have a measured cranking compression of 190 psi, can I calculate Dynamic compression using that number (190)?

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Sort of. They are related but not exactly the same thing.

Only if I know the exact cam and on what ICL it was installed.

That's why it is dynamic it changes with the velocity of the fluid (air).