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Old 05-26-2011, 11:14 PM
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pontiac heads

will the heads off of a 78 trans am 400 work on a 66 bonneville 389 and will i get any better performance??

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Old 05-27-2011, 12:18 AM
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Yes, and it depends

they will bolt on. You'll need to use the pushrods from the 400 since later heads are a bit taller.

What is the casting number of the 400 heads? They might be great, or they might be terrible. They will almost certainly drop your compression to much lower numbers... maybe too low.
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Old 05-27-2011, 12:45 AM
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well

my grandfather that i got the bonneville from said that the 400 that the heads came off of had 400 hp. other than that i dont know. wheres the numbers ur talking about and ill post em for ya. im kinda new to this stuff.so what all do i need to do to have these heads completely redone?because they sat rusting on the garage floor for 25 years and would i have to do anything else to my motor to make them work well on it. and can i get new pushrods because i dont have the old ones anymore.
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Old 05-27-2011, 07:15 AM
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Pontiac Parts

Valves, valve springs, pushrods for Pontiac heads are still available from aftermarket suppliers. The best perfomance heads are the 1967-1970 high compression heads that have 2.11"/1.77" valves. Those heads were mostly used on GTO and Firebird 400 engines and finding a pair that are rebuildable will be difficult.

The 1978 low compression heads can be used on any 1965 - 1979 Pontiac engine. They will not create a problem using pump gas when used on a 389/400 engine.
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Old 05-27-2011, 08:14 AM
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The head casting numbers are on the top of the center exhaust ports. They are usually a two or three digit code with numbers and/or letters. If they are 1978 400 heads, they should be 6X castings which are quite good as far as flow is concerned. The 66 389 should have 91 or 93 casting numbers.

Ironically, I did the exact same swap on my 66 Bonneville, but here's the tough part. Your 389 came from the factory with 68cc chamber heads and 10.25:1 compression. Swapping to 6X heads with 101cc chambers will drop that to around 8:1. That's great for cheap gas in a low-rpm torque motor, but not good for performance.

On mine, I had the head decks milled to reduce the chamber size. About the most I could mill them was enough to get 8.5:1 which was enough for my cheap rebuild, but still not good for performance. To really get performance-level compression from 6X heads on a 389, you need to change pistons as well.
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Old 05-27-2011, 08:38 AM
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Quote:
Originally Posted by ted200008
will the heads off of a 78 trans am 400 work on a 66 bonneville 389 and will i get any better performance??
They will bolt but DO NOT USE THEM. You will bend valves...unless your pistons have reliefs for both the early and late style heads.
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Old 05-27-2011, 08:40 AM
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66 and back heads have a 22 degree angle on the valve to deck relationship and the 67 later haeds have a 14 degree angle. Sealed power makes a replacement piston that has reliefs to accomodate both sets of heads.
Be careful how you mix parts and you ill be OK.
66 pistons are all there own also 67 pistons are different from 65 back.
65 back have reliefs for 22 degree valve angle and a shorter piston wrist pin, 66 have 22 degree valve angle and the longer wrist pin(same as 67 up) and 67 up pistons have the longer pin and the 14 degree angle reliefs in the pistons.
I had my 389 bored to 4.120 (stock 400 bore ) and use 400 pistons and late model heads. Boring a 389 to 4.120 takes it up .05725 from 4.0625 and worked out well . I zero decked the block afterwards( I also used forged sealed power pistons and 5140 forged rods.)
I CC e d my heads and am machining (surfaceing) them to get 82 CCs in the chambers. Cutting that much off the heads will necessatate using the correct pushrod to get the valve head and rocker arm contact patch (geometery) correct.

Last edited by latech; 05-27-2011 at 08:47 AM.
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Old 05-27-2011, 08:59 AM
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Valve angles

The valve angles in the Pontiac heads changed in 1967 so the valve reliefs in the pre-1966 pistons don't match the valve angles of the 1967-1979 heads. However, you can use 1967-1979 heads with 1965-1966 pistons if you keep the valve lift less than .460" and valve duration less than 226 degrees. If that still bothers you or you want to install a mega-lift camshaft, bore the engine .030" and install 1967-1979 pistons. Cast aluminum rebuilder pistons have both valve reliefs and are compatible with all 389/400 Pontiac heads.

I had a 1963 421 Pontiac engine with custom made Ross forged pistons, 1967 GTO/Firebird 400 heads, Lunati Bracket Master camshaft with .460" valve lift and 230 deg. duration. Those Ross pistons did not have any valve reliefs at all and they checked at .300" piston to valve clearance without head gaskets.

If you over-rev the motor and float the valves, the valves can hit the pistons no matter where the valve reliefs are and regardless what valve lift and duration your cam has.

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Old 05-27-2011, 09:30 AM
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I thought the valve relief was well worth the mention as he was planning on milling the heads. From what I understand it can cause an interference problem even on a stock build, probably at high RPM s as you suggested.
With the different valve angle the piston to valve head relationship changes, (contact point of piston to valve changes on piston top) so running 66 back pistons with 67 forward heads is a bad Idea, no matter what.
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Old 05-27-2011, 10:47 AM
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Good catch on the valve reliefs... but going from 93s to 6Xs he shouldn't have an issue. I milled the max off my 6Xs and still had tons of clearance. I was using a 204/212 cam on a 111 LSA and had no issues because of the huge chambers.

but, it should be checked nonetheless.
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Old 05-27-2011, 11:22 AM
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chamber size has nothing to do with it. Valves are the same from 64-71 ( actually more years than that) with respect to length and chamber size does not affect where they are in relation to the top of the piston. If chamber size affected valve posistion in the head then it would require different pushrod length.

Last edited by latech; 05-27-2011 at 11:38 AM.
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Old 05-27-2011, 11:53 AM
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You can run the big valve small chamber heads on a 389 as long as you don't get crazy will the cam lift. There is a noticeable difference in the performance with the big valve small chamber heads over the 389 heads.
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Old 05-27-2011, 03:50 PM
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Quote:
Originally Posted by latech
chamber size has nothing to do with it. Valves are the same from 64-71 ( actually more years than that) with respect to length and chamber size does not affect where they are in relation to the top of the piston. If chamber size affected valve posistion in the head then it would require different pushrod length.
Sure it affects clearance. There is more distance from the deck up to the valves in the large chamber 6X heads than there is in the 93s. The chamber is cast larger and deeper, so where the valve seats is further away from the piston crown. Not enough to affect how the factory selected pushrod length, but definitely different. You're assuming that factory pushrod length made perfect geometry on all engines, but it doesn't. It was well within acceptable ranges, but check it out sometime and you'll see that they are different.

If I had some pontiac heads I'd measure for you... anyone have some to try?
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Old 05-27-2011, 04:07 PM
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You know Curtis 73, Youre right. I am sorry... my apologies. Wow, now that is something right there I DIDNT KNOW, and I am glad you pointed that out. I made the mistake of assumming all pontiac used the same valves(lengthwise at least) I see there are several different lengths and they are listed according to the cylinder head marking.
Knowing that it would be safe to say as long as you are using the proper spring, you could correct valve geometry after milling heads by using different length valves and different springs (if necessary). Mill some off the heads use a slightly longer valve, or you could do what others do and get custom length pushrods.
That must be the way the factory corrected valve geometry when running different heads and using the same pushrods. I do know the 22 degree heads (closed chambered) used a shorter pushrod than the later models for obvious reasons.
Its the stuff you learn after you know it all thats important.
Thats just good lookin out there curtis.
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Old 05-27-2011, 05:39 PM
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Pontiac info gets recycled every few weeks. This is the difference between a factory 389 piston and a 400 piston. The valve notches are not as deep on a 389. If you look at the heads the 400 heads have deeper chambers.
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