What compression ratio?

Hello, I'm new here. I have a 427 I took out of a 68 corvette. Had it in my 55 chevy for quite a while. It is a 390hp with 10.25 stock compression with oval port closed chamber heads. I was wondering what the compression would be if I installed a set of 69 open chamber large port heads that I have laying around. I would like to drop the compression down to 9.25 to deal with the crummy gas at the pump. I have a few large port intakes laying around so if just changing the heads would get me where I want to go that would be great.

here is the trw line up l2268f 106.9cc=11.6cr with 116 cc =10.14cr ...l2300f 100.9cc=9.86 116cc=8.73 .030overwhat is the piston number you have should be the l2300f that is a 66-69 427.335hp and 390 hp 060 over bore is closer to you listed as 100.9cc=10.10 cr and at 116 cc=8.83 cr if bore smaller then so is your CR what bore is it ?

The bore is 4.25 standard. The heads are the stock heads that came on the engine, 390hp, 1968 corvette.

yep well then it is the l2300f 335/390HP 427..it is not as much is you think look at the list i posted with a 116 chamber head your be at 8.62 CR thats going by Federal mogul and the TRW