Gear change TH 125 [Archive] - Forum


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06-22-2005, 05:24 PM
95 Grand AM dual overhead cam L4.
I was wondering if anybody has ever changed the Drive/Driven gears and chain to something other than stock, and if so did the computer give them problems after. I know this car has the OBD 1 computer in it, and as far as know it doesn't control much of anything with the tranny besides converter lock up.
Rite now the teeth are 35/35 and i want to change it to a 33/37 ; this would give me a final drive ratio of 3.18, rite now its 2.84.
Also i would like to change to a higher stall converter. I know some of the other smaller engines stall at 2795 RPMs and as long as the bolt patterns are the same it should be a "bolt on". Rite now the stall i beleive is either 2060 or 2365 RPMs.
If i change the Drive/Driven gears it wont mess up the speed sensor because thats runs off the driven shaft; it will only affect RPM of the engine.
So, what do you people think? Thanks , Tom D.
I am going to post this also in another forum too. :slayer

06-25-2005, 02:00 PM
I've heard of it beeing done. I dont know what happened to the computer or anything like that. If your looking for a higher stall, I think I'd look for a converter from a single cam 2.3L engine. My guess would be around 2500 RPM. I am sorry I cant be much help.

06-25-2005, 11:18 PM
Didn't you post this earlier this week?

06-26-2005, 08:18 AM
looks more like he posted them at about the same time and this one finally got an answer.

btw, where did you get different ratio gears for a th125c? Is the tranny from another vehicle or is there an aftermarket?

06-26-2005, 10:17 AM
Well, as far as I know, GM sells the parts. All you got to do is tell them the teeth that you want on the drive and driven gears and you can get them. They only make like 3 ratios though so your very limited.

07-30-2005, 01:10 PM
well i got it finished!!!! i changed the diff from a 2.84 to a 3.33 and changed the drive and driven to 33/37. plus a changed the TC to a 2795 stall.
it works fine! except that because the govenor is driven off the diff it still thinks it has the 2.84 gear ratio(now it is 3.73) so it will try to down shift all the way to 1st gear at 45 MPH! owned: it does hit the rev limiter! the diff housing still rotates at the same speed as the axles. so now i have to find a way to modify the govenor to shift earlier, maybe heavier weights or a lighter spring if i can find one.
If anybody wants the info on how to go about getting the parts just let me know, i'll be glad to help out!

07-30-2005, 01:21 PM
they make 3 diff ratios: 2.84, 3.06, 3.33.
they make 4 sets of drive/driven ratios:38-32, 37-33, 35-35, 33-37.
i went with 3.33 diff and 33/37 drive/driven for a 3.73 ratio at the axles. :thumbs: :D

07-30-2005, 03:23 PM
I beleive you can change the gearing of the govener shaft, just requires a different gear set. We had to fix that with my buddies car when we swapped rear ends.

07-30-2005, 05:48 PM
I thought about that but i believe you also have to change both gears(drive/driven) so the clearance between them would be correct. the govenor gear rides next to the diff.(theres a ring gear pressed onto the diff.). and the end of the govenor gear has a shaft that runs through it into a pilot hole in the case. so if you only changed the gear on the gov. shaft it would throw off the clearnace between the two gears. Plus the speedo runs off the top of the govenor; this one happens to be electric. so you kind of have to keep the same gearing or if it does change you have to figure out how many points you need on the reluctor for the speedo pick up. its all sooo much fun ;psycho