3400 Rear Manifold Outlet Small

[90GTZHO]

I have been searching around the site and a couple other places with no luck of finding an answer to that im searching for.

I recently bought a 99 GA SE 3400 to which i planned to drop into my wifes new 95 GAGT. After discovering that the yard sold me a bad motor with a snapped cam, i went looking elsewhere for a motor. I found a zero mile 2000 3400 for a real good deal that had been rebuilt by a local chevy dealer (aside from heads and block, everything was new ).

Anyways, looking at the rear exhaust manifold, i noticed the outlet on the 99 3400 rear exhaust manifold was really tiny. And upon looking at the 95 3100 rear manifold it looks to have a bigger opening for the downpipe.

My question is, is there something that can be done about the outlet on the 99 3400 rear exhaust manifold, its so smalli dont see how it flows better than the 3100.

Can someone help me out? I dont have a pic of the 3100 manifold, but its a center exit pipe like the 3400 one is. Heres a pic of the 3400 rear manifold downpipe outlet.



I cant port the the hell out of that cause it will burn thru the walls. Is there a better alternative to porting this, like a better manifold to use?

Sorry so long, and thanks.

[Weatheralls Auto]

Well could you not just use the Manifolds that you had in the car in the first place?

[90GTZHO]

i could but i heard sticking the stock 3100 manifold on the 3400 heads defeats the purpose of the swap

[robertisaar]

now you see why you can gain so much power from a better downpipe/good headers. i'm guessing this is GM's prefered method of power limiting.

[bszopi]

i could but i heard sticking the stock 3100 manifold on the 3400 heads defeats the purpose of the swap

Where did you hear that? I don't think I've ever seen that said on here. Granted, all stock manifolds suck, and the 3400 one may be slightly better, but they still suck. The only difference between the 3100 and 3400 heads is a larger intake valve, so I don't see that being a reason for your statement either. Perhaps I am mssing something...

[IsaacHayes]

Strange, the 3400 manifold I have was ported out a lot and matches the exit size, while my 3100 one looks like the picture above, and can not be ported any larger.

Regardless, the 3400 rear is better because it has short runners instead of being just a log.

[90GTZHO]

yeah but too bad its just on the rear instead of being like that in front too lol

[90GTZHO]

maybe i will just use 3500 exhaust manifolds lol

[Superdave]

maybe i will just use 3500 exhaust manifolds lol

That's the best solution. There are no better stock manifolds then those.

[19Cutlass94]

Anyone have a picture of those manifolds? Im not so sure it would be worth the effort for me to swap on 3500 exh. manifolds over what I have...

[1988GTU]

You can buy these, they are forsale. Just shoot me a PM with your offer.

[19Cutlass94]

Do you have the x-over pipe as well?

[1988GTU]

No. The xover would be easy to make though. I'd advise a custom one anyway.

[90GTZHO]

The 3500s dont have crossover pipes anyways do they? I mean, they just have downpipes off each manifold and meet back under the firewall?

[IsaacHayes]

Stock cross over goes under oil pan. I know on L bodies the notch in the oil pan has the subframe going there... W body might have that space free though. I'd run it up over where stock cross over is though...

On the 3400 rear, you can taper/port the opening better. That's how the one I have is. Should help a bit at least.


How about this, 3500 front manifold, 3400 rear, and join the 3500 front to the 3400 like the stock crossover? You wouldn't have logs, but you'd have some restriction where they all merge in the 3400 outlet... But you could just make a Y pipe and block/chop off the 3400 crossover area....

3500 front manifold on the rear works too, but on some cars like L bodies, the AC lines in the rear hit it..