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Compression test absolute values

Discussion in 'Street/strip 400/430/455' started by Electra Sweden, Mar 16, 2025.

  1. Electra Sweden

    Electra Sweden Well-Known Member

    So I was watching this video:

    The guy seems to rely a lot on absolute values on compression tests for assessing the engines dynamic compression. Which in its turn is an important predictor for engine characteristics. The guy uses 180 psi for a rule of thumb target value when using pump gas.

    What is your view on this? In my experience it has been hard to obtain high repeatability on compression measurements. As such the absolute values obtained would not be as relevant for a predictor. Maybe repeatability is much better on a freshly rebuilt engine?

    I also note the approach varies between engine manufacturers. Buick does not list any target absolute values in its 1970 shop manual, but other manufacturers do.
     
    Electra Sweden, Mar 16, 2025
    #1
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Static compression ratio modified by valve timing (camshaft choice) See attached.
     

    Attached Files:

    LARRY70GS, Mar 16, 2025
    #2
  3. Electra Sweden

    Electra Sweden Well-Known Member

    Interesting read, but does not address the original question regarding compression test results.
     
    Electra Sweden, Mar 16, 2025
    #3
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    My view is that, yes, Buick does not specify specific target values. They instead state that the results must be balanced such that the lowest reading cylinder is at least 75% of the highest reading cylinder. If you want absolute values, start a thread soliciting member's compression pressure results on a completely stock 1970 455. Not sure if you'll get a lot of results, but it is worth a shot. Most rebuilt 455's will have different static compression and different camshaft timing, so their results will be different. You need to compare readings on several stock 1970 engines to come to a consensus on what the specific target values should be. An average is probably the best you'll get. Maybe ask Jim Weise. He has probably seen a lot of stock engines.
     
    LARRY70GS, Mar 16, 2025
    #4
    Electra Sweden and Dadrider like this.
  5. Schurkey

    Schurkey Silver Level contributor

    So many variables when doing cranking compression tests.

    Battery voltage affects cranking speed. Altitude affects intake vacuum. Cam timing changes dynamic compression.

    And nobody ever thinks about gauge accuracy/repeatability. I test my compression gauges occasionally using "shop air". Most compression testers use Industrial Interchange quick-couplers. (Some don't.)

    Compression_Tester_Tester_01.jpg

    Compression_Tester_Tester_02.jpg
     
    Schurkey, Mar 16, 2025
    #5
  6. Mark Demko

    Mark Demko Well-Known Member

    To me compression tests only show how much pressure you can achieve after 4 or 5 cycles of cranking.
    In reality, the piston only has one stroke/chance to squeeze the air, that’s after the intake valve closes on the upstroke, not multiple strokes as seen during a cranking compression test.
     
    Mark Demko, Mar 17, 2025
    #6
  7. Schurkey

    Schurkey Silver Level contributor

    True.

    OTOH, we know from cranking compression tests that cranking speed has a dramatic effect on compression pressure generated...and a running engine at idle is going to be spinning about four times faster than when it's cranking. That means there's 1/4 as much time for the compressed gasses to escape via poor valve seal or leaking rings. As engine speed goes up...that time decreases further.

    Which is one reason why a cylinder with 50 psi of cranking compression may be dead at idle, but contribute somewhat at 2000+ rpm.
     
    Schurkey, Mar 17, 2025
    #7
    Mark Demko likes this.
  8. 87GN_70GS

    87GN_70GS Well-Known Member

    185 ish psi cranking compression equates to roughly 8.0 ish DCR, which is about the limit for 93 pump gas
     
    87GN_70GS, Mar 17, 2025
    #8
    Mark Demko likes this.

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