Cranking compression vs. static compression

Discussion in 'Street/strip 400/430/455' started by 70 GMuscle, Jun 30, 2020.

  1. 70 GMuscle

    70 GMuscle Plan B

    My engine is approximately 9-9.5 to 1. Closer to 9.5 I believe.
    My cranking compression is 195 lbs.
    In reading and speaking w a friend. He asked about my vacuum and detonation.
    I have low vacuum no more than 12” at idle around 900-1000.
    Car has detonated I believe. he said to retard cam to get 150-160 pounds. Car will not lose bottom end but will pick up tremendous on top. Have more vacuum and drivability.
    It does sound bad Azz now and when warm drives just fine.
    he thinks car will be faster. Comp extreme energy. 230/236@.050
    501/501
     
  2. Schurkey

    Schurkey Silver Level contributor

    Cranking compression pressure is related to compression ratio, but there's two dozen other variables that affect the results, among them cranking speed, altitude, cam profile, etc. Was the cam "degreed" when it was installed?

    9.5:1 compression ratio and 195 psi seems...unlikely...if that cam is installed "straight up". That or you're way below sea level. Around here, depending on overlap, I'd expect more like 130 psi.

    I would test the compression gauge to see if it's accurate. Maybe it's reading higher-than-true pressure. Most compression testers with quick-couplers use the very-common "Industrial Interchange" "Milton M" style coupler (there are exceptions) and they can be plugged into an air compressor tank fairly easily if there's a "Universal" or "Industrial Interchange" coupler on the hose. See if the compression gauge reads the same as the gauge on the regulator.

    Or just use a different gauge to test compression, and see if both gauges read the same.
     
  3. 87GN_70GS

    87GN_70GS Well-Known Member

    How do you know if it's detonated? Is it audible?

    If so I would be inclined to retard timing a degree or 2, instead of the cam.

    With 195 cranking psi you are right at the limit of pump gas. Could always mix in a little E85 or racing gas.

    Those comp XE lobe profiles have short seat timing for their 0.050 durations. That plus any advanced intake center line would account for the higher cranking psi (intake valve closing early.)
     
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Sounds to me like you aren't sure what your static compression is. Do you know what the cam was degreed to? That cam has a 110* LSA, so the cam card probably said to degree the intake lobe to 106*. If your static compression was 10:1, the DCR would be getting close to Race Gas Territory.
     
  5. 70 GMuscle

    70 GMuscle Plan B

    Yes I do not know exact static compressions
    I do know my cranking is 195 verified.
    With second tester. Both snap on.
    I have speed pros .035 approx in hole.
    73 heads milled .0200.
    .027 head gasket.
    Cam appears to be degreed advanced.
    so intKe closing early. Which makes sense high crank compression.
    Need to pull timing cover and check it.
     
  6. 87GN_70GS

    87GN_70GS Well-Known Member

    An easy first step would be to retard timing a degree or 2 and see if you are satisfied with that, then go from there.
     
  7. 70 GMuscle

    70 GMuscle Plan B

    That’s the plan.
    Wondered if anyone else has tried it.
     
  8. 70 GMuscle

    70 GMuscle Plan B

    Was told it will pick up on top.
    And not hurt bottom.
    Which I hope not.
    Goes consistent 1.70s 60 ft.
    Want to start whittling down to high 11s.
     
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I think Scott meant ignition timing since you mentioned it was detonating. When you retard the cam, you close the intake valve later reducing DCR. You do transfer some low end to top end. I'm not sure you are going to pick up 1/2 a second like that though. The most important part of a 1/4 mile pass happens in the first 60'. A BBB has some low end to spare though.
     
  10. 70 GMuscle

    70 GMuscle Plan B

    Thanks Larry.
    I know I’m on cusp of detonation.
    I have heard it. Put Holley on for now and don’t hear it.
    Seems to run nice. I need to contact cliff eventually for parts for qjets.
    I know it won’t get me 1/2 second. Just a first step.
    Maybe 373 and a nice converter from Jim.
    Would love to get heads but other projects come first.
    One thing I did miss when intake and heads were off the port size of intake. And you showed some pics of a plenum opened a bit.
    Lots of great info here.
     
  11. 70 GMuscle

    70 GMuscle Plan B

    I was thinking cam.
    I need to check the degree it’s installed.
     
  12. 87GN_70GS

    87GN_70GS Well-Known Member

    When you heard the audible detonation, did you try reducing your distributor ignition timing, you know by turning your distributor housing and use a timing light?
     
  13. john.schaefer77

    john.schaefer77 Well-Known Member

    If it went away with a holley possibly your other carb is too lean? The holley may be jetted fatter.
     

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