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How to come up with a compression ratio?

Posted by BLACK'98DSM, Aug 13, 2019

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  1. BLACK'98DSM

    BLACK'98DSM Proven Member

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    Like the title says. Stock is 9.8:1 (420a) I wanna see where I'm at with my current modifications. I have a compression tester that measures psi. Thank you
     

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  2. kanusky1990

    kanusky1990 Proven Member

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    Hey,

    Static compression ratio is the ratio of the maximum to minimum volume in the cylinder (volume when the piston is at the bottom of the cylinder versus volume when it is at the top).

    This can only be changed by changing the physical components of the engine (crank throw, rod length, piston shape/design, head resurfacing, etc).

    If you changed any of these things from stock, watch this video to learn how to do the math to find out your new static compression ratio. Hint, you can't do it with the engine together.

    If you did not change those things, your compression ratio is unchanged from stock.
     
  3. stillrunning

    stillrunning Proven Member

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    Compression tester will basically tell you engine starting compression not the actual compression ratio.
    As mentioned above static is basically predetermined by the hard parts used and is not changed by mods externally of the engine.
    Dynamic compression can vary based on cam timing and VE of the engine.
     

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  4. BLACK'98DSM

    BLACK'98DSM Proven Member

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    I have "performance" pistons and the head might be a little lower than stock. Thanks for the video.
     

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    kanusky1990 likes this.
  5. silvreclips

    silvreclips Proven Member

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    Are they stock replacement pistons or are they forged JE, wiseco etc
     

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  6. BLACK'98DSM

    BLACK'98DSM Proven Member

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    They are forged, and advertised as "racing grade". They came with the car from the previous owner. Not sure of the brand but as far as I know most performance pistons are designed to improve compression.
    And the heads have been decked at least twice.
     

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  7. delta448

    delta448 DSM Wiseman

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    The head will have to come off to measure. How bad do you want to know? It's arbitrary anyway. Not like there's a formula to plug CR into to get an optimized ignition timing map. As long as it makes as good or better static compression than the stock parts, I'd be absolutely fine with not knowing the exact ratio.

    http://www.guy-croft.com/viewtopic.php?t=129

    ^This explains.
     
    Last edited: Aug 14, 2019

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