The Ford Barn - compression

A rule of thumb is to multiply the compression ratio by the atmospheric pressure. For example, the Model "A" stock compression ratio is 4.2:1, thus
14.7 psig x 4.2 = 61.7 psig. Generally, Model "A" engines will run well with cylinder pressures of 50 psig or more. A newly rebuilt motor should about 76 psig.

The difference between cylinders be none, or no more than 10% (about 7 psig).

There are two types of compression tests, dry and wet. In a wet test is done with 2 or 3 squirts of motor oil in in each cylinder. Wet compression readings should be higher than dry readings because the oil improves sealing of valves and rings.