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Old 05-01-2012, 03:32 PM   #1
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Thumbs up Compression Test Results

As referenced in my previous (and as it turns out, more than a little paranoid) thread about a potentially blown head gasket, my 06 S was in the shop today to be checked out.

The shop called me this afternoon to say that everything looked normal, no hydrocarbons in the coolant and my compression test came back: 5 cylinders @ 150# and one @ 145#. Consistency-wise I'm pretty happy with that, but the raw numbers do seem a little low for a high compression engine. Does anyone know what the spec is for the 3.2 motor?

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Old 05-01-2012, 04:51 PM   #2
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A compression ratio of 11:1 yields a maximum 161 psi compression measurement (11 times the atmospheric pressure of 14.7psi).


**EDIT**

It seems that the conversion from compression ratio to expected PSI and how the actual measurements are taken is quite complicated and not at all straightforward. Hence, I am not sure about my statement above. Will investigate further.
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Last edited by thstone; 05-02-2012 at 09:19 AM.
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Old 05-02-2012, 02:10 AM   #3
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Temperature must play a big part, at my last service had a compression check on my 2.7l - 2001,
all cylinders 210 except #6 which was 190 ambient about 25 deg C
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Old 05-02-2012, 03:55 AM   #4
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Hopefully there wasn't any coolant in the oil. A cylinder leakdown test would be good to perform.

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Old 05-02-2012, 09:43 AM   #5
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Way low.. My guess is this shop wasn't keen enough to do the compression test with the throttle locked open.. The throttle plate being partially open in the idle position effects chamber filling nd seriously effects the compression test.

Any healthy Boxster S will produce over 200 PSI on a compression tester, even here at 1765', as long as the throttle plate is locked open. 150# is very low with the throttle plate open, if it was.

Its pretty simple, air that isn't inducted can't be compressed, so the readings are always low with a closed throttle plate and they are also impacted by intake system characteristics more. Ask them how they performed the test.

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