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Old 10-24-2012, 10:34 PM   #1
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Default Cometic head gasket question

Does anyone know how much compression will be lost between a .040 thickness head gasket and the thicker .070 head gasket?
BP 1.8 84mm bore. Supertech 1mm over valves with 9:1 Supertech pistions.
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Old 10-24-2012, 10:47 PM   #2
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Thicker head gaskets are a bad idea. It screws with the squish and decreases detonation resistance. Use an OEM head gasket.
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Old 10-25-2012, 04:56 AM   #3
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Please explain.
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Old 10-26-2012, 12:18 AM   #4
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Update, I talked to Ron at race Engineering. He said the stock gasket is .031, the Cometic's are .040 and .075. He said increasing to the .075 would lower the compression ratio by a full point down to 8:1. I wouldn't mind a few tenths but not a full point. If the difference between stock thickness and .040 Cometic is true, then there is a few tenths lost anyway.
I've been trying to research on squish. There is some good info talking about the distance of.070 being used with extremely good results. This distance also promotes better detonation resistance. This info comes from the Speed Talk forum. A lot of professional engine builders post there. So I would think this research is based on 2-valve motors. Yeah, really powerful 2-valve motors with 9k red-lines and sprayed. I don't know if squish is effected by the fuel used?
Do 4-valve heads make for a completely different scenario? Does the larger cylinder bore make .070 proportionally the same?
More questions than answers.
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Old 10-28-2012, 02:24 AM   #5
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Squish or quench is where the piston comes up against the flat part of the head (the parts not cut out to form combustion chamber). The air being squeezed out of this area creates turbulance in the mix. This in turn decreases detonation and makes for a more complete burn. To be effective the squish area measures about .035. Any bigger than .050 it is no longer effective. Thicker head gaskets will eliminate any squish the motor has. To be fair I don't think the BP motors have much squish. In a Harley motor there is about 25%-33% of the surface area of the piston for squish. I bet the BP has less than 5%.
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Old 10-28-2012, 05:03 PM   #6
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Been a while since I built my motor, but iirc I had squish at .015", maybe even .010", and the motor regularly sees the 7800rpm rev limiter with 10.5:1 and pump gas.

Getting the squish correct, can make a world of difference. Not using squish makes worlds of difference the wrong direction.
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Old 10-28-2012, 06:43 PM   #7
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Lol @ a professional, respected company suggesting quench pads do nothing.
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Old 10-29-2012, 06:15 PM   #8
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plus 1 for supertech valves
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