Christopher Grimes

I beg to differ. The 6X-8 heads were on the 180 HP 400 motors, the 6X-4 heads were on the 200 HP 400 motors, starting in '77. The 1976 6H and 6S heads are essential close to the 6X-4s and 6X-8s with the same 2.11/1.66 intake and exhaust valves. The 6H, 6S, and 6X-8 heads were designed for motors with 7.6:1 compression ratios. The 6X-4 motors had a compression ratio of 8.0:1. Not statistically significant if you are moving up to 9.5:1 compression in the first place.

I will say that given the number of Trans Ams and Formulas built in '77 and '78, the 6X heads should be plentiful and cheap....assuming the 455s original heads are gone or junk to begin with.

When you look at the cc difference of the 6H at 124cc and the 6X-8 at 101cc, a simple head swap will make a difference.

If you bolt on a set of 6x-8 heads from a 400, you'll get a nice little hp bump from the smaller chambered heads. And still have decent valves for it to breathe.