You need to use absolute pressure (gauge reading +14.7psi). But you have the wrong calculation method. And for earth air nR=53.3 (with "normal" atmospheric conditions). If you switch a stock cam to a high performance cam while doing nothing else to the engine, you end up with lower gauge reading for the compression test because of increased valve timing overlap.Ĭorrect. But you can't tell directly from the pressure gauge reading, as the amount of air in the cylinder depends greatly on the cam profile and the amount of valve timing overlap. Maybe close to 13:1 (by sheer coincidence). "I'd expect the theoretical compression ratio would be higher than 13:1." To make a proper comparison between compression test pressure and theoretical compression ratio you have to know the cam profile. Modern cams have considerable valve timing overlap, so the I and E valves are never both closed at bottom of stroke. But that has not much to do with pressure gauge readings. "If you take volume at bdc and divide it by volume at tdc you get a theoretical compression ratio." Have patience (and have mercy on me for eating bandwidth with small details). Since compression ratio and test pressure comes up periodically I will expand on the theory a little, hopefully enough to get the basics across. I can't let this bit of misinformation circulate in public. Gauge indication is not linear with compression. if you take the pressure indicated on the compression tester and divide it by 14.7 which is atmospheric pressure you have actual compression ratio." today the readings are 1-4: 40 psi 30 psi 190psi 50 psi." Compression ratio vs pressure The MGA With An AttitudeĪt 09:47 PM -0400, someone who shall remain anonymous wrote:
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