Originally Posted by 77pontolds
The 71 engine has been rebuilt, but never installed in a car. I've heard it run, and things seem good, oil press, etc. The rebuilder said it now has flat top pistons and a valve job and all the regular stuff. It has 99 heads. I was thinking of installing roller rockers to help out, but since I would have the valve covers off anyway, well You know, one thing leads to another. I wonder what, if anything I should do to the 6X heads to have aC.R around 9. to1.
The 6X-4 heads have a smaller chamber. Makes upping the CR easier.
Using the -8 heads you need to remove a little over 0.050" from the head decks to get about 8.5:1. These are rough estimates- the heads have to be CC'd to know what the volume actually is. I have never tried to get 9:1 using -8s, so someone else can fill you in on the plausibility of that. My thoughts are it will take too much milling, but that's me.
The '75(?) and newer 400 pistons have a large chamfer around the outer edge, plus a dish and valve reliefs. Your earlier engine doesn't have these pos slugs, so that's a plus as long as they weren't replaced w/the later pistons or those godforsaken 8 valve relief "one size fits all" pistons.
Use straight wall studs (ARP p/n 190-4003 w/hex nuts, or p/n 190-4203 w/12-point nuts, or stock BBC studs will work) to get away from the weaker bottle neck studs.
Some Pontiac info:
• Pontiac engine info