# 389 vs 400



## X~Nightstalker (Mar 28, 2009)

If you were looking to upgrade a 66 and you had a choice between a 400 and a 389...which would you choose and why?


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## Koppster (Jul 27, 2008)

Personally...I'd chose the 389 because it is the correct motor for the 66 and there isn't much difference in the motors, I'm pretty certain a 400 for the most part is just a 389 block with a larger bore.

If you are looking to "upgrade" to a larger motor, consider 421s, 428s or 455s.


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## VettenGoat (Jun 19, 2007)

I chose the best of both worlds. I kept the 1965 WJ 421 SD, then punched it out for 428 pistons and used 1967 670 heads and 67 RA exhaust manifolds. Even though it is not factory correct, I still get a period corect motor but with the benefit of big valve heads and better exhaust. If you 389 is correct, I'd keep the block and "upgrade" it with teh better heads. Just remember the valve reliefs on the 389 pistons will not work with the 400 heads. Good luck//Mike


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## auburnconsulting (Nov 3, 2008)

my engine was sussposed to have balanced and blue printed by a shop 10 years prior to me buying it. the previous owner only drove it 2 times 2nd time engaging the starter into the flywheel while running, then parked it for 8 years with it locked together. i have the 389 block with the 670 heads, so i should assume i have replacement pistons so the valve reliefs match? it runs great and is a tire burner. he is trying to track down the speed shop that did the work, so then i will not have ot assume.


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## geeteeohguy (Feb 2, 2008)

Auburn: not to worry, you'c have a bunch of bent valves if the wrong pistons were used. Obviously, it was done correctly. Nightsalker: I would use a 389 with the better heads and appropriate pistons, as stated above. "389" just SOUNDS way cooler than "400". That said, good luck finding one......


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## even steven (Feb 27, 2009)

whats the best thing i could do to upgrade a factory 400 350 hp motor in a 70 gto with out detracting to much from the originality?


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## 66CAT (May 29, 2015)

X~Nightstalker said:


> If you were looking to upgrade a 66 and you had a choice between a 400 and a 389...which would you choose and why?


old thread but worth reviving...

hello everybody. I've been building Pontiacs for 35 years now.

a 400 is nothing more than a 389 punched .060" over
the 400 also had better heads, i.e. better valve angle, bigger ports, more flow, bigger valves, screw in studs, guide plates, better combustion chamber
400 was 1967-up
389 was 1959-66

if you want to keep the OEM look but upgrade, bore the 389 to 400 CID
have screwed in studs installed the heads
and guide plates (there was a guide plate for early heads, it was over the counter)

install bigger valves if you can, see how much room there is between the valvers and to the bore/valve clearance and piston/valve clearance

port the OEM heads

but keep the OEM engine parts, because if you use the later heads, it won't look right


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## 66CAT (May 29, 2015)

VettenGoat said:


> I chose the best of both worlds. I kept the 1965 WJ 421 SD, then punched it out for 428 pistons and used 1967 670 heads and 67 RA exhaust manifolds. Even though it is not factory correct, I still get a period corect motor but with the benefit of big valve heads and better exhaust. If you 389 is correct, I'd keep the block and "upgrade" it with teh better heads. Just remember the valve reliefs on the 389 pistons will not work with the 400 heads. Good luck//Mike


you had good intentions but you're wrong here, the first thing a purist will notice when the look under the hood, is you have 1967-up heads on a 1966 block. the valve angle is different in the early heads- and it gives the engine noticeably different look. the intake is flatter, and the valve covers are flatter, not angled down as much to the exhaust side. 

what I'd notice instantly is, you have late model heads on a 65-66 series engine.

Pontiac guys are notoriously the most casting-number conscious hot rodders in the entire musclecar hobby. we live and breath casting numbers, and it's the first thing we look at.

use the original heads, and port/work/improve them. then it will look right, but make more power too.


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## oldskool (Apr 26, 2015)

X~Nightstalker said:


> If you were looking to upgrade a 66 and you had a choice between a 400 and a 389...which would you choose and why?



400 block stroker, with 6x heads. Hardened valve seats--pump gas friendly--easy 400+ hp. If you have a 389, keep it--especially if it's numbers matching.


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