how rare is factory AC?

[Mike C]

There were no high performance high compression big blocks in the trucks. The only real difference between the 396 and 402 is a .030 overbore. And on the GMC trucks, they list the big block as "400" on the SPID. Same motor. The combination of pistons and combustion chamber dictate compression ratio. Closed chamber heads used different domes than the open chamber but the were all 8.5:1.

I just built a 390hp 427 for my '69. It's also a '69 from a 'vette. 512 2 bolt block and 290 101cc heads. Should be right at 10:1 with the SpeedPro pistons that are the stock replacement spec. I have a second 427 with the same pistons and a set of 781 heads that should put the motor at 9.3:1 because of the 118cc chambers of the later open chamber heads.

Originally Posted by 71CHEVYSHORTBED402

You know if that had the heads with larger ports? The heads are 10:1 compression, while the 402 is 8.5:1. At least those are the numbers I recall.

Those are in strict terms. For all I know some 402s had the larger heads & vise versa. That and I'm all ears if someone knows better. The guy who built my engine (1993) explained this, and sold me the heads off a 396. He said my heads had been gone through & were toast. I always questioned that. There was only 99K on the truck when it threw a bearing. Sure built a great engine though. Supposedly the best in the area, I can't argue that.