Thread: Understanding cylinder compression

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I am trying to figure out which cylinder heads to buy for my 76 Chevy Nova. The Nova came stock with a 8.5:1 compression rate which I would like to increase. I am not trying to create a drag car but I really would like to be able to burn some rubber What compression rate would you recommend I should aim at without changing pistons?

Bore = 9.489 cm
Stroke = 8.849 cm
Current C.R = 8.5:1

From these specs I calculated that my current cylinder heads would have a 83.5 cm2 combustion chamber. Does this sound big? I used the following method to calculate my heads combustion chamber.

http://www.e30m3performance.com/myth...comp_ratio.htm

I have been browsing summit for suitable heads and so far the 72 cc heads looks good. This would (i think) give a 9.7 compression rate. Should aim at a lower compression?

Regards Jonas

Looks like you've got yourself a 305. 83.5 cc's is way big for a combustion chamber on that motor. 58 was stock. I believe you forgot to take into account the dish in the pistons, the volume added by the head gasket, and the volume created by the piston not travelling all of the way to the top of the cylinder (deck clearance .025" on a stock motor).

Any head you buy is going to have larger combustion chambers as they are all designed with the 350 in mind and will have combustion chambers ranging from 64-76 cc's. That will drop your compression rato significantly. If you were to put flat top pistons in and use the 64 cc heads, that would bump your compression ratio to 9.1:1 which is a good start. After that, you can put in some thinner head gaskets of .015" to .020" which will put you between 9.6 and 9.7:1 and help squish. Not bad. The only concern is will the 1.94" and 1.5" valves fit in your cylinders.

76GMC1500 you are absolutely right. Flat top pistons is definitly something I would like to install but at the moment I am not able to pull to engine. All modifications will be done with the engine in the car.

But thinner head gaskets and the World Products S/R Torquer Cylinder Heads (58cc) will probably be a start. Later on flat top pistons might be the way to go.