considering swap out my th350 trans and install th400
 

i have a 78 Trans Am numbers matching survivor car. original paint, original interior etc.. in really nice condition with 70,000 miles. i recently pulled original engine and have it in storage to maintain the original engine. i now have a 72 400 block with stroker kit from butler 461 now.. has 6x-8 heads milled for a smaller combustion chamber.( 96 cc) flatop Pistons. roughly 9.3 to 1 compression. Comp Cams XE274. Comp Cams Roller tip Rockers 1.65 ratio.. R.A.R.E. exhaust manifolds. 2 1/2 inch collector. full PYPES 2 1/2 exhaust with Violator mufflers. has a semi rebuilt q-jet which will need more tuning.. has a th350 now which is leaking a bit. current rear is 2.56 and planning on 3.08 soon. also currently have a 2200-2400 stall. my thought are if i pull the th350 to at least regasket it i might as well rebuild it.. and if gonna rebuild it im thinking th400.. so i can preserve original th350.. thoughts?? and if i do the swap what can i expect as far as problems.. what needs to bedone to make the swap.. thanks as always for the info.

If you go TH400 you will need different drive shaft, different crossmember, find some way to utilize the TH400 electric kickdown deal.
Yes TH400 is stouter but may not be needed in your case.
Improvements can be made your th350 during a rebuild. I'd stay with the th350.
If you had a serious 500 plus HP, it would be different.

I also recommend staying with a TH350. But, with the torque of your new engine, I'd go with a heavy duty build, using the 36 element sprag/drum/race upgrade.

http://www.coanracing.com/Catalog.as...uctDetail=1573

http://www.jakesperformance.com/th350/

I will echo mr trashcans thoughts. Sticking with the th350 would be the most cost effective and least amount of labor. My friend and I have been going back and forth on this same issue on his 77. One thing were not sure of is parking brake cables might also be different as (mentioned) the drive shaft length and cross member too... If you really want to save the numbers matching trans, just get another one and build that one. Also, think the 3.08 is good gear for all around compromise. Were considering a little more gear as we don't do any highway driving. Just some thoughts as our build isn't exactly like yours.

ok great info.. i was also considering get another to keep original in storage.. ok i feel a bit better about the th350 rebuilding one to a heavy duty build.. didnt think there was that much involved to swap to the th400.. and agree i think the 3.08 is good.. i did purchase a 3.42 first and decided its gonna be too much gear for what i want to use it for.. which is alot of street driving and some highway crusing.. more so then track time. so i also purchased the 3.08. that will go in over the winter. along with the transmission swap..

actually looking on craigslist and just found what looks to be a good rebuildable core for $50.. i think thats the route to go.. i most likley will looking for advise on a master rebuild kit.. and what other little goodies i might need to beef it up.

Nobody wants th 350's or th400's anymore. Everyone has to have an overdrive.
Big torque Pontiac's can handle the high rear end gears. Not so, in Chevyworld.

LOL.. this is true.. i almost was thinking to go over drive route.. but i think would be way too much effort and cost.. with not much benefit..

Another vote to stick with the 350 trans and just build it up a bit. Not worth all the hassle to swap to a 400 in this case. Been there, done that with a 76 Formula. Ended up switching back to a 350 later on anyway.

great.. thanks for input... the 400 eats a bit more horsepower also right?? heavier mass to spin..

The TH 350 in my 78 has spent lots of times in high 10s and low 11s.

Years ago my engine builder told me 10-12 hp more needed to turn a th400 vs a th350.

Skip, what modifications have you performed on your th350?

I've never had anything resembling a powerful engine in my car, but I installed a B and M transpack at about 20,000 miles. I wonder if that has helped it's life expectancy. My trans has about 120,000 on it, it's never been out of the car. Wish I could say that about the engine (number 7 each lobe started go flat around 75,000).

You know I do not know all of them. It was built by Phoenix for one of my local Buick buddies that ran NMCA with me to Jim Turner's specs. Jim ran a Stage 2 GS in the the old classes we ran.Not sure if it got the better 32 element parts in it or not. heck I ran a bone stock rebuilt one in the low 12s for a lot of years before my first E head motor tore up a Trans King convertor in it after I put my first 455 E head motor in it. The Phoenix tranny and a 5000 stall A-1 dropped the car 0.75 seconds!

One I just had built for the 409 project has all the good stuff and was close to $2K.

There's a time and place for a 400. I'm just not a fan of them in my cars and even dumped the one in my 70 GTO many years ago.

The 350 is a great trans for many reasons, especially in 75-81 F cars. If I were to ever go back to a 3 speed trans for some reason, it'd still be a 350.

If you are not already a TH350 rebuild expert, you might wanna acquire some written help. Cliff has a TH350 Book. There are others, and I'm sure there is lots of online help.

https://www.amazon.com/GM-Turbo-350-.../dp/1613251890

There are several sources for the 36 element sprag/drum/race assembly. I think the TCI 327900 assembly is the cheapest. Not sure which brand is the best quality.

https://www.tsr-racing.com/th350-dru...ent-sprag.html

https://autoplicity.com/394905-tci-a...xoCTYgQAvD_BwE

https://www.amazon.com/TCI-327900-St.../dp/B000NZYNC8

http://www.coanracing.com//Catalog.a...uctDetail=2276

https://www.summitracing.com/parts/a...hoCVt0QAvD_BwE

https://transmissioncenter.net/shop/...mediate-sprag/

http://www.jegs.com/i/J-W-Performanc...RoCeZUQAvD_BwE

This set-up claims to be better than the 36 element sprag deal. I can't verify.

http://www.ckperformance.com/View/SU...CHANICAL-DIODE

I think most agree that Trans-Go is the best brand for a shift kit.

https://www.summitracing.com/parts/t...BoCv0UQAvD_BwE

http://www.ebay.com/itm/TransGo-TH-3...38.m2548.l4275

Here's a popular rebuild kit.

http://www.altousa.com/am/high_performance

http://www.ebay.com/itm/TH350-TH350C...26.m2548.l4275

https://www.amazon.com/TH350-Eagle-K.../dp/B00ZB6CN5I

http://dragtrans.com/dragtrans_html/HPOHK.html

this is awesome info..!!!! thank you..

i have rebuild engines.. but only minimal trans work.. and was planning on cliff's book.. also a good friend who has rebuilt transmissions before is on the hook and lives like 1 mile from my house. sent message to the guy with the th350 local that i found on CL.. for $50.. that would be perfect.. and close by.. like 15 minutes away..

Make sure your actually buying a bop 350. Can't tell you how many people brought me cores to build that were later lock up units, or Chevy bell. 2 weeks ago a guy brought me a metric 200 he was told was a 350 from craigslist, I told and showed him what to look for and he brings another 200.

That's actually funny!:D

But if he wants to unload one or both of the 200's, PM me.

If you have enough bottom end grunt, you can run a taller rear gear and still use the 3 speed.

There may be some ET loss due to the greater weight of the 400. But I don't believe there could be a significant internal power loss between the two. Everything is locked in hi gear on both.

A bit cleaner look under the hood without the 350 TV cable.