Can you use a TH125 with an L67, 3800 Series II SC engine? (Page 1/2)
Mike in Sydney DEC 02, 10:51 PM
The subject says it all: Can you use a TH125C with an L67, 3800 Series II SC engine?

I have a opportunity to purchase a low mileage, L67, 3800 Series II SC engine (used Down-under in RWD Holden Commodores) for a reasonable price. (I'm thinking about using it in one of my '86 GTs.) The problem is 460T transmissions are almost non-existent in Australia so if the TH125C will handle the torque and power of the 3800, I may have to buy this engine.

BTW, I don't want to swap over to a manual but if I did, there aren't too many transmissions available because most GM cars using the 3800 are rear wheel drive.

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Mike in Sydney

Dennis LaGrua DEC 03, 11:56 AM
It will bolt up but let me qualify that answer. A 35 yr old automatic transmission is probably well worn and not all that strong. They are really not that strong even in new stock form. The solution is to find a shop that knows how to beef up the transmission with a shift kit and high performance discs but even with a freshly modified TH125H the drawback will be that the 3 speeds won't provide very good mileage. You'd be better off with the 4 speed 4T65eHD that was designed to go with the 3800SC.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, 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 "

Blacktree DEC 03, 12:54 PM
The TH125 will bolt up, but won't live very long. They weren't made to handle that much torque.
Dennis LaGrua DEC 03, 01:26 PM

quote
Originally posted by Blacktree:

The TH125 will bolt up, but won't live very long. They weren't made to handle that much torque.



Stock yes. Modified no. Years back there was a racer by the name of Pat Ciarcia from PA. He ran a drag Fiero with a modified SBC w nitrous. Pat ran a 10.4 sec 1/4 mile with that engine and a modified TH125H. He was originally running the 5 speed but he was blowing them up. His transmission was built by Jims Performance Transmissions in Eaton Rapids Michigan. I believe its now owned by Brian's Performance Transmissions.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, 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 "

Blacktree DEC 03, 01:44 PM
The OP didn't mention anything about having a built transmission.
Honest Don DEC 03, 01:48 PM

quote
Originally posted by Mike in Sydney:

The subject says it all: Can you use a TH125C with an L67, 3800 Series II SC engine?

I have a opportunity to purchase a low mileage, L67, 3800 Series II SC engine (used Down-under in RWD Holden Commodores) for a reasonable price. (I'm thinking about using it in one of my '86 GTs.) The problem is 460T transmissions are almost non-existent in Australia so if the TH125C will handle the torque and power of the 3800, I may have to buy this engine.

BTW, I don't want to swap over to a manual but if I did, there aren't too many transmissions available because most GM cars using the 3800 are rear wheel drive.




How angry will the L67 be?

I have the stock, 120k mile 125C surviving so far behind my turbo 2.8

Some things that are working for me:

Transgo shift kit (easy enough to install "on car" but leave out replacement governor spring)
Use a big transmission cooler
Replace all fluid with Type F

YMMV

Depending on how much power the engine is making and where, the WOT shift points may move higher than you'd like due to converter "slip." On mine, I had to add weld filler for weight and make my own governor springs to bring it back down.





[This message has been edited by Honest Don (edited 12-03-2020).]

Dennis LaGrua DEC 03, 02:37 PM

quote
Originally posted by Blacktree:

The OP didn't mention anything about having a built transmission.



That's correct but the open exchange of ideas and opinions is usually helpful. I run a modified TH125 behind my modified 3.4L turbo. That engine is all out and has numerous full throttle runs on 13 psi boost. Its almost as fast as my 3800SC The trans shifts hard and is absolutely bulletproof. IMO, relaying suggestions based on experience is a good thing.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, 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 "

Mike in Sydney DEC 03, 05:40 PM
Thanks for the feedback, folks.

I'd love to have a 4T60eHD but they aren't generally available in Oz. Commodores with the 3800SC had 4L60eHD transmissions. The Holden Commodores are rear wheel drive vehicles.

If if go this route, the 3800 will be pretty much stock. I'm not planning any changes for more power or additional torque. Just looking for something to raise the fun quotient a bit. I would be taking the opportunity to rebuild and upgrade the TH125. The car I'm thinking of putting this in has 106,000+ miles (~171,000 km.) on it. The engine and transmission are in excellent shape but there's always the chance something could go bad, eventually.

I'm not too worried about fuel economy. Both of the GT's are on club plates and historic vehicle registration so they aren't driven every day. They get around 2-3000 mile per year, max and the trips are usually under 200-miles and max speed limits in NSW on the motorways are ~70 mph (110 kph).

Another question: Would I need to change the flex plate for the TH125 transmission if I go with the 3800?

(edited for spelling)

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Mike in Sydney

[This message has been edited by Mike in Sydney (edited 12-03-2020).]

cam-a-lot DEC 03, 06:59 PM
You can get an HD transmission shipped from North America. It won't be cheap, but will be a lot less hassle than installing the 125 and having it blow up a week later.....

I shipped several skids of Fiero parts to Australia a few years back. The shipping cost was not "that" bad...
Mike in Sydney DEC 03, 07:53 PM
Who did you use to ship? I've got loads of stuff in storage at my house in North Carolina. I've looked at pallet shipping but the costs were not cheap. Almost cheaper to buy another car, load it in a 20-ft container, and then use the excess space for spare parts and other stuff. (did that before.)

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Mike in Sydney