Differentials - 3.62 - 3.91 - 4.10

[M3mike]

I currently am sitting with a stock diff and considering going to a 3.91 car is N/A.

Curious whether anyone has done the swap on here and your thoughts.

How is the driving experience/improvement? Butt dyno/fun dyno... Costs? Where can you get it swapped Diffs online?

[spnsprt]

I got a 4.10 installed at Performance Technic in the Bay Area. I have sort of mixed feelings about it. 1st gear is pretty useless and it feels like it cruises at a pretty high RPM now on the freeway. On the other hand, I've taken it to the track twice and the butt dyno thinks it's pretty funny.

[Inizes]

I got a 4.10 installed at Performance Technic in the Bay Area. I have sort of mixed feelings about it. 1st gear is pretty useless and it feels like it cruises at a pretty high RPM now on the freeway. On the other hand, I've taken it to the track twice and the butt dyno thinks it's pretty funny.

1st gear already feels borderline useless on 3.62 gearing, really just meant to get you rolling. I’ve been interested in a swap, but I think 3.91 is the way to go since I have a decent amount of highway driving near me, Midwest sorta sucks.

[kermmmit7]

I have 3.91 and I really like it but you have to rev on 1st gear at least to 4k because is short . It's also still good for highway driving. I never tried 4.10 but I think it's too short for me .

[Apex_80]

Overall I think 3.91 is the sweet spot for this car. It is long enough that most tracks you'll still have plenty of RPM left in 5th gear and it definitely feels a lot peppier around town.

[Obioban]

IMO BMW nailed it with the stock gearing. Perfectly matched to back road bombing or tracking-- both of which have you almost entirely in 3/4, which minimizes the chance of money shifting.

If I could change anything about the stock gearing, it would be to make 6th longer-- but 3.91s or 4.10s do the the opposite, soooo...

[Tbonem3]

Ya gears are a waste. Stick with stock unless your local track is best with lower gearing (elevation change, higher altitude, tighter track, not as high speed) or due to the shifting sequence (if you want to cut out a gear change or add one). You can accelerate at a lower RPM with lower gearing so as to avoid a downshift. But you can also run into the problem of redlining before needing to let off for the next corner.

Nothing is free, everything's a compromise. You pick the compromise for your situation. All else equal, BMW already got that compromise right with the 3.62 Maybe you could argue 3.85 is perfect, IDK.

I hated the 4.10s after a while. And I don't even care about cruising RPMs or MPGs.

[Apex_80]

There is pretty much no argument that on a track that you don't see 6th that 3.91 is not better vs. 3.62. Every second you are in 5th you are higher in the power band. Now gearing in general is highly dependent on what kind of corners you have. I also find that once you have slicks and aero on an e46 2nd gear is just about useless even with stock gearing - so that puts you more into the power band in 3rd on corner exit.

[Obioban]

There is pretty much no argument that on a track that you don't see 6th that 3.91 is not better vs. 3.62. Every second you are in 5th you are higher in the power band. Now gearing in general is highly dependent on what kind of corners you have. I also find that once you have slicks and aero on an e46 2nd gear is just about useless even with stock gearing - so that puts you more into the power band in 3rd on corner exit.

Eh... If you're in 5th gear with 3.91s vs higher RPM 4th gear with 3.62s at the same speed, the 3.62 car is making more power AND has a more aggressive total gear ratio at that speed-- plus the time you're shifting instead of accelerating.

One of the best mods we did to the dedicated race car we owned for a while was move it from the 4.10s it came with to the stock 3.62s.

[Feffman]

I switched from 4:10 in the race car to 3:91 and feel it's a good set up. I simply ran out of gear at longer tracks like COTA, Road America, etc.with the 4:10 but it still comes out of the corners nicely with the 3:91. I also shift a little less which has slightly improved lap times. As noted above the 4:10 made first gear all but useless.

Feff

[usdmej]

i had 3.91s on street tires and it let me redline in 2nd on most auto-x courses
switching to 3.62 and r-comps fortunately resulted in the same outcome

OP there has to be a specific reason you'd want to change your gearing and only you can really answer that

[DoubleSidedTape]

I love my 3.85 gears (e9x m3 manual ring and pinion), but I mostly drive on curvy back roads rather than on the track.

[oceansize]

I went 3.91 and enjoy it on the street. The difference on the street is slight, noticeable but not annoyingly so. I like to experiment on my car so this was just something I had to do. Great time to do a bushing refresh and install ARCAs. I used diffs online, they send you the new one and you return yours as a core and the core charge is steep plus you pay the shipping charges to send back and our diff weighs 90-95 pounds.

[Tbonem3]

Diff weighs 82lbs sans fluid and cover. Weighed 2 back to back both 82lbs. Not to be pedantic, Oceansize, but in case someone is creating a shipping label. With fluid and cover it must be 90?

[bmw m3 s50]

I would have to disagree, I have had 3.91 and swapped to 4.10 diff on my supercharged m3 none the less. I felt it was one of the best mods I ever did to the car. It gets the car into the higher rpms quicker which the s54 needs. You feel the torque multiplication instantly. To all arguments that say 1st gear is useless, I would like to ask when are you actually driving in 1st gear? You use 1st gear to get you out of the hole that’s it or maybe in a parking lot. I feel the 4.10 diff keeps the m3 in the powerband without having the need to downshift as much. The 4.10 diff simply provides the low end grunt that the s54 needs in IMHO.