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Old 05-20-2005, 07:42 AM
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Combustion chamber question

I am building up a small block 350. I have seen mixed results as to what the compession ratio is using 64cc heads with flat top pistions. Does anyone know the general ratio I would achive? Also has anyone noticed any performence differences between a 70cc and a 64cc head?

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Old 05-20-2005, 08:01 AM
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There`s too many varibles, Head gasket thickness, head gasket bore size, block deck height, piston overbore size (if any), piston compression height, piston volume in minus cc`s for the valve reliefs, actual chamber size in cc`s. There are many compression ratio calculators online free of use and you can add it up there.
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Old 05-20-2005, 11:57 AM
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StevenC,
For what it's worth, I just changed from 64CC Cast iron heads to aluminum 70CC heads with no other changes. This was on a 350HP 327, but it should be representative of any similar sized engine.

I was surprized at the decrease in power. The engine seems soft compared to before, and I swear I can hear the starter turning the engine faster than before. I'm seriously considering going to .015" head gaskets to get most of the compression back.
Johnny
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Old 05-20-2005, 01:12 PM
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327's are tough to build compression with flattop pistons, I recently built one with my Son using 64cc 462 castings and flattops with two valve reliefs and also had the block zero decked. With a .042 gasket we ended up at 10.1-1
With alluminum they say you should go higher with the compression to make up for the loss in heat retention. So you lost compression plus lost burn efficiency do to the slower burn with the cooler heads.

With a 350 with stock height block, rebuilder pistons at .020 in the hole @tdc, and 64cc heads, and rebuilder head gaskets you'll probably end up at between 9.3 to 9.5-1. With good pistons two valve reliefs, .038 piston to head clearance, and 64cc chambers you'd be around 10-10.5-1. If your compression is up around 9.5 or 10-1 the difference in chamber size from 64-70 cc wouldn't be very noticable IMO.
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Old 05-20-2005, 01:18 PM
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I have heard/read (as a very general guideline) every 4cc's of change in head volume equals .6 change in compression ratio,either up or down. Maybe others on this Board can verify or refute this.
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