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  1. #1
    monaroman76's Avatar
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    Recalculating compression ratio

    Hey guys, I got to thinking the other night about how much getting your head decked affects your engines compression ratio.

    When I had my head off a while back, I had a machinist take 7 thou off to get it flat. I did the following calculations to recalculate my comp. ratio.

    Stock bore is 100mm
    stock stroke is 63.4mm

    cylinder displacement is 498cc

    Stock head volume can be easily calculated given that we know stock comp. ratio is 8.4:1

    (498 + x)/ x = 8.4

    This comes out for x = 67.3cc
    This represents the volume in the combustion chamber with the piston at TDC.

    Taking 7 thou off the head removes 1.4cc of volume from the head.

    The new comp. ratio is

    (498 + 65.9)/65.9 = 8.55:1

    As a pretty accurate estimate, for every 5 thou you remove from either the head or block, you increase comp by 0.1:1.

    I dont know if this is helpful to anyone, but I couldnt find any info using the search.


  2. #2
    Thanks for the good info, but I don't know if your approach can be a good approximation or not. Your calculations assume a linear volume reduction when decking the head, based upon the diameter of the piston. This isn't the case; the combustion chamber is spherical profile, with a complex shape (cloverleaf) and as you remove more head deck, the volume decreases faster. And the piston crown is not flat. The bottom line is that the only feasible way you can get a good estimate is to actually measure the change in volume. (or if you were really good at math you could model it with a computer using a cloverleaf-shaped spheroid and slice it horizontally in .001 steps and see the change in volume.)

  3. #3
    Stockrxp's Avatar
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    Slow91 assured me it wouldn't raise comp enough to matter

  4. #4
    monaroman76's Avatar
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    He assured you what wouldn't increase comp? Decking the head?

  5. #5
    Quote Originally Posted by Stockrxp View Post
    Slow91 assured me it wouldn't raise comp
    Well, that is wrong, if that is really what he said. He probably meant that it won't raise the compression enough to make any difference. (it won't)

  6. #6
    monaroman76's Avatar
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    Quote Originally Posted by Sea Dood View Post
    Well, that is wrong, if that is really what he said. He probably meant that it won't raise the compression enough to make any difference. (it won't)
    +1. I'm thinking that is what he meant. Any time you remove material from the surface of your head, you are definitely going to effect your CR.

  7. #7
    Stockrxp's Avatar
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    PM slow91 he will answer you questions

  8. #8
    Stockrxp's Avatar
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    he did for me

  9. #9
    monaroman76's Avatar
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    Quote Originally Posted by Sea Dood View Post
    Thanks for the good info, but I don't know if your approach can be a good approximation or not. Your calculations assume a linear volume reduction when decking the head, based upon the diameter of the piston. This isn't the case; the combustion chamber is spherical profile, with a complex shape (cloverleaf) and as you remove more head deck, the volume decreases faster. And the piston crown is not flat. The bottom line is that the only feasible way you can get a good estimate is to actually measure the change in volume. (or if you were really good at math you could model it with a computer using a cloverleaf-shaped spheroid and slice it horizontally in .001 steps and see the change in volume.)
    Hey mate, I understand what your saying there, and I didnt really take into consideration the combustion chamber profile on the head. This would affect the accuracy of the calculations a bit for sure, but I would still think it would be a reasonable estimate.

    Im not sure that the piston top would affect the calculations at all, as the volume in the chamber at TDC (Not necessarily just in the head) has to be given as 67.3cc to achieve the stock CR of 8.4:1. This would mean that the head volume + head gasket thickness + Piston top volume would be 67.3cc.

    Given that the HG is a perfect cylindrical shape, removing 2 layers from the stock gasket (which I believe is 20thou) would increase your CR by 0.4:1.

  10. #10
    Eastside Powersports Slow 91's Avatar
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    Quote Originally Posted by Sea Dood View Post
    Well, that is wrong, if that is really what he said. He probably meant that it won't raise the compression enough to make any difference. (it won't)
    What i meant was, decking .003-.004" which is what it usually takes to clean up most heads isnt going to raise your compression a whole point or two. I have decked heads as much as .015" and saw a 3psi increase.

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