Finding the Compression Ratio
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  1. #1
    Senior Member ronnydog's Avatar
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    This is beyond me on how to come up with the compression ratio.

    What I do know is the motor is 71mm piston x 62mm stroke and the top dead center head volume is 26cc. I could use some help.



    Thanks, Ronnydog

  2. #2
    Senior Member lizrdbrth's Avatar
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    In raw numbers it's 10.4:1 with a flat top piston and 26cc head.



    Not accounting for head gaskets or domed pistons though.



    I can explain it to you, but I can't understand it for you.

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  3. #3
    Senior Member ronnydog's Avatar
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    Quote Originally Posted by lizrdbrth View Post
    In raw numbers it's 10.4:1 with a flat top piston and 26cc head.



    Not accounting for head gaskets or domed pistons though.




    Head gasket .90mm and the piston is domed, stock piston 9.5 to 1, domed piston 10.25 to 1.



    Ronnydog

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  5. #4
    Senior Member lizrdbrth's Avatar
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    The gasket adds 3.56 cc's to the chamber volume, which brings it down to 9.3:1 with flat tops.



    With gasket the Wiseco dome would need to take up at least 3cc's to bring you back to 10.25:1. I'm pretty sure it will, but I don't have a decent way to measure it.



    Assuming it's 1mm tall it takes up 4mm, minus the edges.



    In any case you should be well over 10. Maybe Sebastian has calculated the dome.



    Those Nikasil XT's are killing any chance of anyone bothering to make us anything bigger. Not that I want you to get your hands on anything bigger , but I'd be pretty tickled with a 72mm at 9.5:1.



    I can explain it to you, but I can't understand it for you.

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  6. #5
    Senior Member srs713's Avatar
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    the hardest part of compression calcuation is the TDC volume. The surest way of finding that is with the head off. Close the valves and see how many ccs of liquid it takes to fill it. Same with the piston head @TDC.





    Once you have that the rest is simple math.
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  7. #6
    Senior Member xdac's Avatar
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    Probably bigger than mine
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  8. #7
    Senior Member lizrdbrth's Avatar
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    Quote Originally Posted by srs713 View Post
    the hardest part of compression calcuation is the TDC volume. The surest way of finding that is with the head off. Close the valves and see how many ccs of liquid it takes to fill it. Same with the piston head @TDC.

    Once you have that the rest is simple math.


    Everything is known but the volume of the dome. In a motor this small being off by a cc can mean half a point in compression ratio. Even simple math gots to be right.



    I can explain it to you, but I can't understand it for you.

    Powdercoated '87 frame, extended swingarm, YZ fork legs, ATV tire, 14/55, XT350 tank, spliced quick-release seat, disc brake conversion, beeg headlight, beeger rack, Lizrdcooler, Lizrdventz and bunch of other stuff all covered in invisible ink.

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