TH 350 or TH400?

[SRSCOUT]

Since I am deep into my rebuild, I am debating swapping my current 4speed set up ( SM465/205) for an automatic with a 230/205 doubler. I already have a TH350 with a 203 range box that came with it. I also have several 205 transfer cases that came on automatics. In reading I know the TH400 is some stronger than the TH350. But is it enough of an upgrade from the TH350 that it's worth scrapping the TH350/203 that I already have and rounding up a TH400 with a 203?

I'm running a stock TBI Chev 350 and 35" tires. Just not sure if there is enough difference to justify the additional cost considering the rig I am running.

 

[Jinx]

I would say no, the TH400 isn't worth it. The TH350 with a tranny cooler should be fine for what you are doing...

Plus the 400 will rob a bunch of horsepower by the time it hits the wheels.

 

[BCGPER]

The TH400 has a slightly lower first gear and is a bit stronger. Since you already have the 350, I'd just run that with the doubler.

 

[SRSCOUT]

The TH400 has a slightly lower first gear and is a bit stronger. Since you already have the 350, I'd just run that with the doubler.

OK From what I was able to find the TH350 was slightly lower than the TH400. The specs I found were 2.52 for the TH350, and 2.48 for the TH400. I also have a TH700R4 with a 203 that will bolt up. Now it does have a much lower first gear (3.0:1 I believe) but from what I have been able to find it requires major internal modification to get it to hold up to serious Off Road.

 

[SRSCOUT]

I would say no, the TH400 isn't worth it. The TH350 with a tranny cooler should be fine for what you are doing...

Plus the 400 will rob a bunch of horsepower by the time it hits the wheels.

If I run the TH350, I was planning on a large external cooler and a deep pan for extra capacity. Years ago I ran a TH350 with a 350 Chev in a 1980 Toyota 4X4 pickup. It held up great, but was much lighter and only had 33's on it.

 

[BCGPER]

It's been a while, I might have the 350 and 400 gears confused. If you comfortable rebuilding a trans, the 700R4 is a good transmission once you cure the weak points of it. I'd still go with the 350 though. Easy to rebuild, just make sure to support the transfer case well, the tail housing on the 350 tends to crack under abuse.

 

[SRSCOUT]

Yes I agree that transmission/ transfer case support is critical. Where this is a complete custom swap, I have a very heavy trans mount already for the SM465/205 that is currently in it. I was planning a dual mount with skid plate for the automatic swap.
I have very little experience with automatics, but fortunately I have a good friend who is a 30yr mechanic and owns his own shop. He has rebuilt numerous TH350's/400's and has kindly volunteered to help me rebuild (I'd like to learn how to do it myself) whatever trans I decide to run for the cost of the parts.

Finally I'm thinking an RV type converter (approx 1500rpm stall) and was wondering what you automatic guy's thoughts are on this? Am I thinking right, or should I go with a lower stall converter?

 

[Jinx]

Just chiming in. I have run a 700r4 with a 4 spd atlas behind a medium hot tpi 350 for the last 15 years.

Love the 3:1 first gear.

 

[sawtooth4x4]

I like the 700R4 I put in my Range Rover. Its always nice to have atleast a 4 speed auto.

 

[SRSCOUT]

I thought about the 700R4, but this rig has seen such limited street use and no plans for any more in the future, that I don't think that lack of OD will be an issue for me. When I calculated it out my crawl ratio with the doubler will be right about 82:1 so I'm not sure I need much lower gearing. The 350/400's are just so easy to rebuild (for a novice like me) that I think I will be OK with either one. Since I have a TH350 with the 203 range box already, I think that is what I will use.

Continuation of this. The two Doubler kits I have looked at are from Off Road Design and Northwest Fab. Anyone here have any experience with either one of these kits, and if so what were your impressions?

 

[I Lean]

I do not like a low-stall converter, especially with low gears like you have planned. It will pull a lot harder at idle that you'll want, and it will be difficult to hold back with the brakes. I had a stock converter in my 4.3/TH350, and because of that I chose a high-stall converter in my second try, which was an LT1/TH400. I liked the high stall converter a lot better.

 

[SRSCOUT]

So what kind of stall are you talking about? I was thinking about something in the 1500rpm area. Is this going to be to low?

 

[I Lean]

So what kind of stall are you talking about? I was thinking about something in the 1500rpm area. Is this going to be to low?

In my opinion, yes. 1500 will be too low. IIRC stock converters will be something like 1800 RPM stall speed, and the one I used was closer to 2100-2200 or so.

 

[Agility Customs]

I am currently running a 1600 rpm stall in my promod buggy and it crawls through the brakes so hard I cant stop it in low range I have a 2200 stall converter waiting to swap in.

 

[Greg]

I'm with Carl, a higher stall speed will fight the deeper crawl ratio with an auto.... I ran a higher stall convertor in my TJ with a TH350 and it didn't push thru the brakes. But a higher stall convertor will introduce more heat into the ATF, so a properly sized trans cooler becomes even more needed with a higher stall convertor.

 

[larrybenny]

The output shaft is what breaks in a 700R4 or TH350. You can get a stronger, hardened one from advance adaptors. Otherwise both units are similar internally and should be fine with a proper cooler. My 2 cents.
If you were to use a 700R4 make sure you have torque converter lockup control or you will overheat it for sure.