Cranking Pressure to Compression Ratio

I don’t know how accurate these figures are but i have always used them as a guideline only. They are on the dial of an old gunsons compression gauge that i used for many years.

A lot depends on cranking efficiency which would be around 60%, engine temp etc. You are supposed to have all plugs out, a warm engine, throttle wedged open to get the best readings. There’s a lot of differences from car to car depending on the camshaft profile – on high performance cams the inlet valve opens earlier giving a lower reading which will give conflicting readings.

The main thing is to have a reasonably consistent readings between cylinders.