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Old 10-24-2012, 11:34 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.
thanks
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Old 10-24-2012, 11:47 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, 05:56 AM   #3
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Please explain.
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Old 10-26-2012, 01:18 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, 03:24 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, 06:03 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, 07:43 PM   #7
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Lol @ a professional, respected company suggesting quench pads do nothing.
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Old 10-29-2012, 07:15 PM   #8
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plus 1 for supertech valves
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Old 10-30-2016, 07:21 AM   #9
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bump from the dead, but I would like to ask why are the cometic gaskets better than stock MLS ?
Iīm from europe, so I can source OEM one for 55$ compared to cometic at 103$ plus shipping and taxes from states. So letīs call it more than twice the price.
my car is NA8, with NB8 head, shaved, ported, with unshroud valves, 10.3:1 CR, 10mm lift custom cams with ITBs if that change anything.
my quetion is why should I buy more expensive gasket ? I see that this is "the only gasket" the big names in miata world uses, but I do not know why is it superior to OEM one ?
I donīt care pay twice the price for it if that makes things better, but if there is no profit from it for N/A car, I will stick with OEM one.
Also I noticed that 949 sells only the oversized 85.5mm one for all customers, while othere shops selling 83,84,85 mm, so is that best to get 83mm if I have stock pistons and match valve unschrouding to the gasket right ?
Thanks for your answers in advance !

Last edited by AndrewG; 10-30-2016 at 09:20 AM.
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Old 10-30-2016, 12:33 PM   #10
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I put OEM head gaskets in every motor I build, except for 85.5mm-bore NA builds which get the 85.5mm Cometic. There's no reason to shell out for a Cometic on an <84.5mm build.
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Old 11-01-2016, 08:26 AM   #11
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Thanks for your reply ! much appreciate.
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