Mismatched Heads on a 1972 YX engine
 

I'm looking at buying a complete running '72 Catalina engine for my GTO project, and noticed it has a 6X-4 on the driver side and a 6X-8 on the passenger side.

From what I read on the Butler site, the 6X-4 has a ~93cc chamber and the 6X-8 has a 100cc chamber. No idea if the heads have been milled down or what, but I'll likely want to replace one of them to match.

I assume the 6X-4 would be more ideal on a stock YX 400 engine, to bump the compression up a bit. Or is there a better head to look for?

I'll be installing my Tri-Power intake on it, and probably install a roller cam. It ran smooth and quiet, and would easily boil the tires (Posi), so hoping the bottom end is good. Pontiac stuff is quite scarce around here!

Cheers,
Brian

Go with another 6X-4 head.


:)

How does a tripower match up on the 4Xs?

6X-8 heads maybe 8:1, 6X-4 will be 8.5:1 at best.

Choose your cam accordingly.

I’d be looking for 1970 #13 400 heads, usually around 77 to 80cc chambers although advertised as 72cc. This will bump you up around 9.5:1.

Bonus - #13 heads won’t need any finagling to match up the center exhaust crossover to your Tri-Power, unlike any 1972 and later heads like 4X or 6X.

6x-4 heads on a 400 wwa 8.1:1 according to 78-79 W72 specs... 6x-8 was like 7.6.

The dash 8s can be milled down to that of a dash 4 but bolting up an intake and getting it to seal on that side can be a pain once you hit a mill amout of .060” without milling at least .045” of of the intake flange side of the hesd.

Also the exh crossover of a tripower does not exactly match up with the crossover in the 6 series heads so there’s a gasket mod that you need to do.

I guess you will need to pull that -8 head so you can CC to find out if it’s milled.

Thanks B-Man, I'll be on the look out for some #13 heads I guess. I was not aware of the crossover issue.

This is kind of a "placeholder" engine until I can get a proper 389/421 together. The car came with a non-original 389 block, crank and tripower intake, but was missing the heads, and front accessory brackets, bolts, etc. I figured it would be easier to just buy a complete drop in, known running engine with all that stuff attached, rather than try to piece something together.

Oh, and I should mention that I bought the Ames 1967 Ram Air manifolds, so any head choice will need the proper bolt holes to mount them.

Cheers,
Brian

Not sure Tom, but it sounds like there is an heat crossover issue to deal with.

I'm a rookie with this Pontiac stuff, if that wasn't apparent LOL.

Cheers,
Brian

That was supposed to say on a 400 "was"....

I am taking off from work tomorrow to go first thing in the morning.

#15 heads from a 1970 455 are not too bad for a little more compression either. Supposed to be 87 cc. Just have press in studs though.