3.91 gear upgrade for 2013SS M6

[Oboy66]

I’m going to start by saying there is no chance that I can do this myself or know someone that I trust enough to do it so I’ll be paying for the job.
I see threads from 2019 where guys were paying $1500 to upgrade to the GM 3.91 gears, this does not seem unreasonable to me but I can’t find anyone near me to do it anywhere close to that number. They’re all saying it’s very labor intensive.
I know I can buy the ring and pinion from Janetty for $425, not sure if should I should get a gear installation kit but that’s $350. Or I can have them build a whole new differential with gears, Eaton TruTrac posi, bearings, shims, seals etc. for $3200.
My question is what do you think the labor difference is between just swapping the entire differential and pulling and rebuilding the existing? I truly have no idea although I like the idea of just swapping and trusting Jannetty to send me a professionally built unit.
I don’t track the car I’m just looking to upgrade the fun factor for my weekend rides.
Hope this kind of makes sense and I appreciate any advice you have to offer,
Tim.

[CamaroCracka]

It takes a few hours to pull the diff out of these cars. I find it a bit more effort than old school sold axle cars, but you can rebuild a lot of solid axle diffs right in the car. In other words, yes, it cost more to get a gear installed on an IRS diff. It is also significantly more labor to R&R and rebuild the diff vs just the R&R. My guess would be at least double the labor, but I'm a hobbyist and don't do it for a living.

The cheapest route is to find a good 3.91 and then sell your 3.45, but these cars are getting old and good low mileage 3.91s are pretty hard to find. I prefer using low mileage factory units because if you buy a diff someone else built and it makes noise, you will be paying the labor all over again.

The effort to put in new bearings etc. during the rebuild is not much more. In fact, many mechanics prefer it because they don't have to use a bearing puller to get the old bearing off the pinion or even the carrier.

If it were me and I was paying to have it done, I would put in new bearings and TruTrac in while I had it apart. That will yield a much stronger 218mm diff, but if you don't beat on it, it's probably overkill.

[Oboy66]

CamaroCracka, thanks for the reply. What you’re saying makes sense to me. Finding a used 3.91 will probably be tough but I’ll look into it over the winter. I talked with someone at JRE and he seemed very honest concerning buying the entire differential was probably the best route for me but I’ll have to save a bit more dough before that happens. Appreciate your time! Tim

[acammer]

I'll add some more input - I've had my diff in and out more times than I'd care to admit - you get pretty good at it after a few times, and if your exhaust comes apart nicely it really doesn't take all that long, less than an hour for sure with experience, so maybe 1-2hrs if you're learning for the firs time. I used to pull JUST the passenger wheel, drop the upper control arm to spindle bolt, pop the axle out of that side, undo the driveshaft, and then lower it down and pop it off the drivers side axle while leaving that all in place.

As far as doing it yourself, you've got the right idea - don't. This rear-end is complicated for a few reasons - it requires a case spreader, and the tolerances are extremely tight. They are honestly tough to get right, and if they aren't perfect, they are noisy, and that noise transfers into the car very easily because of the IRS design with the diff firmly mounted into the subframe.

I also agree with Cracka - if you're going that far, it would be a fools errand not to install a TruTrac - the factory LSD is notoriously weak in terms of it's ability to transfer power to the tire with the most grip, and gets worse as it wears. The differential itself is also a failure point, many a spider gear have been sacrificed in the name of burnouts, drifts, and drag racing shenanigans. The TruTrac solves both problems, it's very tough, and it performs excellent.

I think you should also consider 4.10s over 3.91s. Rather than explain it here, I'm going to give you a link to an article I wrote that's on our site that'll give you a bunch of information to consider. Be happy to chat more with you about it as well, or any questions you've got about getting the rear end built. It looks like you're semi-local, maybe we can even work out a place that might be able to help with the build. Feel free to reach me directly at andrew@gwatneyperformance.com. We can certainly chat here in this thread as well!

https://gwatneyperformance.com/geari...1-manual-cars/

[CamaroCracka]

Never seen a diff taken out without removing both axles, but I have a ZL1 diff so I can't even imagine trying to work that beast back up in there and on to the axle. Do you unbolt the mount points and point the nose down to slide it off the passenger axle?

[joshnmn]

What’s it take to add a tru-trac? Don’t mean to hijack the thread, it applies

[CamaroCracka]

You are replacing the carrier with a Torsen style instead of the factory clutch style. It's essentially a complete rebuild as you would not want to put old bearings and gears on the new carrier.

[acammer]

Originally Posted by CamaroCracka

Never seen a diff taken out without removing both axles, but I have a ZL1 diff so I can't even imagine trying to work that beast back up in there and on to the axle. Do you unbolt the mount points and point the nose down to slide it off the passenger axle?

Necessity is the mother of invention - I broke quite a few of these and got pretty good at doing them. You imagine it correctly - pull the passenger axle, driveshaft, driver front mount bolt, rear mount bolt, grab a strong friend, drop the passenger front mount bolt, begin lowering, and give a little twist to the passenger side to pop out the driver side axle. Keep in mind, it's less than half the weight of the ZL1 rear-end - I can't imagine man-handling one of those like this, but for the SS guys, it's a proven method that works well. Of course you can substitute a transmission jack for the strong friend if you're working alone.

You are replacing the carrier with a Torsen style instead of the factory clutch style. It's essentially a complete rebuild as you would not want to put old bearings and gears on the new carrier.

Agreed - if you could press off the bearings and still have them serviceable (hurdle #1) they are still used bearings. These things are tough enough to keep alive without adding recycled, used bearings to the mix - spend the money and do it right so you hopefully only do it once.

If you're doing a gear change and a TruTrac the work actually gets easier, because you don't need to pull old bearings off the carrier or the pinion, just press on your new ones.

One point that'll trip you up in the future - have a close look at the clips on your axles before reassembly. I have seen these start to get mangled - and chances are you can get it back together like that - but heaven forbid if you ever have to then get that axle out again. Again, been there, done that, still having flashbacks. If they are looking like trouble clean them up or replace them.