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.