This is something I have been curious about for a while and have never been able to figure out. On the majority of automotive engines, cranking compression generally reads way higher than expected for the engine's compression ratio and I'm wondering why this is.
For example, my 05 Civic has a compression ratio of 9.9 to 1. With simple math (14.7 PSI atmospheric pressure X 9.9), we can calculate that the compression should theoretically be 145 PSI not accounting for imperfect volumetric efficiency or inevitable leakage. But when tested, that engine actually produces about 215 PSI across all 4 cylinders, which is a typical result for that engine despite seeming scientifically impossible. In theory, assuming no leakage at all and perfect cylinder filling (which is obviously impossible), it would need a compression ratio of about 14.6 to 1 to generate 215 PSI, which it definitely does not have.
Does anyone know why this is? The only potential explanation I can think of is gauge inaccuracy, but I doubt that explanation is the answer since seeing significantly higher than theoretically possible compression is typically expected on healthy engines, not just some anomaly in a few test results.
For example, my 05 Civic has a compression ratio of 9.9 to 1. With simple math (14.7 PSI atmospheric pressure X 9.9), we can calculate that the compression should theoretically be 145 PSI not accounting for imperfect volumetric efficiency or inevitable leakage. But when tested, that engine actually produces about 215 PSI across all 4 cylinders, which is a typical result for that engine despite seeming scientifically impossible. In theory, assuming no leakage at all and perfect cylinder filling (which is obviously impossible), it would need a compression ratio of about 14.6 to 1 to generate 215 PSI, which it definitely does not have.
Does anyone know why this is? The only potential explanation I can think of is gauge inaccuracy, but I doubt that explanation is the answer since seeing significantly higher than theoretically possible compression is typically expected on healthy engines, not just some anomaly in a few test results.
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