WTB: 4.10 or 3.91 LSD

[Bimma360]

Going to be ordering a 3.62 Diff from Diffsonline for the M3, probably sometime in the next month or so.

If someone local(ish) has a 4.10 or 3.91 LSD that they are regretting, I may be interested. Looking for something that's been rebuilt with low mileage. Willing to spend about $2000-$2500 on this, otherwise the stock rebuild from diffsonline makes more sense to me.

Thanks!

[LSB4Me]

Good luck with the search.

I would rethink the stock rebuild. 3.91s are barely noticeable in the best way possible.

[oceansize]

I really like my 3.91 as well, just the slightest bit more grunt down low. Nothing over the top but noticeable enough that when I bought my most recent slick top M3 I'm moving it over to that car.

[Bimma360]

Thanks for the feedback guys. It helps. I would have figured go big (4.10) since its on an SMG car.

[maupineda]

I have a 4.1 and there are a few things to consider.

It is indeed a super nice mod when you take it for a spirited drive, it is wonderful, the extra punch is very exciting indeed. However, as hard as I tried and as careful and anal I was with the setup, there is a small whine in the 50 to 62 mph range that at least for me is somewhat of a disappointment. I did it three times to the point I made minor adjustments.

i tried 0.05mm thicker shim, 0.1mm thicker shim and factory. Best was factory, yielding the best pattern, but even then there is that whine.

Dan from diffsonline said the later sets from BMW are not consistent and some whine more than others, the best is to find a black setup, those seem to be original equipment in some models. Whether this is reality vs myth, who knows. Outside that range is noise free, to up to 120mph that I have tested. But 50-60 is the speed I am very often when cursing on urban sections of interstate highways.

the gearing itself is fine, but is more like a motorbike and you need to go to the next gear sooner. This is something you will either love (which I do) or not.

driving on winding roads is an absolute BLAST!!!

[Cubieman]

My 4.10's don't whine luckily, I am not sure who did the work on the diff as the PO installed it.

I bought a stock diff to replace the 4.10's and I find around town, where I do 90% of my driving, its just not really enjoyable for me personally.

I am a parts hoarder and intend to keep the 4.10 unit at least until I decide whether or not I like stock gearing better, so this is just another opinion I guess.

[LSB4Me]

My 4.10's don't whine luckily, I am not sure who did the work on the diff as the PO installed it.

I bought a stock diff to replace the 4.10's and I find around town, where I do 90% of my driving, its just not really enjoyable for me personally.

I am a parts hoarder and intend to keep the 4.10 unit at least until I decide whether or not I like stock gearing better, so this is just another opinion I guess.

FWIW OP, I read a lot of reviews like this when researching which diff to build. Stock gearing was too lazy IMO, but I didn’t want something frantic.

In the end, the 3.91s were the right call for me. A high revving motor should want to rev more than it did as stock. It does that now.

The decision seems to depend greatly on your comparable vehicles and experiences. I have a car that sits around 4200 at 70mph on the freeway, so the e46 remains my comfortable errand runner. Comparatively, as mentioned.

[Bimma360]

My 4.10's don't whine luckily, I am not sure who did the work on the diff as the PO installed it.

I bought a stock diff to replace the 4.10's and I find around town, where I do 90% of my driving, its just not really enjoyable for me personally.

I am a parts hoarder and intend to keep the 4.10 unit at least until I decide whether or not I like stock gearing better, so this is just another opinion I guess.

Welp if you potentially want to sell it, sometime in the near future, keep me in mind.

FWIW OP, I read a lot of reviews like this when researching which diff to build. Stock gearing was too lazy IMO, but I didn’t want something frantic.

In the end, the 3.91s were the right call for me. A high revving motor should want to rev more than it did as stock. It does that now.

The decision seems to depend greatly on your comparable vehicles and experiences. I have a car that sits around 4200 at 70mph on the freeway, so the e46 remains my comfortable errand runner. Comparatively, as mentioned.

I appreciate all of the input/perspectives from everyone. I am a bit torn about the whole thing honestly, haha. I had a 4.1 in my e30 325i back in the day (stock is 3.73). Loved it. It was a very fun car. But it did get old as a manual car. If I was driving more relaxed, I'd shift through 3rd very quickly. In an SMG car, I like doing a lot of shifting so I don't think it would bother me in the same respect. It would probably add to the experience. And I love the idea to have the opportunity to redline it more often.

Having said that, the 3.91 sounds like a very nice (less is more) solution. I would probably do that if I were to just order one from diffsonline. The crux of the matter is though, a 3.91 or 4.1 from diffs online is going to be about $3400 delivered, whereas a stock build from them would be about $1800. I am also going to do new axels and rear bearing while I am in there, which will add another $2500 to all of this. So what I am unclear on is if any of this is worth the difference between $4200 and $6000.

Which really brings me to my price range in this thread. I'd be willing to split the difference knowing that I could ultimately rebuild my stock diff and just sell the 3.91/4.1 to get my money back.

[elrichmeister]

I just went through a 3.91 rebuild upgrade. I was referred to a local offroad shop that builds diffs, they let me source parts for the 3.91 upgrade and went with Peakyparts on IG from Portugal which got me 3.91 Ring and pinion plus all shims and a rebuild kit. Only thing was the clutch disks for OE LSD, couldnt find too much info on sources new ones. I opted to upgrade to a Wavetrac LSD recommended by the offroad shop and went with that.

After the job was complete, my axles on both sides started flinging a bunch of grease from outer cv joints. Ended up just buying new genuine BMW axles, just to get rear end complete didnt want to deal with rebuild.

If i was to do it again, i would just go through Diffsonline and get the complete unit for easy swap in.

[Bimma360]

I just went through a 3.91 rebuild upgrade. I was referred to a local offroad shop that builds diffs, they let me source parts for the 3.91 upgrade and went with Peakyparts on IG from Portugal which got me 3.91 Ring and pinion plus all shims and a rebuild kit. Only thing was the clutch disks for OE LSD, couldnt find too much info on sources new ones. I opted to upgrade to a Wavetrac LSD recommended by the offroad shop and went with that.

After the job was complete, my axles on both sides started flinging a bunch of grease from outer cv joints. Ended up just buying new genuine BMW axles, just to get rear end complete didnt want to deal with rebuild.

If i was to do it again, i would just go through Diffsonline and get the complete unit for easy swap in.

That's my thinking too. I just want to bolt the stuff up and move on.