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Leakdown test is more reliable/repeatable.. (archive)
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Posted by JamesM on March 29, 2001 at 09:08:59:

In Reply to: Q: Is 150-155psi acceptable compression - see more posted by Patrick on March 23, 2001 at 09:06:44:

Those numbers are OK according to Brett Anderson, a little low, but OK. A leakdown test will give you a more accurate look into the engine. So will a wet compression test. Squirt light oil into the cylinder and repeat the compression test. If the numbers go up, you need new rings. If not, you have pitted valves/seats, most likely on the exhaust side.

Does your mechanic have a long hose or large diameter tube connected to the spark plug adapter? If you increase the volume of the system, the numbers will be too low. That combined with a low-reading gauge could be a problem. Make sure the engine cranks over fast, too. Helps to have all the plugs out at the same time.

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