Differentials - 3.62 - 3.91 - 4.10

[bigjae46]

More accurately, it's sometimes faster sometimes slower, depending on the speeds possible in a given area-- both in the real world and from numbers. If you have to shift into a higher gear because you're running a more aggressive diff, you have a less aggressive net gear ratio and lose time to shifting. If a 3.62 and 4.10 car end up in the same gear in a given area, the 4.10 car will have no extra shifts required and a more aggressive net drive ratio.

I think the extra 1 or 2 shifts hurt less than the extra acceleration out of every corner.

The other thing is if, today, you do 100 mph on a straight which requires an up shift with a 4.10 and 275/35R18s. You can stay in 3rd with a 3.62.

A couple of events later...you gain 1 mph of exit speed down the same straight. You’re going to have to upshift whether it’s a 4.10 or a 3.62 once you get over 101mph.

The point is if you make hearing calcs based on today’s data, they are likely invalid after some improvement.

I’ll take having more power out of every corner with a 3.91 or 4.10.

Street driven car? Stick with the 3.62.


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[Obioban]

no

I think the extra 1 or 2 shifts hurt less than the extra acceleration out of every corner.

The other thing is if, today, you do 100 mph on a straight which requires an up shift with a 4.10 and 275/35R18s. You can stay in 3rd with a 3.62.

A couple of events later...you gain 1 mph of exit speed down the same straight. You’re going to have to upshift whether it’s a 4.10 or a 3.62 once you get over 101mph.

The point is if you make hearing calcs based on today’s data, they are likely invalid after some improvement.

I’ll take having more power out of every corner with a 3.91 or 4.10.

Street driven car? Stick with the 3.62.


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It’s not more power out of every corner. E.g. If the exit speed necessitates 4th with 4.10s but allows 3rd with the 3.62s, the 3.62 car will have more power coming out of the corner.

And, on average, including as speed goes up, the 4.10s always require more shifting.

[rbg]

driving 3.91 for a couple of years now. What I miss the most is endless 3rd gear :-)

[BMWahba]

I installed a 3.91 a few months ago. I only have headers+tune+19" wheels. It woke the car up and it's definitely more fun/lively on the street -however- my car is SMG so if i'm being completely honest, it's just a bit of a band-aid for not being a manual...

That being said I like it but I don't love it. No regrets trying it but it won't stay in my car either. It's not a huge compromise in the grand scheme other than highway cruising, so some people will be ok with that but it matters to me. Cannot imagine going 4.10 i'd hate it. The intangible effect of the car being a bit less relaxed at 85-90mph and more E36-like feels like a downgrade IMO. It definitely traded away some signature S54 character for how long you can run the gears out.

Not a terrible mod but i'd wholeheartedly insist putting the car on a 250lb diet instead is a better decision.

Good feedback - I feel the notion about it making it feel more like an E36 is the exact opposite effect of what I would want. I do enjoy that endless 3rd gear.

[maupineda]

I am surprised to see how most prefer a longer gearing car, shorter gearing makes the car quicker, and peppier enhances the experience, who cares about lap times, that is what got the industry in a down spiral chasing high horsepower and lightning-fast DCT's diluting the experience, why we want 6 speeds if you can't go past 3rd at full song, at that point all you need is a 4spd tranny :P . Just look at 993 Porsches gearing, it was perfect, look at 911's of today, you can only use 2 gears before asking for jail, where is the fun in that? I'd rather use 3-4 gear ratios enjoying a winding road than staying in one gear forever.

the gearbox ratios are indeed spot on, but I do feel the 3rd and 4th gears are unnecessarily long due to the final ratio.

[Obioban]

I am surprised to see how most prefer a longer gearing car, shorter gearing makes the car quicker, and peppier enhances the experience, who cares about lap times, that is what got the industry in a down spiral chasing high horsepower and lightning-fast DCT's diluting the experience, why we want 6 speeds if you can't go past 3rd at full song, at that point all you need is a 4spd tranny :P . Just look at 993 Porsches gearing, it was perfect, look at 911's of today, you can only use 2 gears before asking for jail, where is the fun in that? I'd rather use 3-4 gear ratios enjoying a winding road than staying in one gear forever.

the gearbox ratios are indeed spot on, but I do feel the 3rd and 4th gears are unnecessarily long due to the final ratio.

To be clear, the reason I dislike shorter gearing is it ruins the car on the highway.

Regarding lap times, just having the discussion because it was brought up-- and I don't think it's actually faster, on average (some tracks faster, some tracks slower).

[maupineda]

To be clear, the reason I dislike shorter gearing is it ruins the car on the highway.

Regarding lap times, just having the discussion because it was brought up-- and I don't think it's actually faster, on average (some tracks faster, some tracks slower).

This is in my view a conflicting dislike, people considering a shorter diff also tend to modify other things that make the car more feel-some, stiffer suspension, stiffer mounts, etc. A 4.1 make the car rev 10% higher at 75mph, that is only 300rpm, how can that be is so bad when probably the car is already plagued with feedback from the other mods, 300rpm on an engine that redlines at 8k are pocket change.

[oceansize]

I spend zero time driving my car on highways (where is the fun in that?) or the track (vert). It is 100% a country/mountain road when the sun is shining vehicle so the 3.91 for me is really fun. I won't be going back to stock.

[Obioban]

This is in my view a conflicting dislike, people considering a shorter diff also tend to modify other things that make the car more feel-some, stiffer suspension, stiffer mounts, etc. A 4.1 make the car rev 10% higher at 75mph, that is only 300rpm, how can that be is so bad when probably the car is already plagued with feedback from the other mods, 300rpm on an engine that redlines at 8k are pocket change.

I think 6th gear is already too short, stock, so 10% more RPMs is seriously annoying to me. And, my average cruising speed on the highway is generally 100-120, not 75, so we're really talking about 4000-5000rpm with stock gearing, vs 4400-5500rpm with 4.10s.

I don't think I'm inconsistent in my NHV desires-- my drivetrain mounts are 100% stock, I have a full interior, my suspension isn't stiff compared to... almost anybody with aftermarket suspension, I refuse to run a light weight flywheel, I have a quieter than stock muffler, any parts that make noise are immediately removed from the car, etc.

I spend zero time driving my car on highways (where is the fun in that?) or the track (vert). It is 100% a country/mountain road when the sun is shining vehicle so the 3.91 for me is really fun. I won't be going back to stock.

Often one must drive to or from the good driving.

I also think the car is amazingly perfectly geared for back road bombing/mountain roads, stock. 3rd and 4th are the correct gears 99.9% of the time, so no need to worry about missing (or money shifting)-- just push straight forward and pull straight back. That's also part of the reason I think they're perfectly set up for track-- almost never is 2nd required (don't think I've used 2nd on track in the last decade) and very few tracks require you to get into 5th. Going between 3 and 4 are worry free, easy shifts, with a low chance of blowing up your engine.

[oceansize]

Often one must drive to or from the good driving.

True. I forget sometimes that I really do live out in the sticks and I can be country/mountain road driving by walking out my front door. LoL.

[maupineda]

... my average cruising speed on the highway is generally 100-120...

Man, that is brave, I do fear being caught. I am from Mexico, and I would not care about speed limits there, however, even our tollways are not "safe" for more than 100!!! , but as I am in the states and my car is driven here, I rarely can do +100 stints, so for me, 6th is looong, at 75 I do 3000rpm, and at that engine speed my CSL is barely making its magic present.

One thing that is consistent throughout the thread, the gearing is a question only one can answer based on how we use the car and our preferences.

[bigjae46]

My car is track only.

Street - stay with stock gearing unless you’re willing to deal with higher revs in the highway.

Track - If I can put more power down coming out of a corner which gives me 1-2mph more exit speed. That multiplies as you go down the straight.

Even if I top out in 3rd and bounce off the limiter for a bit before the next braking zone...I’m getting to X mph faster.

I’ll post the data plots. The advantage the 4.10 has is not insignificant. I never really thought a 4.10 made much of a difference until I drove it on the track. It’s comparable to when I installed the BW exhaust. Probably good for 1-2 seconds a lap on a 1:47-1:50 lap.


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[heinzboehmer]

I do fear being caught.

Eh it depends on where you are in the US. When I lived in Colorado and drove between Boulder and Denver, traffic would move at around 90 mph (during weekends). If you stuck with the faster moving cars, you could easily be going at 100+. I've found Bay Area highways to move between 70-80 mph now and at about 0 mph pre-covid.

The only place I would even think about going past ~70 mph in Mexico is the new highway between Monterrey and Saltillo. Every other "high speed" road I've driven on is absolutely not suited to higher speeds (I've seen speed bumps on highways with a speed limit of 100 km/h lol).

But yeah, as long as traffic is moving as fast as you are, then it's no big deal.

[BBRTuning]

I've had the 4:10 diff for about 4 years now and absolutely love it. I have 288/280 cams and since almost all the gains with these cams are over 6000 RPM, the 4.10 gears will be much better at keeping the engine in its best powerband. I use my car almost exclusively on canyons and track, and especially in canyons where there's a wide variety of speeds, there's NO doubt having shorter gearing means you'll be able to find the "right" gear more often. There's a reason hill climb and rally cars have extremely close ratio gears.

I find the 3.62 gears WAY too high for most usage personally, although on certain tracks around here there are a few situations where they come in handy, or if you're on the Nurburgring for instance. Of course I wish 6th gear was significantly higher, but I don't mind it too much. I've even driven the car to and from Laguna Seca multiple times (700 mile round trip not including on-track). There's always going to be a lot of subjectivity on this type of thing. It really comes down to what sort of speed ranges you're most often in, and whether or not you enjoy shifting. If you like reading a book or having a coffee between gear changes, then the stock 3.62 is for you.

[old///MFanatic]

*If you like reading a book or having a coffee between gear changes, then the stock 3.62 is for you.

🤣 lol