3900 top-swap (3.1 Turbo)

[pontiacjeff]

I successfully completed a "3.1 I/C Turbo" topend swap onto a 3400SFI with a T3 upgrade a couple years ago. It was realtively easy and became a real monster on the street with only 10psi boost. I am thinking of doing the same thing with a 3900 shortblock. Will this be possible with the 3900? I read somewhere about offset bore centers and some other things. Anyone know or point me to a detailed info on the 3900. Thanks.

[robertisaar]

so you stuck Gen2 intakes and heads on a 3400 block?

[pontiacjeff]

so you stuck Gen2 intakes and heads on a 3400 block?

Yes, the only issue was pushrods and I used the stock 3100 pushrods after bleeding down all the 3400 roller lifters to make sure they were toppped-out. Pre-load was less than stock, but just enough to quiet the valvetrain. It was a turbo beast! It made more on 4psi than the 3.1 did on 7psi with the same turbo and i/c setup. Way more difference than I expected.

Now I want to do the same thing, 3.1 Turbo heads and intake ("3.1 Intercooled Turbo") on a 3500 or 3900, whichever will work, the bigger, the better. But I fear issues with the 3900.

It is for my 1990 6000 S/E AWD. Pontiac built two 3.1T AWD cars for testing. I want this to look bone-stock from the outside, as if installed by Pontiac. But I want the biggest bang for the buck. Can you blame me for that?

[WrathOfSocrus]

A Gen 2 top end isn't as bad as some people make it out to be. The main restriction is the intake, and a turbo helps defeat that problem. Granted the exhaust manifolds could be used on the Gen 3 top end with minor modification.

Before I started reading this forum I had purchased a set of rebuilt Gen 2 heads and gasket matched the top end. I'm not sure if it is worth rebuilding the old Gen 2 bottom end when I can pick up something Gen 3 with less miles and no bottom end work needed for fairly cheap. Get some new piping and a bigger turbo and it should move pretty good for a junkyard motor. When it dies I can take all the turbo specific parts off and move to a complete large port motor.

That being said I'm fairly certain it would not fit on a VVT 3500/3900 block. It probably could work on a 3500 LX9 and make more power than stock but would be rather choked compared to the 3500 top end. The TGP exhaust manifold ports could be made into a D shape to fit the 3500 heads.

[robertisaar]

that "bone-stock look" is killing power... biggest bang for the buck would be to use a full 3500 motor and just stick a turbo right to it and a TCE Crank Trigger and you're pretty much done, aside from exhaust/intake plumbing.

and from what i read here, only the VVT 3500/3900 have the issue of the bore offset, so a non-VVT 3500 could work... after all, people do stick non-VVT 3500 heads on a 3400.

http://en.wikipedia.org/wiki/Gm_high_value_engine

[IsaacHayes]

3900 won't work for this. It's considered gen4.

LX9 3500 (gen3.5) might work, not sure about the heads etc from a gen2 though working on it. And for sure putting gen2 heads on a 3500 is a huge reduction in power.

[asylummotorsports]

By putting gen 2 heads on an LX9 3500 you would need the damn turbo just to replace the lost power.

[pontiacjeff]

So a 3500 (non-VVT) will work? I already have a 3400, so I might just go ahead with that. Restriction on the intake side will not be an issue. I will be using a T3/T4 hybrid turbo. I understand the "loss" of power, but the "stock appearing" is alot more important to me. If I have to turn up one more pound of boost to make up for the possible loss of power (as opposed to just building a stock 3500 with turbo), then so be it.

I'll be looking for about 350FWHP. The limit will be the power has to go thru a TH125 transmission into the AWD transfer case. That, I am not worried about, nor am I concerned about the rear differential in this car. It is a front diff from an S10/T10/Astro AWD that was turned backwards. It is a 7.25" R&P. More than adequate. I'll be building some custom parts fo the TH125 to keep the 2nd band alive and well.

[Superdave]

3.1 heads on a 3500 would yield about 10.7:1 compression.

[pontiacjeff]

I measured the chambers on the 3400, but I don;t rememebr what it came to, do you recall the difference int he 3.1 and 3400 chamber size?

[Superdave]

I measured the chambers on the 3400, but I don;t rememebr what it came to, do you recall the difference int he 3.1 and 3400 chamber size?

gen2 is 26.4 and 3400 is 28.6, 3500 is 32.4

[pontiacjeff]

that explains why the 3.1T topend ran so good, the slightly smaller chamber gave a slight boost in compression, not to mention the base compression was higher, too. The 3.1T is (IIRC) 8.0:1. The 3400 engine is ~9.25:1, so I bet my hybrid was about 9.5:1.

I have decided to simply build another version of that hybrid for my 6000 AWD. It will be a simple swap with only a/c lines needed customizing. I will run it on a 3.1T computer with the 8F definition that I have tuned many times. I am looking forward with great excitement. I'll start a new thread soon so anyone interested can follow along.