I have a few questions about the TH125 that comes stock with the fiero. How much can these handle? I've heard that they are pretty stout and rarely have issues with power. I wonder because Im trying to do an LS swap and I want a temporary transmission to pair it with until I can afford a better manual (F40 possibly) and if it does survive, what kind of upgrades can and/or should I do on it to improve the slow shifting?
SInce im removing the stock ECM would I need to install a manual TC lockup switch as well?
Good luck. I think they are quite capable, when paired with a V6. (They also came in FWD 3.1 cars.) Beyond that, I'm not so sure. I know at least one person who used to post here who had one bolted to a SBC. It was supposedly "built up", but still failed.
But please don't take my word for it. I'm sure others will chime in.
[This message has been edited by Raydar (edited 01-30-2024).]
Not sure I would go through all the work to "temporarily" install it especially if you want to go manual. It means modifying or changing the adapter plate.
I know people have built a 4T60 to handle gobs of power, but not sure if the 3-speed can be built the same. Fuel mileage will suffer big time with the 3-speed.
Not sure I would go through all the work to "temporarily" install it especially if you want to go manual. It means modifying or changing the adapter plate.
I know people have built a 4T60 to handle gobs of power, but not sure if the 3-speed can be built the same. Fuel mileage will suffer big time with the 3-speed.
well the reason I want to go with this trans is because it doesn't need an adapter plate. Its gonna take me a good year just to get the trans and an adapter plate (broke teen) and the LS4 wont be hard to get and is pretty cheap. So I want to run with it until that time comes. Im not planning on pushing it super hard and hopefully it will blow up when im ready. I do plan on getting a shift kit and some other stuff to improve the trans before I drop it. Also it's just a fun project and gives me more experience working with automatics. Its kind of stupid reasons but I also want more switches on the dash, makes it seem cooler lmao.
[This message has been edited by BruhMans06 (edited 01-30-2024).]
Originally posted by BruhMans06:well the reason I want to go with this trans is because it doesn't need an adapter plate. Its gonna take me a good year just to get the trans and an adapter plate (broke teen) and the LS4 wont be hard to get and is pretty cheap. So I want to run with it until that time comes. Im not planning on pushing it super hard and hopefully it will blow up when im ready. I do plan on getting a shift kit and some other stuff to improve the trans before I drop it. Also it's just a fun project and gives me more experience working with automatics. Its kind of stupid reasons but I also want more switches on the dash, makes it seem cooler lmao.
The Th-125 will definitely handle the V8. It's pretty solid, but that's assuming it's in good shape / recently rebuilt. Keep in mind that any used Fiero transmission you hook up to the V8 will be already ~30 years old and will likely be worn out and more prone to failing than when it was new. You can totally do this, I just don't recommend that you get too crazy with it. No dead-stop burn-outs and stuff like that... at least I wouldn't. I wouldn't waste money on a shift kit, or anything else like that. To be completely honest... why not just get yourself a used junkyard 4T60E? Swapping in a 4T60 in an 84-87 is basically as simple as buying new transmission mounts and driveshafts.
Originally posted by 82-T/A [At Work]: The Th-125 will definitely handle the V8. It's pretty solid, but that's assuming it's in good shape / recently rebuilt. Keep in mind that any used Fiero transmission you hook up to the V8 will be already ~30 years old and will likely be worn out and more prone to failing than when it was new. You can totally do this, I just don't recommend that you get too crazy with it. No dead-stop burn-outs and stuff like that... at least I wouldn't. I wouldn't waste money on a shift kit, or anything else like that. To be completely honest... why not just get yourself a used junkyard 4T60E? Swapping in a 4T60 in an 84-87 is basically as simple as buying new transmission mounts and driveshafts.
When I bought the car it seemed to be in pretty decent condition. The car had less than 80,000 miles on the clock and when I swapped the Iron Duke out for an 85' Iron Duke, a lot of the seals looked somewhat new, and the fluid was still a nice cherry red with very little contamination. the problem with getting a 4t60 is the same as getting the F40, whiles its leagues cheaper, it still requires money that I don't have, at least with the Th125 I can retain the half of the stock mounts and axles. BUt I have definitely considered the option of a 4T60 instead of an F40 or Getrag transmissions just because of how cheap it is.
Originally posted by BruhMans06:When I bought the car it seemed to be in pretty decent condition. The car had less than 80,000 miles on the clock and when I swapped the Iron Duke out for an 85' Iron Duke, a lot of the seals looked somewhat new, and the fluid was still a nice cherry red with very little contamination. the problem with getting a 4t60 is the same as getting the F40, whiles its leagues cheaper, it still requires money that I don't have, at least with the Th125 I can retain the half of the stock mounts and axles. BUt I have definitely considered the option of a 4T60 instead of an F40 or Getrag transmissions just because of how cheap it is.
Thanks for the input
So I guess the reality then is, going with the Th-125 is just your time... but no cost. And there's nothing wrong with that. You can definitely hook up the V8 and drop it in, and then not have to worry about driveshafts since you can just re-use the ones you have. I'm assuming you've already got an ECM and wiring harness... so at this point, the only thing you're going to have to deal with is exhaust... which you'll have to do regardless of whether you have a manual or an automatic in there. So... when you design the exhaust, just keep in mind where the shift cables WOULD go with an F-40... that way you don't design an exhaust and then have to re-do it again when you get the manual.
But yeah, you can use the Th-125, but I wouldn't spend any money on it... just use it and be gentle with it.
Most of the thread is the same response I just gave (swap to a 4T60) which doesn't answer your question. But nested in there, I found a post that talks about torque numbers. This specific response: https://www.fiero.nl/forum/...02-2-104381.html#p34 ... states the following:
"The TH-125, even as built as it can get, is very torque limited. The steep chain ratio of 33/37 puts 1.12 times engine torque through the transmission. This means that if the transmission can handle 300 ft-lbs (for example), the strongest engine you can put in front of it makes 267 ft-lbs. With the 37/33 chain set, you can put a 336 ft-lbs engine with the 300 ft-lbs transmission. With the 38/32 chain set, you can go to 356 ft-lbs at the crank."
The rest of the thread has examples of people using it behind really built engines, and them all starting to fail. Biggest issue seems to be that most of them start slipping in third gear after being abused.
A couple of years back I had a coversation with Dennis LaGrua about this very subject. I was considering a 3800 swap into an '86 GT. Dennis is a very knowledgeable PFF member. Dennis said:
"If the modification is done correctly the TH125H will last. A local guy Pat Ciarcia a drag racer ran an 500HP SBC w nitrous with a TH125 in his 87 GT and made many 10 second 1/4 mile runs with no blow ups. The key is getting all the slippage out . Jims Performance Transmissipn in Michigan used to build these boxes and I have one on my 3.4L Turbo GT. That box is rock solid. Jim retired and sold the business but occasionally one pops up for sale here in the USA. Over your way the 4T65eHD transmissions used with the 3800SC FWD cars are probably not that common but you might find the non HD version. Whichever way you decide to go I just would not advise using a stock TH125H with a 3800SC."
He also said:
"The first thing with these transmissions is to up the line pressure and eliminate slippage. A good TransGo shift kit will firm up the shifts.Do not use the B&M kit as its no better than stock. The internal modification is simple. You just add another friction disc to the first and second clutch packs. This process is done by using slightly thinner discs so that the overall length stays the same. Finally you can take some load off the trans by using a higher stall speed (2500 RPM) torque converter and a 3:33 FDR."
and
"You can purchase a complete TH125H rebuild kit here in the USA and the TransGo shift kit. If you decide to ship parts, you are not talking about that much weight except for the torque converter. You can also buy the ATSG rebuild manual. Look on eBay. There are also some rebuild videos on You Tube. If rebuilt for performance these transmissions will work."
[This message has been edited by Mike in Sydney (edited 01-30-2024).]
I wouldn't attempt to put a 125c with the LS4... sure there used to be people who build these up, but those people are no longer doing it and the parts to do so are likely long gone as well.
The other issue with the LS4/125c combo is the LS4 is DBW and the 125c uses a TV cable which normally connects to the throttle body.
The LS4 comes with a 4 speed auto, so just swap in the motor and transmission. Mounts are easy, and it keeps you from giving me $$$ for a starter mount.
Really not trying to rain on your parade, and it is great to see a teen on the forum among us grumpy old guys... But if you are a "broke teen" as you say, then maybe stick a 2.8 or 3.1 into it and just enjoy the car. Once you start getting into LS4, F40, etc.. there are a lot of things to buy, modify, hours, blood sweat and tears.., and they all need money.
For most of us, these are toy cars that we throw extra money at- but if you are young and really tight for cash, you may end up with a car that you cannot drive because you bit off more than you can chew
Not trying to discourage you, just sharing some life experience
Auto Trans Torque rating is On the Input Shaft. TH125c also know as 3T40 is one of the Weakest auto trans GM made & rated ~ 200 ftlb.
Torque Converter puts ~ 1.5 to 1.8 or ever more X Engine Torque & w/ "big" engine will easily overload the trans. If Engine puts out 150ftlb X 1.5 = 225ftlb, X 1.8 = 270ftlb, in to TH125c rated ~ 200ftlb
Worse, TH125 have different parts internally over the years made & more... Examples: units for Dukes may have different clutch packs etc vs units for V6. later units have bigger chain & sprockets regardless.
So saying the unit will work w/ whatever motor is often just luck even when someone have this unit "behind" that motor...
And This is Common for Auto Trans & not just GM. Ford C4 had many changes for given motor attach to them. Including 1 piston pack to drive one clutch pack had different sizes & had Letter on it to tell from outside which piston set you have in a car. "Code" A for Small Block V8, other letters for L6 etc.
