comp. ratio
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comp. ratio

  1. #1
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    Default comp. ratio

    im putting together a tall block that picked up as a running moter but needed a new block. rotating assembaly is all the same that was in the old block. all i did was get a block and bore it to match the pistons. started to assemble it and the pistons are about .135 down in the hole. rods are 6,535, pistons are BME flat tops and measure 1.4 pin heigth and the crank is 4.25 if my math is correct its about 10.065 and block is 10.20. and with 119 cc headsand 8.2cc gasket it comes out to be about 7.34-1. does anyone have any input on this combination ? I know the motor ran strong before with an 871 and about 10# boost. I can get some more compression by milling the heads and decking the block or should i just up the boost for more power?

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  3. #2
    gn7
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    The normal pin heigth for that combo would be 1.52. But some piston manufactures raise the pin heigth to lowewr the compression sounds like thats the cases here. This would put your piston .120 further in the hole than a normal 496 piston with that rod would. When a piston manufacture does this, its a pretty good intication that it a stout piece. Ross does the same thing, and there blower piston has a much thicker deck on it than their normal flat top.what ever the case, those pistons were built to take some boost. Thats why they are that low in the hole. Normal blower lake deal would run flat tops maybe .020 in the hole, or higher.

    You can up the boost, but the thing is going to be lazy anytime you run it in a unboosted condition. You can take .040 to .045 of the deck of a truck block with no problem. They are thicker than a passenger car block by almost that much. I personally don't like milling heads for the sould reason to raise compression on a blower motor. I know there are lots of people who have. I don't.



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