Just wondering if its possible to run the stock transmission behind a NA 3800. I know people have made decent power behind 3.4 engines and the tranny has been fine so I'm not worried about power levels.
The car will be used for daily commute and no performance driving. I have to get the car running in two weeks and I have a NA 3800 that I may just swap in. I don't really have time or the money to pick up a 4 spd auto and I have a few 125s. This isn't my car and the 2.8 just quit. I'd be doing it for free as a favor if it would be possible.
I know the bellhousings are the same but will I have any problem with the factory torque converter and the 3800 flexplate? I would be controlling the engine with an OBD1 pcm as I ran my SC3800 in my 84. Also, are there any physical issues with the tailshaft or the crossover pipe perhaps??
I would think the 125 would be fine behind the plain 3800 if it's not romped on hard. But there are those that are making over nicely over 200 HP at the flywheel and that trans holds up. I'd just keep an eye on the fluid for color and scent. Make sure it's not burning up, which I doubt it would if just driven like a sane person.
IP: Logged
09:11 AM
82-T/A [At Work] Member
Posts: 23828 From: Florida USA Registered: Aug 2002
The Th-125 is extremely strong. Many many people push it far beyond it's torque ratings. More often than not, when an old GM car makes it way to the junkyard, about the only thing that DOES still work, is the Th-125 transmission.
I've owned my 87 Fiero SE / V6 for 12 years now. I went through a phase (12 years ago), where I would rev the engine in neutral, and then dump it into drive. Hah... I wasn't aware at the time that what I was doing was called a neutral drop, and that it was bad. Then, a few years of going triple digit speeds to the point where the car simply didn't have enough power to go any faster (meaning the wind resistance was too great).
I'm a bit less immature now, and the transmission is the best part on the car. I've rebuilt the engine already, still original transmission.
Sounds good guys. Like I said earlier, not looking for anything spectacular, just need to get the car running. It seems that the 95 bonne ECU will run the engine just fine with this transmission and a few 'tricks' wired in.
IP: Logged
08:50 PM
darkhorizon Member
Posts: 12279 From: Flint Michigan Registered: Jan 2006
Why not just upgrade while you have it apart ? I understand I had a high hp 383 with my custom built 125s, but I had 4 failures in 4000 miles...all while just cruising down the interstate at 75 mph. Started by slipping, and within 50 more miles, burned out completely. From my point of view the only trans worse is a 50s Chevy ' Powerslide' .
IP: Logged
08:41 AM
Dennis LaGrua Member
Posts: 15350 From: Hillsborough, NJ U.S.A. Registered: May 2000
I must respectfully disagree with Roger in that a quality TH-125H build will take a considerable amount of power.My modded TH-125H is bulletproof having survived many WOT runs with my 3.4L turbo engine. A few years back there was a race Fiero owner that ran a 10 sec. 1/4 mile with one in a drag only Fiero. He was one of Archies customers. He raced for a whole season using a very high HP smallblock and the TH-125 without a failure. He was blowing apart Getrags before he switched. Although the 3 speed box has shown its durability, you are limited in RPM range vs. FDR vs mileage. I have the 3:33 FDR trans and I'm at 3200 RPM at 65MPH or there about. As a result I've never gotten better than 20 MPG on my turbo Fiero. True my engine has an agressive cam but the TH-125 without the forth gear does affect mileage. You can definitely use it behind a 3800SC but you will need to choose the FDR for mileage or acceleration. You won't have both with this box.
------------------ 87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles 87GT - 3.4L Turbocharged engine, modified TH125H " I'M ON THE LOOSE WITHOUT THE JUICE "
[This message has been edited by Dennis LaGrua (edited 08-02-2008).]
I just needed to know IF it would work. I'm sure the girl I'm doing the swap for won't care or know the difference and as long as it runs she/we will be happy. The car is her daily driver and just needs it running again. I am putting the engine together on a spare cradle and just swapping them out. Once this is done, she can save up money for the appropriate 4spd and we can take the time to make all the necessary adjustments at that time. I am hoping for less than 1 week downtime once i start.
IP: Logged
11:06 AM
PFF
System Bot
Dennis LaGrua Member
Posts: 15350 From: Hillsborough, NJ U.S.A. Registered: May 2000
I just needed to know IF it would work. I'm sure the girl I'm doing the swap for won't care or know the difference and as long as it runs she/we will be happy. The car is her daily driver and just needs it running again. I am putting the engine together on a spare cradle and just swapping them out. Once this is done, she can save up money for the appropriate 4spd and we can take the time to make all the necessary adjustments at that time. I am hoping for less than 1 week downtime once i start.
May I suggest that you then go for the TH-125H with the 2.76 FDR and use the 1800 RPM stall speed torque converter You can tell the FDR by the color of the gear in the trans speedometer sender, I can look this up for you if you don't have the info. I also have the codes for the Torque Converters
------------------ 87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles 87GT - 3.4L Turbocharged engine, modified TH125H " I'M ON THE LOOSE WITHOUT THE JUICE "
IP: Logged
01:59 PM
Aug 3rd, 2008
Mark A. Klein Member
Posts: 608 From: Pleasantville IA Registered: Aug 2002
