Thread: compression

Tweet

I am about to buy a set of world product heads for my SBC, and have a quick question. Right now I have stock heads (76cc) and I am going to a 64cc head. The pistons are stock style dished/forged pistons. What should the new compression ratio be (approx).

Tech Inspector- Is this going to hurt the fact that I just bought that cam you recommended, it was CRN-100172. I thought that you said the duration was low to compensate for the lower compression.

All things being equal (and they rarely are!) your change in combustion chamber volume (cc's) is 16%, so if you were running a 10-1 motor before, you now should have 11.6-1. I think I figured this right, if not someone please chime in.

Anybody happen to know if it works this way? When I did the math I came up with 9.4 compression! Any help appreciated!

well I can deff tell u that a lower CC head will up the compression not lower it.. here is my guess 11:1 if u were at 10:1

why I say that is cause my setup I am buying is a 8.5:1 at 68cc heads.. if I go to 72cc heads my compression drops to 8:1

so if u started at 72cc heads and were at 10:1 and u lower thre chamber size in effect upping the compression my guess is 11:1

if u went to a 68cc head u would prolly be at like 10.5-10.8:1

Dave

Using a 0.030"-over 350 as an example, cylinder volume would be 727 cc's, chamber volume would be 76 cc's, eyebrows in the stock-type flattop pistons would be 7 cc's and the gasket volume at 0.039" compressed would be 9 cc's. We'll assume a zero deck. Adding all volumes together (819) and dividing by compressed volumes (92) yields a c.r. of 8.9:1

A change to 64 cc heads with all else being equal would result in dividing (807) by (80) and would yield a c.r. of 10.08:1

Here's a paper I wrote on figuring c.r.
http://streetmachinesoftablerock.com...opic.php?t=124

As far as the cam goes, let me try to explain....
The most important aspect of any cam is the intake closing point on the intake lobes. This is the point that determines how much of the mixture that has been drawn in by the piston on its downstroke will be trapped and compressed as the piston moves to top dead center and the spark plug fires. This intake closing point is directly tied to the static compresssion ratio of the motor.

Contact your favorite cam grinder for a recommendation when you have all your figures together and know exactly what you want the motor to do and when you want it to do it.

Thanks alot guys....nothing but great advice so far