Compression Ratio Formula
Thread Starter
Instructor
Joined: Dec 2006
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From: Roswell NM
Compression Ratio Formula
Any one have the formula to calculate CR when changing combustion chamber sizes on heads. Say piston x has 10.5 cr on a 64cc heads obviously i would like to know how much cr it would have on the stock 54cc lt4 heads
Burning Brakes
Joined: Aug 2006
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From: Seattle WA
Racer
Joined: Sep 2005
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From: New Freedom PA
try this...
try this link , it has some other cool features too , forget that is a import web site...
http://www.europartsinc.com/comp_calc.htm
Desert
Team Owner
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From: Summerland B.C. Canada
Safety Car
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From: Hinckley Ohio
So stock LT4 has 10.8:1 compression ratio, 54.4cc heads......
What are you trying to do?????
Racer
Joined: May 2006
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Originally Posted by GulfCoastEagle
Compression Ratio = (Displacement + clearance volume) / clearance volume.
Clearance volume includes the volume of the heads and the volume of the head gasket.
Clearance volume includes the volume of the heads and the volume of the head gasket.
Now here is the quick, down, and dirty, way to get the figures you want. Rather than pull the engine down to measure the deck clearance, piston dome, or valve notch, volumes, etc. use your head instead of your tools. You know the (from above) Displacement, and you know the stock compression ratio. Figure backward to determine the clearance volume. Then adjust the clearance volume by the intended changes and re figure the CR by the formula above.
If your old combo has a clearance volume of 83 ccs, subtract the new head ccs of 54 from the old head's 64 ccs, and recalculate using a clearance volume of (64 - 54 = 10) 73 ccs.
Safety Car
Joined: Sep 2000
Posts: 3,524
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From: Hinckley Ohio
Now here is the quick, down, and dirty, way to get the figures you want. Rather than pull the engine down to measure the deck clearance, piston dome, or valve notch, volumes, etc. use your head instead of your tools. You know the (from above) Displacement, and you know the stock compression ratio. Figure backward to determine the clearance volume. Then adjust the clearance volume by the intended changes and re figure the CR by the formula above.
If your old combo has a clearance volume of 83 ccs, subtract the new head ccs of 54 from the old head's 64 ccs, and recalculate using a clearance volume of (64 - 54 = 10) 73 ccs.
For example, stock LT4 is 5733cc total or 716.625cc per cylinder.......
volume = 716.625/10.8=66.3541cc (for 10.8:1 Compression ratio)
since head is 54.4cc, 66.3541-54.4= 11.9541cc.....is clearance volume
plug in new head data and get a pretty good result for CR..
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From: Hinckley Ohio
10.8*(383/350)=11.82:1 should be pretty close
10.8:1 is the compression ratio a 350 lt4 using these heads...
