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Old 02-04-2004, 06:11 PM   #1
TIMSPEED
 
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Difference in compression ratio going from 76cc to 64cc??

If I'm running 8.5:1 Compression right now, with a 76cc Head, and I swapped to a 64cc Head, would that bump my compression ratio up to 9.5:1 or 9:1?
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Old 02-04-2004, 07:55 PM   #2
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With flat heads, more like 9.8 maybe 10 : 1
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Old 02-04-2004, 08:20 PM   #3
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with a .040 quench..figuring 11cc piston dish..with 64cc heads..the cr is 9.54-1 If you have flat tops with a .065 quench(.039 gasket+.025 piston down) and add 64cc heads the CR will be 9.66-1 if you add a shim gasket(.015) The cr will be 10.2-1
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Old 02-05-2004, 01:50 AM   #4
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I don't know what kinda pistons I have...just stock I guess??? I think they're like flattops..but like, they have a lip around the edge. Like, the center is taken down lower than the outer edge.
9.6:1 would be perfect...
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Old 02-05-2004, 08:14 AM   #5
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JMO, General rule of thumb is 8.5cr @ 76cc - 64CC =12CC + 8.5 should get you pretty close to 9.7CR, without getting real technical with head gaskets, quench, etc. This should be fine with cast iron heads depending on Cam with out having to worry about pinging, with Aluminum, you can usually get away with 10.2cr Cam Pending. Too short of a cam duration bumps running CR up, long duration lowers running CR, too long a Duration cam with out other mods results in low performance.
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Old 02-06-2004, 03:35 PM   #6
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Quote:
Originally posted by bigjimzlll
with a .040 quench..figuring 11cc piston dish..with 64cc heads..the cr is 9.54-1 If you have flat tops with a .065 quench(.039 gasket+.025 piston down) and add 64cc heads the CR will be 9.66-1 if you add a shim gasket(.015) The cr will be 10.2-1
I'd say Jim's #'s look spot on!! The 'rule of thumb' I've heard for SBC's is 1.1 pts. difference when going either to or from 64cc - 76cc.
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Old 02-06-2004, 03:40 PM   #7
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timspeed those arent flat tops they are dished pistons
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Old 02-06-2004, 09:45 PM   #8
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Try this!

If you know all the numbers, this works great!

http://www.classictruckshop.com/garage/shopmathcr1.asp
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Old 02-06-2004, 11:21 PM   #9
lukecp
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What Cabelguy said. Flattops are completely flat, except most have small valve reliefs in the middle. You proably have like 8.0:1 compression right now.....64cc heads would put you into the 89 octane gas range. I run 89 octane in my flattop-pistoned 64cc chamber 350 and it works pretty good.
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