How to perform a compression test.
I like to have a lightly warm engine(a few minutes of running), but not a too hot to work on/around engine. Any difference over 20% between cylinders is a concern and you will generally have performance/running issues. Where as engines within 10% of each other are in good condition. If you are comparing the data for your engine to new factory compression specs, remember that there is a 3% per 1000ft in elevation drop(160 psi new rating will only be 136psi at 5000ft above sea level). You can remove all of the spark plugs to do the test and the engine will spin up faster, but I've seen no difference so I prefer to do one at a time.
The compression tester I use allows me to hook a standard air compressor line to to fill the cylinder with air for on engine valve work or cylinder leak detecting.
This is the tester I've used for the past 15+ years: http://amzn.to/2mXDmMo
