SurfnSun

Say I went from a 58cc head to a 68cc head or even 74cc? My current compression is 7.5:1. Thanks.

65Z01

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.

SurfnSun

wow that completely lost me

VenkmanP

Try this:

http://www.csgnetwork.com/compcalc.html

CFI-EFI

Or more simply, just add the difference of the chamber volumes to the fixed volume, and refigure. Of course, you subtract if your going to a smaller combustion chamber.

RACE ON!!!

65Z01

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.

SurfnSun

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.