How this calculator works
If air is compressed quickly enough that little heat escapes, its pressure follows the adiabatic relation between pressure and volume. The compression ratio is the ratio of the volume before and after compression, so pressure rises by the ratio raised to the polytropic exponent. The result is an absolute pressure; a compression gauge reads gauge pressure, which is the absolute pressure minus atmospheric pressure. Real engines read lower than this ideal because the intake valve closes late, the charge warms up, some gas leaks past the rings and heat is lost to the cylinder walls. The exponent input lets you match a known engine instead of assuming perfect air.
Formula
P_absolute = P_atmospheric x (X / Y)^n, where n is the polytropic exponent. Gauge pressure = P_absolute - P_atmospheric. To reverse it, X / Y = (P_absolute / P_atmospheric)^(1 / n).
Worked example
A 10:1 engine at 101.325 kPa with an exponent of 1.4 reaches 101.325 x 10^1.4 = 2,545 kPa absolute. That is 2,443.7 kPa gauge, or about 354 psi, which is the ideal upper bound rather than a typical compression test reading.
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Frequently asked questions
Why is my compression test much lower than this?
A cranking test runs at low speed, the intake valve closes after bottom dead centre and the gas loses heat to the walls. Those effects lower the trapped volume and the pressure. A typical warm engine at 10:1 often reads around 150 to 200 psi, well below the adiabatic ideal.
What exponent should I use?
Use 1.4 for a quick ideal estimate. If you are matching a measured pressure from a known engine, tune the exponent until the calculated pressure matches; values around 1.25 to 1.35 are common for real cranking.
Is this the same as dynamic compression ratio?
No. Dynamic compression ratio corrects the trapped volume for valve timing and then applies the compression formula. This page takes the ratio as given, so enter the dynamic ratio if that is what you need to convert.
Should I enter psi as gauge or absolute?
The measured pressure field is absolute. If your gauge read 180 psi, add atmospheric pressure, about 14.7 psi, to get about 194.7 psi absolute before finding the ratio.