Just how strong is the auto tranny in my 86 2.8 Fiero? Will it handle my 3.8NA swap engine or should I convert the car to manual tranny? 210hp/220tq. Lotsa spirited city driving weekend type of car.

Thanks.

Unless you're partial to manuals, I'd suggest just swapping in a 4T60 that's mated to that engine in most cars.

And you'll want that OD also.

And you would need an adapter plate for the 3.8L with TH125c since it has a different bolt pattern than a 60* V6.

EDIT: Bad info on my part...

[This message has been edited by Hudini (edited 11-01-2009).]

quote Originally posted by fieroall: Just how strong is the auto tranny in my 86 2.8 Fiero?

For your engine swap, a new or well-rebuilt TH125 presumably shouldn't have a problem in terms of handling the power.

However, if you've neither a new nor a well-rebuilt TH125 for which you've already paid a healthy sum of money --- but just a 23-year-old TH125 transmission --- I suggest giving strong consideration to the 4T60 that others have suggested here. The RPM-reducing overdrive gear of the 4T60 also very likely will be something you'll appreciate when having to pay the bill for your Fiero's gasoline consumption.


By the way, in regard to a "well-rebuilt TH125," I don't mean slapping on a shift kit, which even a shift kit manufacturer will tell you, is a bad idea for an old transmission that isn't in great shape in the first place.


Take care, and good luck to you with your engine swap project.

quote Originally posted by Hudini: And you would need an adapter plate for the 3.8L with TH125c since it has a different bolt pattern than a 60* V6.

Uh... if that was the case then all the 3.8SC manual swaps would need an adapter, and yet they don't...

Well it's a 23 year old tranny but it's in great shape and shifts great.

quote Originally posted by Hudini: And you would need an adapter plate for the 3.8L with TH125c since it has a different bolt pattern than a 60* V6.

3.8 rwd needs a plate/adpter
3800fwd doesNOT

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Question wonder and be wierd
are you kind?

Go with the newer tranny if you want an auto. I spent LOTS of cash trying to get T125s to work with my V8. 4 times, the tranny didnt last 1,000 miles just driving on street and hiway. It was built each time by a local shop that builds drag racing trannies for drag racers here. They did every trick in the book including putting a 8 disc clutch pack in place of OEM 4 disc. You might get away with it on a V6 as long as you dont crank up the hp. They work fine on original 2.8 V6.

I've had only good experiences with reliability on the TH125H but mine is beefed up. I know of two Fiero owners one who ran 10's with a V8 and another who ran 11's and their modified TH-125's also held up. This trans will only bolt directly to FWD engines like the 3400, 3800, 3.4, 3400, 4.9L Caddy etc. While it may survive in your application, a better choice is the 4T60 4 speed auto. You'll cruise at much better RPM's on the highway and also get better mileage. Howeverr, this trans will not bolt directly to a 3.8L V6 either.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Will the TH125 torque converter bolt up to the 3800 flywheel?

Never mind found all I was looking for here -----> https://www.fiero.nl/forum/A...090907-2-093783.html

[This message has been edited by fieroall (edited 10-31-2009).]

quote Originally posted by fieroall: Will the TH125 torque converter bolt up to the 3800 flywheel?

It should.

The T 125 is tougher than the 4 speed autos built before they went to electrical shift ..This is one tough tranny..this is not appreciated on this forum ...The 4 speed 4T is more versatile because of the extra speed making it a better choice in 90 % of cases..the 3 speed is a gas hog.. There are 100s of cases of failure of the T125..on this forum..but it is tough..neither transmission is for drag racing
the only reson to swap to a 4 speed is it is more user friendly,,not tougher..
GM built this transmission to take the abuse that millions of average owners would dish out
When you backup,,come to a stop before you shift to drive,,this is a tranny killer..change the fluid and filter ..
My information comes from internet research,,posting about T125 on this forum..talking to people and post where the 2 trannies were argued..And radio mechanic shows.... I quote ""that is one tough transmission"
The 4 speed is better ,,the T125 tougher...My arogant centavo,s,worth

[This message has been edited by uhlanstan (edited 10-31-2009).]

OK now can someone help me identify the gear ratios I have in my 86 2.8l Auto fiero GT? I want the tallest gears available for the autos. Something I read about swapping 3.73 gears? Anyone have more info on this?

Thanks.

quote Originally posted by fieroall: OK now can someone help me identify the gear ratios I have in my 86 2.8l Auto fiero GT?

Here you go; the gear ratios of your TH-125 transmission are:

• 1st Gear = 2.84:1
  
• 2nd Gear = 1.60:1
  
• 3rd Gear = 1.00:1

So does that make it a 3.33 FDR? Also does anyone have the taller gear info swap procedure?

Thanks.

can you put the 4T60 on the fiero 2.8?

quote Originally posted by kwaidonjin: can you put the 4T60 on the fiero 2.8?

Yes. See http://www.fieroaddiction.com/4T60a.html .

It may be a tough tranny for a 2.8 v6. It is not for a 300+ hp v8. Otherwise I would have gotten more than 1,000 miles out of one cruising down the interstate. All of mine failed just cruising. All the clutches and low band self destroyed. Car never even did a burnout. It was a show car, not intended to be raced. It was more of a rice intimidator. Give me a Fiero with anything bigger than 2.8 and I can blow it up in a few minutes without power slamming it from neutral from my experience.

OK so I just ordered the 33/37 sprockets and chain combo.

Here's the ACDelco #'s for anyone interested:

33 Teeth sprocket # 8643209
37 Teeth sprocket # 8643224
Chain # 8660099

All parts cost me $125 Canadian. Bought locally. Now to see how hard it is to swap them. I will go to the junk yard this weekend and will poke around some TH125's.

using the search feature at the top of tech type in 33/37 auto transmission swap ..

you should come up with some good reading (with exception of my post)

Plenty of bad stories on T125 but it is tougher than T460,, the 4 speed is better ,,not as tough

[This message has been edited by uhlanstan (edited 11-02-2009).]

quote Originally posted by uhlanstan: using the search feature at the top of tech type in 33/37 auto transmission swap .. you should come up with some good reading (with exception of my post) Plenty of bad stories on T125 but it is tougher than T460,, the 4 speed is better ,,not as tough

I think I've read everything their is on the topic but noone has actually done a step by step write up ... which is what I was actually looking for. O well does anyone know what final drive ratio I have with my 86 2.8l 3 speed? Is it a 3.33 or a 3.06? Any way to tell? Which tags and where?

Thanks.

quote Originally posted by rogergarrison: Go with the newer tranny if you want an auto. I spent LOTS of cash trying to get T125s to work with my V8. 4 times, the tranny didnt last 1,000 miles just driving on street and hiway. It was built each time by a local shop that builds drag racing trannies for drag racers here. They did every trick in the book including putting a 8 disc clutch pack in place of OEM 4 disc. You might get away with it on a V6 as long as you dont crank up the hp. They work fine on original 2.8 V6.

IMO it sounds like who ever built your trans did not sufficiently increase the line pressure to account for the greater holding force that the larger clutch pack provided. If they had your shifts would feel very hard. I theorize that excess slippage of the clutches caused your failures.
The fastest SBC V8 Fiero on record built by Pat Ciarcia of Mt Pocono PA. in the late 90's used a TH-125H. Pat ran a JPT modified TH-125H behind a 500HP nitrous fed SBC V8 for three seasons (running the 1/4 mile in the mid 10's) with no failures. Pat ran large racing slicks on his Fiero and I believe had Mark Williams axles. Point is that correctly built the TH-125H has been proven to be very strong as long as it is not slam shifted. I have the 3:33:1 FDR trans in my Turbo GT and while it is not a V8 it's certainly being run behind a strong engine. It is a gas hog as I get 20 hwy MPG (less than my 3800SC at 25 MPG) but other than that it works well. For drivability the four speed 4T60e and 4T60eHD autos are better

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

There are always exceptions Dennis. Maybe his was a high dollar cryoed setup. The shop that did all of them for mine specializes in building racing transmissions for drag cars and circle track cars and been in business for 30 years. Each time, they upped the build to make it better. They tried all the diff packs, kevlar/stainless band, diff valve bodies, custom tc, pumps...you name it. Even tried a manual valve body.

quote Originally posted by fieroall: I think I've read everything their is on the topic but noone has actually done a step by step write up ... which is what I was actually looking for.

I've looked for the same thing, and unfortunately, I haven't found such a step-by-step write-up either.


However, I think I do have some answers for your questions below:

quote Originally posted by fieroall: ...does anyone know what final drive ratio I have with my 86 2.8l 3 speed? Is it a 3.33 or a 3.06? Any way to tell? Which tags and where?

If it has a "GX3" somewhere in the RPO codes' sticker located on the top side of the black, driver's-side fenderwell of your automatic-equipped `86 Fiero, then it presumably has a 3.33 final drive ratio, not the 3.18 indicated in the GM-provided, "Pontiac Fiero 1986 Do-It-Yourself" manual that accompanied each new `86 Fiero.

The 3.18 is designated by the RPO code, "F75" rather than "GX3" in that same sticker.

This topic is discussed in more detail within the thread titled, "Are all 3 speed auto trannies the same?" accessible via this Pennock's Fiero Forum link: https://www.fiero.nl/forum/Forum2/HTML/102213.html .

Well mine is a 3.06 according to the FW2 code on my fendertag which is kinda unfortunate cause the highest I can get the ratio is 3.43 without messing with the inner gears. Still not too bad tho.

BTW the Fiero Cave says their never was a fiero available with the 3.18 FDR. Your tranny must have been swapped.

http://home.comcast.net/~fierocave/

[This message has been edited by fieroall (edited 11-03-2009).]

quote Originally posted by rogergarrison: It may be a tough tranny for a 2.8 v6. It is not for a 300+ hp v8. Otherwise I would have gotten more than 1,000 miles out of one cruising down the interstate. All of mine failed just cruising. All the clutches and low band self destroyed. Car never even did a burnout. It was a show car, not intended to be raced. It was more of a rice intimidator. Give me a Fiero with anything bigger than 2.8 and I can blow it up in a few minutes without power slamming it from neutral from my experience.

quote Originally posted by rogergarrison: There are always exceptions Dennis. Maybe his was a high dollar cryoed setup. The shop that did all of them for mine specializes in building racing transmissions for drag cars and circle track cars and been in business for 30 years. Each time, they upped the build to make it better. They tried all the diff packs, kevlar/stainless band, diff valve bodies, custom tc, pumps...you name it. Even tried a manual valve body.

Sounds like you never had the TV cable adjusted right.

[This message has been edited by Will (edited 11-04-2009).]

They built and installed them all, so I assume they know how to adjust it being only a trans shop. Manual body didnt have a cable.

quote Originally posted by fieroall: Well mine is a 3.06 according to the FW2 code on my fendertag which is kinda unfortunate cause the highest I can get the ratio is 3.43 without messing with the inner gears. Still not too bad tho. BTW the Fiero Cave says their never was a fiero available with the 3.18 FDR. Your tranny must have been swapped. http://home.comcast.net/~fierocave/

My `86 GT actually is a 3.33, as indicated by the "GX3" RPO code on the sticker of its driver's side fenderwell.

quote Originally posted by fieroall: OK so I just ordered the 33/37 sprockets and chain combo. Here's the ACDelco #'s for anyone interested: 33 Teeth sprocket # 8643209 37 Teeth sprocket # 8643224 Chain # 8660099

Yes, I'm very much interested in learning the specific AC Delco part numbers, fieroall, and thank you for posting them. As I understand it, the 33/37 sprockets and chain combination you've ordered will result in a (3.18/[33/37] =) 3.43 ratio for your car, or a 3.33/[33/37] =) 3.73 ratio for mine, which in either case, would be a 12% increase.

Yesss!

I've a question of you, however.

Is the part number you've listed for the chain (# 8660099) a typo? When I input that part number into AC Delco's parts catalog website ( http://parts-catalog.acdelc.../b2c/stock_check.php ), all I get is this error message: "Part Number (8660099) not valid."

[This message has been edited by project34 (edited 11-04-2009).]

The chain # is 8660099. BTW I just got all the parts needed including a tranny filter and the needed gaskets/seals for under $200 can.

Here is all the info you'll need:



I will post picks later.

[This message has been edited by fieroall (edited 11-04-2009).]

quote Originally posted by rogergarrison: They built and installed them all, so I assume they know how to adjust it being only a trans shop.

That's a big assumption.
Some carbeurettor manufacturers refuse to warrantee their products with ovderdrive transmissions (IE, TH-700 using TV cable) because people can't set up the TV cable correctly. This tells me that this seemingly simple operation is misunderstood on a ridiculously widespread level.

Since you had a non-stock throttle cam on your engine, it would not surprise me one bit if you didn't have the right pull on your TV cable.

What I've been told about the high power failure mode of the TH-125 is that the intermediate band doesn't have enough oomph and can't bring the transmission solidly into 2nd gear. The band applies, but doesn't produce enough torque to stop the drum from rotating, which in turn cooks both the band and the drum. If you're cooking *clutches*, especially if you're cooking clutches in light duty driving, then you don't have enough line pressure. 99% of the time, this is a problem with the TV cable.

quote Originally posted by rogergarrison: They built and installed them all, so I assume they know how to adjust it being only a trans shop. Manual body didnt have a cable.

I do not call into question your bad experiences with the TH-125H but we've seen this transmissons successfully used for very high power V8 applications with great success. JPT transmission in Eaton Rapids MI. has built 100's of bulletproof TH-125H's. A good portion are used with V8's and still running fine. I've noticed that on their boxes the shifts are very harsh, leading me to believe that they run very high line pressures to keep the expanded clutch packs from slipping and burning. If your trans burned out from just normal use then I would call into question how the rebuilds were done. If they used a B&M shift kit then I can see the problem. Those shift kits were not designed properly.
I believe that the success of any perfornance trans rebuild lies in the hands and skill of the builder and IMO there aren't many shops that know how to rebuild the TH-125H properly. We won't argue the point on your experience. All we can do is weigh it against the positive expereinces.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

This series of post then requires info on:

Which stift kits are the good ones?
What is the desired target line pressure?
How do you get the line pressure there? specific spring or washers in valve body?
What or how do you get and know the TV adjustment is right. Assume it's not the same as in the factory service manual? i guess?

Any one help or input on answers. About to rebuild my 125C.

quote Originally posted by DandRauto: This series of post then requires info on: Which stift kits are the good ones? What is the desired target line pressure? How do you get the line pressure there? specific spring or washers in valve body? What or how do you get and know the TV adjustment is right. Assume it's not the same as in the factory service manual? i guess? Any one help or input on answers. About to rebuild my 125C.

Answers to the above:
1. Use the Trans Go shift kit. The B&M's do not work very well. We've used both and can say from expereince.
2. Line pressures vary according to gear and position of the TV cable. Maximum line pressures can be up to 175-180 PSI in drive.
3. The base line pressure is set by the pump and control valve assembly . It is regulated by the TV cable position
4. TV cable adjustment is easy. Just go the post on the forum right now.

If you rebuild use the GM manual and try to locate a rebuild video that will guide you through the process.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

I don't think a TH 125 will hold up to a 3800 na they have about the same power as a 4.9 And the th-125 I had lasted less than a week behind a 4.9 The th-125 had no more than 80k miles on it, You will be better off with a 4t60 or 4t60 E I would even take a fieros isuzu 5speed over a TH-125
If your lucky and it last for more than a week you will hate it when traveling on the freeway out of town
I also find it hard to believe ppl ran them behind v8s running 10s without them breaking And if they did they spent a small fortune on it(and still sweat bullets when driving), Those transmissions was only built to handle the 4cyl duke with only 90hp and torc, Rebuilding one is nothing but a temp fix before your replacing a transmission, I know I would hate having to yank the transmission out every week or other week because im trying to use a weaker th-125
And a Th-125 is alot weaker then a 4t60 and 4t60E, If you can run OBD2 on the 3800na engine Why not go with a 4t65E (alot stronger) Or better yet a 4t65E-HD transmission? With the 4t65E you can later throw a turbo on it and not worry.


The 4t60E, 4t65E and 4t65E-HD uses electronic shifting instead of the annoying TV cable ( if not adjustred properly will also kill a transmission) If your running a OBD1 system then do a 4t60E if your running obd2 use the 4t65E or the stronger 4t65E- HD I bet for the cost of a rebuild and buff up on a th-125 will turn out to be a heck of alot more expensive then simply buying a lower mileage 4t60E, 4t65E, 4t65E-HD, I personally got my 4t65E-HD unit for 600bucks and its running behind a 3800 sc 2 engine without any complaints.

[This message has been edited by 86fieroEarl (edited 11-05-2009).]

Technically the 4T60E uses vacuum modulation for line pressure and electronic solenoids for gear selection, so it's not fully electronic and easy, relatively, to control. The 65E series use PCM controlled line pressure and pretty much requires an OBD-II computer or an expensive aftermarket controller to make it work properly.

I have to admit the more I read about the 3T40... TH125C... the more that it sounds like the improper use of the TV is the cause of the failures. A lot of J-body owners use this tranny behind a lot of different swaps and turbos... it is not a weak tranny.