[wooftfd]

Anybody got any pictures of this mod being done, or a good walk-through set of directions? I understand there are cams that can also change the firing order to do the same thing, reduce cross-cylinder back pressure, has anyone used one?
Working on two motors, original 389 from the car, and date-code correct 421 HO... Going to stash the 389, and put the 421 in.

[Tom Vaught]

I see no benefit to butchering up the 389 heads or the 421 heads with the mod you suggest. Please explain why you are doing this, or was it suggested by someone else?

Tom V.

[wooftfd]

My understanding is that the center exhaust passages are linked, causing backpressure on the 4 central cylinders?

[b-man]

All Pontiac cylinder heads other than some rare Super Duty heads have the two center exhaust ports being used to heat the intake manifold crossover.

It’s common to fill these passages with molten aluminum or furnace cement so as to separate the exhaust pulses for performance applications.

[steve25]

The best way to accomplish this is with a 3/8” thick steel adapter plate with the needed T extension welded on it to divide the center two ports.

Yes, you will need six of them to stand-off your exh manifolds or headers.

As posted above, if you’d also not going to also close off the crossover passages in both ports in both heads , then don’t bother going through the effort trying to extend this center port divided to the flange, or make the adapter and especially if your running headers!

To me it has always been a waste of funds to run headers on stock heads if you’d leaving the crossovers open , at least on early heads up to the 4X series when the crossovers where made smaller.