391 gears

[robbiegordon]

Originally Posted by esemjay56

I don't know if anyone else had this issue, but the programmer I bought did not include 3.91 gears as an option to calibrate the speedo... I think 3.73's and 4.11's were the choices.

my calibrator is the same. i changed the tire size to get it where it needed to be with the 3.91. i emailed the company and they said they may be updating to add 3.91 as an option .. that was over a year a go... never heard if they did it or not.

[steakmaker]

Originally Posted by cc-rider

You've got a few options for the 3.91...

Buy a complete 1LE diff (look in the classifieds here...or ebay)
Buy a 3.91 gear set and have it installed (cheapest option)
Buy a 3.91 gear set and Eaton Trutrac and have it installed

If you plan to go with big power build (500+ rwhp)...you should opt for the Trutrac.

And lastly...if you really want a robust rear end...you can go for a ZL1 differiential swap. You need the diff, driveshaft, and axles. This is extremely robust and can handle big power. But you've only got the gear choices of 3.23 or 3.73.

I bought a GM 391 kit,came with everything,including the diff additive.My installer had to call GM(number provided in kit) for a program to alter the MPH.The kit came with a sticker that stated this car has GM high performance gears installed and GM even told the mechanic where to put the sticker under the hood driver's side in the corner by hinge mount.One of the most satisfying mods,for a M6.

[Misfit-ss]

Does anyone have the GM Part # for 391 Rear End Gears. Looked on-line and could not find it!
Thanks!

[blakjak55]

GM Part numbers for the 3.91 kits:

2010-2013 - 19301504
2014-2015 - 19329768

5th Gen SS and 1LE comes with 8.6in/218mm ring gear in 3.23 (3.27?) for auto SS, 3.45 for manual SS, 3.91 for 1LE. http://www.demandaam.com/resources/p...0130101924.pdf

5th Gen ZL1 comes with a 9.8in/250mm ring gear in 3.91. http://www.demandaam.com/resources/p...0130102643.pdf

They were all made by American Axle and Machine (AAM). I don't know if they still make them for GM factory production or not.

[Misfit-ss]

Thanks!!!!!

[arussh]

i installed 3.91 a few years back. Bought the GM kit and had dealer do install included transmission reprogramme. Nice performance difference on street not too big of a penality gas wise on highway. A good bang for the buck.

[Misfit-ss]

I am going to pull the trigger and install the 391s. Will be ready to play by spring.
Thanks again!

[SSyclone]

I see your car is a manual, let me say from experience the 3.91s are incredible. You will love your car all over again. Unless of course you throw a supercharger on it, then it's a handful. But that's another story.

Jegs sells the actual GM gears in a kit with everything GM had in it plus marking dye, lube, and posi additive. Ring gear has left hand threads....lol

[Pro Stock John]

You can swap just the gears and re-use your carrier and axles. You should buy new carrier bearings.

[bses48]

need how to install gears in rear end and specs

[sixty9fordkiller]

Originally Posted by bses48

need how to install gears in rear end and specs

If you have never worked on a differential before, i would not recommend it be your first time on a car whose differential is currently operating properly. From bearing races, to crush sleeves, shims, and case spreaders, to the shimming of the carrier for proper pinion to ring mating/clearances, there are a boat load of things you can do incorrectly due to lack of inexperience that can permanently damage multiple parts of the unit and ultimately make your investment worthless. Best thing to do is to take it to a differential shop or have someone that has experience supervise you through the rebuild/installation procedure. A step by step "how to" guide will not make up for having someone with years of experience guide you through the process.

Even if your a regular home mechanic that does their own brakes, cylinder heads, suspension, and things of the sort that most of us do, dont get into tearing down and rebuilding a transmission or differential if you've never done it before. Its not just about doing the reverse of the teardown. Clearences and tolerances are very delicate and require more experience to achieve correctly as compared to a simple crankshaft endplay check.


Ive built a few motors in my lifetime, (2) TH-350 trannys and (1) 700R-4. After building that 700R-4, i decided i wouldnt get into transmissions and gears. I just leave it up to the tranny/differential guys to deal with that. Too much shit. Dont want any part of that

[mydreamcar]

Originally Posted by Misfit-ss

I am going to pull the trigger and install the 391s. Will be ready to play by spring.
Thanks again!

You won't regret it. Of all the work I have done on my car or had done, the rear gear up grade with the eaton upgrade was by far the best investment I have made. The car finally feels like a 400plus hp car.