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#1
11-19-2025, 07:51 PM
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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
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1965 GTO Coupe project in the works. PHS documented Mayfair Maize, Tri-Power, 4 Speed, 3.55 Posi |
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#2
11-19-2025, 08:36 PM
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Go with another 6X-4 head.
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John Wallace - johnta1 Pontiac Power RULES !!! www.wallaceracing.com Winner of Top Class at Pontiac Nationals, 2004 Cordova Winner of Quick 16 At Ames 2004 Pontiac Tripower Nats KRE's MR-1 - 1st 5 second Pontiac block ever! W.E.C moment "Every man has a right to his own opinion, but no man has a right to be wrong in his facts." "People demand freedom of speech to make up for the freedom of thought which they avoid." – Socrates |
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#3
11-19-2025, 08:46 PM
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How does a tripower match up on the 4Xs?
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#4
11-19-2025, 09:23 PM
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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.
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1964 Tempest Coupe LS3/4L70E/3.42 1964 Le Mans Convertible 421 HO/TH350/3.08 2002 WS6 Convertible LS1/4L60E/3.23 1966 VW Type 2 Single Cab Pickup 140hp 2276 1956 VW Type 2 Panel Van 36hp 1200 1957 VW Type 1 Beetle 180hp 2276 |
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#5
11-20-2025, 09:54 AM
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6x-4 heads on a 400 wwa 8.1:1 according to 78-79 W72 specs... 6x-8 was like 7.6.
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#6
11-20-2025, 11:00 AM
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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.
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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#7
11-20-2025, 11:34 AM
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Quote:
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
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1965 GTO Coupe project in the works. PHS documented Mayfair Maize, Tri-Power, 4 Speed, 3.55 Posi |
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#8
11-20-2025, 11:36 AM
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Quote:
I'm a rookie with this Pontiac stuff, if that wasn't apparent LOL. Cheers, Brian
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1965 GTO Coupe project in the works. PHS documented Mayfair Maize, Tri-Power, 4 Speed, 3.55 Posi |
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#9
11-20-2025, 01:29 PM
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Quote:
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#10
11-20-2025, 02:39 PM
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I am taking off from work tomorrow to go first thing in the morning.
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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#11
11-20-2025, 04:05 PM
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#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.
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Skip Fix 1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever! 1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand 1965 GTO 4 barrel 3 speed project 2004 GTO Pulse Red stock motor computer tune [email protected] 1964 Impala SS 409/470ci 600 HP stroker project 1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs 1964 Corvette Coupe 327 4 speed |
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