PPRuNe Forums - View Single Post - Cylinder compressions
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Old 4th Nov 2007, 18:41
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SNS3Guppy
 
Join Date: Oct 2005
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Cylinder tests are a diagnostic tool, but the average layman puts far too much stock in the results. The same engine given three different differential compression tests, will produce three different results. Change mechanics performing the tests, you'll get different results. Change the temperature, you'll get different results. Change the test set, you'll get different results. The specific numbers aren't particularly meaningful, though typically you should see 75% of the input pressure as a general guideline.

When I perform a differential compression test, I'm more interested in hearing where air is leaking, than what the actual numbers are, and only remotely interested in the number trends. It's not uncommon for the compression numbers to increase with subsequent tests later on.

Ring position has no real significance, though some seem to think so.

Compression testing can revel ring blowby and cylidner wear, but is a useful tool for targeting valve seating issues.

You don't need to interpret numbers you're seeing. What you've posted isn't bad, but to make any kind of judgement on those numbers alone would be pointless. What you have there is a very small piece of a much larger puzzle.

Hopefully you're doing spectrometric oil analysis with every change, and doing regular oil changes, checking mag timing and internal timing, looking for induction leaks, boroscoping the engine, flowing injectors, etc.

Oil color and consumption is a very poor indication of engine health. A healthy engine can be consuming a considerable amount of oil.
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