I recently bought a 402 chevy. It is a 1971 block but it has 1970 domed pistons. I know this set up should have closed chambered heads but it came with open chambered heads. The heads have been ported and big valves installed. What does the open chambered heads do to my compression ratio? I imagine it lowers it but how much? I am putting in my 69 chevelle SS. Any thoughts would be appreciated.

Well, it depends! Do you know what pistons are in it? You have to know the dome volume, or make and model, or something. If the dome is pretty big, your c/r might not suffer too much. What heads are they? (casting #) You'll need to be pretty specific if you expect to get close with your estimations. There are lots of cases of closed chamber pistons with open chamber heads. It just depends on what is used with what.

John, Thanks for replying. The pistons are Sealed Power 2312P.
The heads are 3993820. In the books the pistons are replacements for 69-70 10.5 to 1 350hp pistonns. The heads are 1971 300hp heads.
thx Stan

I looked up the heads on a website. It says they are 113.00cc The correct heads should be 3931063. They are 100.9cc.
That appears to be quite a bit. I just dont know what it means.

I cant find the dome volume on those pistons. I 'll keep looking later, but we're going to breakfast now to meet some friends. I ran a calculator just for the heck of it...a bbc with 113cc chambers, and all other specs about stock, with a piston that gives a 9.5 c/r, when going to a 101cc chamber, will be 10.5, if no other changes are made. There are other variables, but this should give you some idea. I'll get back to you later. John

Johnny,
I think you are right. It should have around 9.5 to 1. I think that should be acceptable for street use. I should be able to run 89 octane with that ratio. Thanks for the help.
Stan

I did it years ago for a guy with a 396 in a 2 ton truck. It worked with no problems.

Thanks Mike I think I am going to leave it alone.