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1. always use different compression testers, never trust 1 tester
2. cranking speed, number of hits, throttle position, air temperature, cam profile, air filter restriction, and other factors influence compression, never take a number at its face value
The most important aspect of the test is the difference between cylinders. If you see a low or high cylinder(s) you should try some basic things to even them out. You can try de-carboning them (water injection or seafoam etc...) or remove the head and manually clean off carbon. You should wash the intake manifold and ports out because there is a chance that oil and carbon deposits in the manifold are causing changes to gas velocity which influences compression. Inspect the pcv system and replace the pcv valve and hoses. Old hard Oil or lots of aging liquid oil hanging out in the cylinder raises compression and causes detonation so washing out the cylinders is also applicable approach. I remove all plugs so it won't hydrolock and literally degreaser them with water and purple power for example. Then immediately re-oil them, pour oil and crank it over with plugs out to spread the oil rapidly so it doesnt rust. The fresh new oil isn't a danger as it lubricates the cylinders and prevents rust, its the old oil accumulating in the ring packs and deposits forming from old oil hydrocarbons that need to be removed.
Next, how do you know when compression is too low.
Answer: when the fuel economy of the engine is too low for your application.
For example say I have an engine that runs fine but it has low compression on every gauge. I tried cleaning out the carbon. I make sure the valves are all fully closing. I cleaned the pcv system and made sure PCV is working fully by measuring crankcase pressure so the engine doesn't smoke or have any cleanliness issues. But the fuel economy is much worse than it should be on the stock tune. Or maybe I tuned it and still the economy is very bad. Like 18mpg instead of 27mpg. Thats how you know maybe its time to get some new rings.
If the economy is good enough, compression is fairly even or at least then engine doesn't shake noticeably and seems to be runs fine, pcv is intact- then ignore the results no matter how low on a gauge. Heaven save us from people that rebuild their engines because a gauge told them to "oh no the gauge says its bad" worst thing you could do. Props to a mechanic that understands that. Results help you diagnose a problem and there is no problem with a perfectly running engine if you are satisfied with the economy.
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