Could My Heads Be Milled Or Something???

[Jeff Chambers]

Doesn't quite work like that Jeb. Static compression tests are a measure of the pumping ability of the cylinder. Many things affect it such as ring seal, ring gap, piston clearances, engine temperature, throttle (open/close), exhaust restriction, etc. You just can't take the static compression test pressure and divide by 14.7 to get the compression ratio. Remember that you've also got the volume of the test guage to consider in the process. That's why you have to crank it 5 to 10 times before the pressure reading peaks.

For example, my motor is above 12:1 so by your calculations I should see about 176psi on a static tests. My normal readings are more on the neighborhood of 250 to 260 psi per cylinder. That would say that I've got a compression ratio of 17:1! God, I wish!!!! The extra pressure comes from many factors, including the fact that there's no exhaust scavenging present to help empty the cylinders, among others. Bow Tie is right in the normal range for a stock motor. There's only 7% difference between what he saw and what he was expecting. Hot/cold, adiabatic heating, inaccuracy in the gauge, and many more things could stack up to that 7% real quick. Static compression test should be used as more of a qualitative test rather than a quantitative test. As long as all cylinders were within a reasonable range of each other, he's OK.