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Engine Compression Ratings

An item of interest from “Thoroughbred & Classic Cars”, April 1996.

Q. When using a compression tester on my engine, can I estimate what pressure it should read, if I know the compression ratio? If so, roughly what pressures should I be looking for from an 8:1 compression ratio and from a 10:1 ratio?


A.   If you cannot find out the manufacturer's recommended compression test figure for your engine, or if you have altered its compression ratio, you can estimate the test pressures to look for using the chart below. The figures assume a thoroughly warm engine, open, and a fully charged battery, to ensure a good cranking speed.


Cylinder pressures should not vary more than 10% from each other. Where readings are low, try again after intro­ducing a few squirts of engine oil into the cylinder. If the readings improve, piston ring and/or bore wear is present. If they remain low, suspect valve or head-gasket trouble.

Approximate cylinder pressures for Compression Ratios between 6:1 and 11:1


Note: Compression ratios for A30 / A35 engines are as follows:

803cc - 7.2 : 1,  948cc - 8.3 : 1,  1098cc - 7.5 : 1.         



Compression Rating            6:1          7:1           8:1           9:1          10:1         11:1



Pressure   (psi)                     108          123          131         152          167          181