Need a math whizzz - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Longhorn Man · 07-17-2002, 11:02 PM

Ok, if I had an engine..oh, say displacing 425 cubes or so, and it is an 8.2:1 Compression Ratio, and it has 96cc heads on it, what would happen if I slapped a set of 76 cc heads...how about 120cc heads?
I know it would go up with the 76cc, and down with the 120cc...but how much?
Can anyone here calculate the C/R with the given head numbers?

bigvinnie · 07-17-2002, 11:07 PM

try this link....

http://www.johnmaherracing.co.uk/enginecalc.htm

Project1970 · 07-17-2002, 11:16 PM

Using http://www.rosspistons.com/CR%20Calculator.htm and some guessing (to get 8.2:1 with the 96cc heads, I had to put in a .039 gasket thickness, .077 deck height, and no cylinder dish/dome), the 76cc heads should boost you to 9.62:1 and the 120cc heads would plummet you to a measly 7:1.

Longhorn Man · 07-17-2002, 11:27 PM

Hmmmm, 9.6:1 is ok for cast postons too. *evil grin* I wonder what 1.4 points of compression, along with better flowing heads will do for pewrformance.
The 500 I am picking up next Friday will have one or the other on those heads...depending on the year. I know it has the old points ignition on it, so that makes it a '74 or older.
Caddy 500...1970 - 1972 has the 76cc heads, while in '72 (what month I don't know) they started the 120cc untill '76.
Being this should be a '74 or older, that means I have a little less than a 50/50 chance in getting the 76cc heads.
Then again, it is automaticly a 50/50 chance...either it has them, or it doesn't.

07-17-2002, 11:02 PM	#1
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Need a math whizzz Ok, if I had an engine..oh, say displacing 425 cubes or so, and it is an 8.2:1 Compression Ratio, and it has 96cc heads on it, what would happen if I slapped a set of 76 cc heads...how about 120cc heads? I know it would go up with the 76cc, and down with the 120cc...but how much? Can anyone here calculate the C/R with the given head numbers?

07-17-2002, 11:07 PM	#2
bigvinnie Registered User Join Date: Aug 2001 Location: escondido, ca 92026 Posts: 1,222	try this link.... http://www.johnmaherracing.co.uk/enginecalc.htm __________________ '72 short step, 350, 700R4, tilt, ps, pdb, a/c, lowered coils, etc., other work in progress... San Diego, CA 72 Stepside Project

07-17-2002, 11:16 PM	#3
Project1970 14.1 @ 96MPH Join Date: Jul 2001 Location: Mesa, AZ Posts: 2,811	Using http://www.rosspistons.com/CR%20Calculator.htm and some guessing (to get 8.2:1 with the 96cc heads, I had to put in a .039 gasket thickness, .077 deck height, and no cylinder dish/dome), the 76cc heads should boost you to 9.62:1 and the 120cc heads would plummet you to a measly 7:1. __________________ Project1970 - LS1 Swap Complete!

07-17-2002, 11:27 PM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Hmmmm, 9.6:1 is ok for cast postons too. evil grin I wonder what 1.4 points of compression, along with better flowing heads will do for pewrformance. The 500 I am picking up next Friday will have one or the other on those heads...depending on the year. I know it has the old points ignition on it, so that makes it a '74 or older. Caddy 500...1970 - 1972 has the 76cc heads, while in '72 (what month I don't know) they started the 120cc untill '76. Being this should be a '74 or older, that means I have a little less than a 50/50 chance in getting the 76cc heads. Then again, it is automaticly a 50/50 chance...either it has them, or it doesn't.



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