I have a question for you engine gurus out there. I have a 390 in a '62 Thunderbird that needs to be rebuilt. I'm curious about the effects of going +30 on the bore size will have on my compression. It is currently supposed to be @ 9.5:1.

Someone mentioned that increasing the bore will lower compression so I would have to deck the block/heads, use a smaller gasket, or use domed pistons to compansate. Is this correct? How can I determine the effects of the rebuild? How can I determine how to correct it? I wouldn't mind bumping the compression up a little bit, but not much. How much is too much considering the use (4100+ LB car)?

I mentioned that since the compression is a ratio, it shouldn't matter the size of the bore if the stroke and everything else remains constant. Yes, the volume is increased but it seems to me the ratio will stay the same. Who's right? What is my best option?

Thanks.

 

Your swept volume will increase much more than your static volume. This increases compression. It will be minor though. Search on engine math in google an you will find the formulas you need.