389 vs 400 Head Differences - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 07-21-2018, 09:27 PM Thread Starter
 
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389 vs 400 Head Differences

The question of head differences comes up every once in a while and most want to know if they can put the 1967 and up 400CI heads on a 1964-1966 GTO 389CI or big car 421CI (the family of engines with oiling through the pushrods and not the block).

I ran across these diagrams/pics that show the differences between the 1966 and earlier heads and the 1967 and up later heads. The 1967 400CI heads were closed chamber except for the mid-year big car "061" heads which were open chamber heads. 1968 and up were all open chamber heads.

The pics show the angle of the valves and their orientation in the head and above the piston - 20 degrees versus 14 degrees. The differences in valve angle, size, and placement clearly point out that the piston's valve reliefs need to match that of the head used.

The larger 2.11" intakes and 1.77" exhaust valves on the 1967 and up high performance heads required more room, so the valves are moved closer to center of the cylinder and you can see the difference in the centerline distance between the valves in the last pic.

The 1967 and up heads use a pushrod guide plate seen in one of the pics while the 1966 and earlier heads used the pushrod hole in the head itself to keep the pushrods in place. And of course screw-in studs versus pressed-in studs when going to the '67 and up high performance heads.

To use the later 1967 and up heads on the 1966 and earlier engines, options are to obtain a set of pistons with either the correct valve relief positions or select a piston designed with valve reliefs that allows the use of either type head. Another piston that may work would be a dished piston used to lower compression. In each case, valve-to-piston clearances should be measured to ensure that there is enough clearance so valves/pistons don't contact - especially with a higher lift and/or longer duration camshaft.

Cylinder bore is another consideration. The large valve 1967 and up heads require a larger bore so the valves do not hit the cylinder wall nor become shrouded by the cylinder wall. The 389CI can be bored .060" over to 400CI bore size or the cylinder can be notched for valve clearance. The 421CI would require a .030" over bore or notched cylinders.

I read that if the lift is not too high, you may get away with installing the later 1967 and up heads on the earlier engines, but I would definitely be checking valve-to-head & valve-to-cylinder clearances on that one.

Here is an older post that details a magazine article in the 389CI to 400CI conversion: https://www.gtoforum.com/f12/389-400-conversion-118561/
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post #2 of 4 (permalink) Old 07-22-2018, 04:27 PM
 
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Good read. Thanks for posting.

I am assuming that the same would apply when using edelbrock or kre aluminum heads on a 65 389 ???
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post #3 of 4 (permalink) Old 07-22-2018, 07:34 PM Thread Starter
 
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Originally Posted by My65goat View Post
Good read. Thanks for posting.

I am assuming that the same would apply when using edelbrock or kre aluminum heads on a 65 389 ???
Yes as they use the 2.11" intake and 1.66/1.77" exhaust valves.

Now with either of these heads being aluminum, it may be possible to start with bare heads (instead of a complete head with valve/springs/rocker arm studs etc..) and pull the valve inserts and install new ones so as to use the smaller 1.92/1.60" used on the earlier heads OR you may be able to go with the larger Super Duty 2.02/1.76" due to the centralized placement of the '67 and up valves. The 389SD used the same 421SD heads with the larger valves (and cannot recall if they notched the cylinders). In this way, you may not have to over bore the 389 or do any cylinder notching to clear the larger valves.

I know most will want the bigger valves, but keep in mind that intake flow is about velocity. The smaller valves will keep flow velocity higher in the lower-mid range lift while the larger valves would work better in the mid-upper range. The Edlebrock and KRE are going to flow some big air, so you need either bigger cubes or higher RPM's to really take advantage of them. Your engine's lower RPM's may be soggy as flow velocity may be low at these RPM's with these higher flowing heads.

An alternative might be to take a look at SD Performance as they now offer their CNC work on the earlier 389/421 heads. They use the stock sized valves and they flow 230-240 CFM's @ .550" lift. SD Performance- Pontiac Performance Specialists
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post #4 of 4 (permalink) Old 07-25-2018, 06:52 PM
 
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Thanks for the information Jim.
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