jevanstintshop

Hi, I'm helping my father in law do a rebuild on a 1970 olds 455. It won't be a serious performance engine or anything other than a fairly small cam (Engle EP-18/20)and "C" heads that to my understanding are 80cc... The pistons that come in the master kit are TRW 369P - 30.. Does anyone know what the approximate compression ratio will be using these pistons????? The dealer selling the master kit isn't sure Thanks in advance!!

coppercutlass

Google compression calculator and you should be able to find one. There are many factors to claculate compression. Bore , stroke , deck height (how far the piston sits in the deck ) gasket thickness , piston cc's .

joe_padavano

Same answer I gave you on the other forum. Approx 13cc dish on those cast pistons. With true 80cc chambers and original shim head gaskets, around 10.0:1. With the more likely 83-84cc as-cast chambers and FelPro 0.040" head gaskets, closer to 9.25-9.5 CR.

Fun71

Be aware that 80cc head volume is an average number. A set of G heads that I have measured 85cc, so you really need to cc the heads to know for certain what yours are.

I found this info for the pistons:

http://www.summitracing.com/parts/slp-369p
Notes: Recessed piston head is 0.120 inch deep x 2.940 inch diameter.

Those piston dish dimensions calculate to 18.16 cc (assuming I did the math correctly).

edit: I didn't - it's 13.3 cc

Assuming the following:
80cc combustion chamber
.045" thick head gasket
4.250" gasket bore diameter
zero deck block

CR = 10.1:1


Increase the heads to 85cc
Increase the piston-to-deck to .025"

CR = 9.26:1

This shows why it is important to actually measure YOUR parts and not rely on average values for determining your CR.

joe_padavano

You might want to recheck your numbers - or show your work.

I get 0.8146 cu in, which converts to 13.3 cc, as I posted above.

Fun71

You are correct. I will edit the CR in the above post.

jevanstintshop

Thanks... Not the greatest with the math but this info helps.. Thanks for replies!