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