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Porsche Crest Compression readings for a 930

Are these compression readings normal




My readings were:

120 psi on all cylinders (cold)

105 psi on all cylinders (hot)





My 1979 930 has "sc cams" in it.

Old 02-26-2007, 04:46 PM
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They are low...but did you do the test right? With WIDE OPEN THROTTLE?
Old 02-26-2007, 05:38 PM
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In all the testing I've done, I've never seen a significant difference in compression figures with the throttle open vs. closed. Also, compression tests are usually used for relative readings, not absolute. So even if there were a difference based on throttle position, as long as the throttle position is kept consistent throughout the test, the resulting numbers would be internally consistent.

That said, I think numbers around 105 psi sound a bit low, but I'd be far more concerned about cylinder-to-cylinder variation than absolute value. If you're concerned, follow up with a leakdown test -- that will tell you more about the extent and location of wear than you can learn from a compression test.

Rob
78 930
Old 02-26-2007, 05:54 PM
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This test was done with the throttle fully open!
Old 02-26-2007, 07:16 PM
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Those numbers don't sound too low for an engine with a 7.5:1 compression ratio...nicely consistent between cylinders, too.

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Old 02-26-2007, 07:25 PM
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