How do you figure compression differences between heads?
Team Owner
Joined: Dec 2000
Posts: 90,675
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From: SE NY
Cruise-In II Veteran
CR=(Vs + Vf)/Vf
where Vs is swept volume and Vf is fixed volume. Vf is the sum of chamber volume (Vc) plus all other fixed volume (Vo). So
Vf =Vc + Vo
Since you know the CR and Vs you can calculate Vf. Then subtract 58cc from Vf to get the sum of all other fixed volumes, Vo.
Now just add Vo to each chamber volume of interest to get Vf for that chamber. Then do the math to get the new CR for that chamber.
where Vs is swept volume and Vf is fixed volume. Vf is the sum of chamber volume (Vc) plus all other fixed volume (Vo). So
Vf =Vc + Vo
Since you know the CR and Vs you can calculate Vf. Then subtract 58cc from Vf to get the sum of all other fixed volumes, Vo.
Now just add Vo to each chamber volume of interest to get Vf for that chamber. Then do the math to get the new CR for that chamber.
Safety Car
Joined: Apr 2003
Posts: 3,793
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From: VA
Originally Posted by SurfnSun
Say I went from a 58cc head to a 68cc head or even 74cc? My current compression is 7.5:1. Thanks.
http://www.csgnetwork.com/compcalc.html
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Originally Posted by 65Z01
Since you know the CR and Vs you can calculate Vf. Then subtract 58cc from Vf to get the sum of all other fixed volumes, Vo.
Now just add Vo to each chamber volume of interest to get Vf for that chamber. Then do the math to get the new CR for that chamber.
Now just add Vo to each chamber volume of interest to get Vf for that chamber. Then do the math to get the new CR for that chamber.
RACE ON!!!
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Ok, here's an example.
Your 350cid SBC has a total of 5,735cc or 717cc/cylinder as displacement or swept volume, Vs.
And with 58cc heads you now have a CR of 7.5:1.
Solving CR=(Vs + Vf)/Vf for Vf, we have
Vf=Vs/(CR-1) = 717/(7.5-1) = 717/6.5 = 110cc
Solving Vf=Vc + Vo for Vo, we have
Vo=Vf-Vc = 110cc - 58cc = 52cc
where Vo is the sum of all other (than heads) fix volumes, i.e. deck height, gasket thickness, piston dishes, distance to compression ring, etc.
So lets check the math to get the CR for your 58cc heads,
Vf=Vo + Vc = 52cc + 58cc = 110cc
CR=(Vs + Vf)/Vf = (717 + 110)/120 = 827/110 = 7.5:1
For 74cc heads,
Vf=Vo + Vc = 52cc + 68cc = 120cc
CR=(Vs + Vf)/Vf = (717 + 120)/120 = 837/120 = 7.0:1
For 74cc heads,
Vf=Vo + Vc = 52cc + 74cc = 126cc
CR=(Vs + Vf)/Vf = (717 + 126)/126 = 843/126 = 6.7:1
Ain't math fun.
Your 350cid SBC has a total of 5,735cc or 717cc/cylinder as displacement or swept volume, Vs.
And with 58cc heads you now have a CR of 7.5:1.
Solving CR=(Vs + Vf)/Vf for Vf, we have
Vf=Vs/(CR-1) = 717/(7.5-1) = 717/6.5 = 110cc
Solving Vf=Vc + Vo for Vo, we have
Vo=Vf-Vc = 110cc - 58cc = 52cc
where Vo is the sum of all other (than heads) fix volumes, i.e. deck height, gasket thickness, piston dishes, distance to compression ring, etc.
So lets check the math to get the CR for your 58cc heads,
Vf=Vo + Vc = 52cc + 58cc = 110cc
CR=(Vs + Vf)/Vf = (717 + 110)/120 = 827/110 = 7.5:1
For 74cc heads,
Vf=Vo + Vc = 52cc + 68cc = 120cc
CR=(Vs + Vf)/Vf = (717 + 120)/120 = 837/120 = 7.0:1
For 74cc heads,
Vf=Vo + Vc = 52cc + 74cc = 126cc
CR=(Vs + Vf)/Vf = (717 + 126)/126 = 843/126 = 6.7:1
Ain't math fun.
Thread Starter
Team Owner
Joined: Aug 1999
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St. Jude Donor '10
Originally Posted by 65Z01
Ain't math fun.
DAMN...I have a headache
I haven't seen that much math since Buisness Cal in 97, and I had to take it twice to get out. 
Thanks man.
