Compression pressure readings are not particularly useful unless the same exact procedure is followed for each cylinder. Throttle open or closed, all spark plugs removed or only one, number of compression strokes measured, and a lot of other details need to be considered. Bottom line- - - -pick a routine and stick with it, no matter what it happens to be. You're looking for a percentage variation between cylinders, as whatever the PSI reading happens to be is totally irrelevant. Use 5 different compression gauges on the same cylinder, and you'll probably get 5 different pressure readings. Also, use a known air pressure such as the tank pressure of an air compressor to test the gauge before you start the compression test.
A cylinder leakdown test is a far more useful test than a compression test. Then there's the most effective test of compression-related performance, a "Power Balance Test" where the engine is run at a constant throttle setting on only two cylinders, and RPM and intake manifold vacuum is recorded and compared. Compression testing is better than no testing at all, but not much better.
Jerry
