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Thread: High compression test readings

  1. #1

    High compression test readings

    Just had a annual service done on my 73 and noted that the compression test results seem high--200--215 pounds. I run premium all the time but now worry this may not be good enough. Engine was rebuilt with 9.5:1. pistons but that's about the only big change.
    Do I need to do anything??

  2. #2
    I would not worry about the absolute value of a compression test reading, as gauge calibration and test procedures can vary widely. You should only be concerned with a wide variation between individual cylinders done with the same gauge and procedure. I believe the Porsche spec for this difference is about 21psi, so your readings seem to be within the acceptable range.

    Even today's lousy CA 91 octane premium pump gas should be OK for a 9.5/1 compression ratio engine. My '67S has 9.8/1 compression and has not suffered any detonation from running 91 octane unleaded for many years.

    Did you actually measure the ratio when your engine was built, or did you accept the manufacturer's number? I have heard that this value can fluctuate pretty wildly (advertized vs. actual), depending on manufacturer.


  3. #3
    That's the quoted ratio. It was an aftermarket piston set by JE. It has run for a year without blowing up so I guess I am just in the Porsche Paranoia mode. My other Porsche with RS pistons is around 170 at the same shop. That seems about right.

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