TH350 or TH400?: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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April 21, 2011 12:38 p.m. rebelgtp SuperDork

Alright guys well it looks like I will be getting the 403 for my Olds and considering that these were never mated to a 4speed from the factory the crank would have to be drilled for the pilot bearing. This means that I can't use my Sag 4spd without modifying the crank on the engine which is more than I want to do at this point.

Currently I have two TH350s sitting in the two different cars and the engine has a TH400 in the Delta 88 that the yard is going to be testing out. I am kind of leaning to just staying with the TH350 considering the driveshaft in the car is already set to the proper length and all. However I figured I would ask you guys is there any reason I should make a deal for the TH400?

Also anything in particular I should do to either transmission to help with performance?

I guess the TPI 355 setup I have will just have to find a home in another car, I have been wanting to find a mid 70's Camaro...
April 21, 2011 12:41 p.m. OldGray320i New Reader

I know some Trans Am's had the 403 in them, and I'm pretty sure they had 4-speeds behind them (in fact, I thought I drove one once - it was a 79, California car)...
April 21, 2011 1:23 p.m. tuna55 Dork

If you're rebuilding it, there is no reason to get a TH400 at those power levels. If you're not, buy it based on condition. Either one can handle the torque in good condition.
April 21, 2011 4:12 p.m. lesabre400 New Reader

TH350. It can handle the output of the 403 and won't sap as much power as the 400. And if I'm wrong and it goes kaboom, hey, you have two of them.
April 21, 2011 8:12 p.m. NGTD HalfDork

I think you may find that getting a B-O-P pattern turbo 400 is quite a challenge. They are rare.

I had one once and was too young to understand what I had and let it go to scrap.
April 21, 2011 10:00 p.m. novaderrik HalfDork

a lot of BOP cars in the late 70's and early 80's had a trans that looked like a TH400 but had mostly TH350 guts.. i think it was called a TH375. but i'd just run a TH350 and not worry about it.. but the crank just might be drileld for the pilot bearing- the only way to know for sure would be to slide the torque converter back and see if you can get a finger up in there to see if it's drilled.. if you can't get enough room that way, then slide the trans back a few inches..
April 21, 2011 11:02 p.m. Vigo Dork

Im with tuna, there's no point in giving away the extra power it takes to turn a th400 if you're not making more power than a th350 can handle. And you're probably not.
April 29, 2011 7:40 p.m. rebelgtp SuperDork

Well I decided to go with the th350 and keep things simple considering its in the car already and all. I think for this build the stock 7.5 rear will stay but get beefed up as I collect the parts and save the 12 bolt for another build maybe (I have always had the desire for a twin turbo V8 Monte SS or GN).
April 30, 2011 7:30 a.m. 44Dwarf Dork

Well you can still put a 4 speed against it. you'll have to mod the trany but it can be done. they make a "button" that goes around the cranks center OD and has a bore for the 4 speed input shaft you have to cut the pilot bushing area of the trans and grind a new area further back to fit in the button. there cast crank 440 running this way.