3.91 Lsd

[jgro1976]

I've done the search thing, with not much luck.

Looking into the 3.91 swap on my '97 M3/4a and doing some browsing, ebay and the like, and wondering if there were more than one 3.91 manufactured. Like, what is a "non-locking" diff? Same as limited slip? This is my first car in 17 years with a diff, so I'm a bit rusty on the subject.

Basically, just need to know what to look for out there, best places to look at/buy from, prices expected, etc.

Thanks!

J

[///M3 CRAZY]

non locking or open is what you want to stay away from. you want an LSD limited slip.

[jgro1976]

Sweet, thanks man.

[Boracay]

I just recently picked up one and I'm waiting for the new diff oil to come so I can change it out before mounting the new diff. Has anyone installed one? DIY's dificulty level?? It's a heavy sucker so I know I'll need a second person to help get it in there.

[DADx2]

Great idea to go with 3.91. I dont know what you searched, but there are loads of threads on the swap. Simply search "3.91" and you will find tons of info. I just did the swap by myself and it took about 3 to 4 hours with me taking my sweet time. To get the unit back in, I just put the diff on a jack and jacked it into place(car was on stands, not a lift). It was a tad difficult, but not too bad. I think I recently saw one for sale in the parts for sale forum. I purchased mine from diffsonline.com for a pretty good price, and I even had them upgrade to a 40% lockup with fresh bearings. They (diffsonline) have great customer service and ship out fast too. Consider them for your 3.91. If you go with them, the diff comes without a housing cover, so you will have to use yours. You will also need a star socket, and a really big allen wrench, both of which I bought from the local big chain auto parts store. Another recommendation I have is to fill the diff with Redline or Royal Purple 75/90 gear oil before you put the unit in. It makes it easier to get the oil in and you have room to tighten the allen plug. PM me if you have any ?'s about the swap. Good Luck

[m3chaser]

diffsonline.com is selling one right now. i did the swap and if you have the right tools then it is not too hard. i actually put the diff in myself cause noone was there to help me and i am only 150lbs myself. i do work out quite a bit so i am not a skinny 150lbs, but the point is that if you absolutely have to you can do it yourself. of course if you can get someone else to help you then that is much better.

oh yea, bimmerdiy.com will help you step by step with the removal and install.

good luck bro.

[Gizmo330iT]

Running a 3.91 LSD too, love it! Its so quick with my 6spd box!!

[BXMPWR]

Hello guys, I Just have a couple of questions. It may sound dumb to some of you, but I don't know too much about it. #1 What diff comes stock in the 98 M3. #2 What is the advantage of replacing the diff with the 3.91 LSD. What does it do, make your acceleration faster etc?:conf

THANKS

[erobinson]

Hello guys, I Just have a couple of questions. It may sound dumb to some of you, but I don't know too much about it. #1 What diff comes stock in the 98 M3. #2 What is the advantage of replacing the diff with the 3.91 LSD. What does it do, make your acceleration faster etc?:conf

THANKS
Stock? 3.23 I think?

LSD means limited slip differential. The idea behind this is that (primarily when cornering) when one wheel begins to lose traction, power is diverted to the wheel with greater traction. A non-locking or open differential will simply spin the wheel without traction, which is no good. Our cars come with an LSD already.

Changing the final drive ratio changes the torque multiplication. Increasing it from 3.23 to 3.91 will yield a fairly decent gain in acceleration. It's the same idea as using a cheater bar on your ratchet to gain better leverage. The downside is that your gears run out more quickly. I don't mean longevity, I mean, 1st gear will end at a slower speed than it did before so you end up shifting earlier (in terms of mph). You also lose top speed. Finally, crusing on the highway will not be so fun because at 65mph you'll probably be around 3500rpm (there are diff calculators online), and gas mileage will go down.

That said, it's a very effective way of gaining acceleration.

[luvdriven540i]

Stock? 3.23 I think?

LSD means limited slip differential. The idea behind this is that (primarily when cornering) when one wheel begins to lose traction, power is diverted to the wheel with greater traction. A non-locking or open differential will simply spin the wheel without traction, which is no good. Our cars come with an LSD already.

Changing the final drive ratio changes the torque multiplication. Increasing it from 3.23 to 3.91 will yield a fairly decent gain in acceleration. It's the same idea as using a cheater bar on your ratchet to gain better leverage. The downside is that your gears run out more quickly. I don't mean longevity, I mean, 1st gear will end at a slower speed than it did before so you end up shifting earlier (in terms of mph). You also lose top speed. Finally, crusing on the highway will not be so fun because at 65mph you'll probably be around 3500rpm (there are diff calculators online), and gas mileage will go down.

That said, it's a very effective way of gaining acceleration.

erobinson, you seem to be learned on the subject, couldn't BronxMpower have his M3 tuned to delete the rev limiter giving the car more of a capacity for higher RPM's per shift, which would translate into more mph per shift, thus having the best of both worlds, sought of speak? Please elaborate, I myself am thinking of swapping the stock 2.81 open diff to a 3.15 quaife for my 98 540/6....thanx.........:alright

[B.Watts]

This is my first car in 17 years with a diff, so I'm a bit rusty on the subject.

Uhh...exactly what kind of car did you drive that didn't have a diff? And, if it didn't have a diff, exactly how did it transfer the power from the driveshaft to the wheels?

[PedroBMW]

i'll have my 3.91 lsd for sale in a few weeks, PM me if ur interested,

to the question about raising ur rev limiter, i suppose this would help, but realistically ur only raising it a few hundred rpm, and your not gonna redline in every gear all the time, I guess it would help your ultimate top speed by a very small amount though, 3.91 will make your car wicked fast off the line, but you really got race through the gears to keep up with the redline coming at you, it gives the car a realy go-kartish feel.

[erobinson]

erobinson, you seem to be learned on the subject, couldn't BronxMpower have his M3 tuned to delete the rev limiter giving the car more of a capacity for higher RPM's per shift, which would translate into more mph per shift, thus having the best of both worlds, sought of speak? Please elaborate, I myself am thinking of swapping the stock 2.81 open diff to a 3.15 quaife for my 98 540/6....thanx.........:alright
PedroBMW answered your question but I'll add a little. Raising the rev limiter can only go so far. I'm not very familiar with the engine in the 540, but it's probably not built to handle much more than 7000 rpm or so (maybe 7400 like the M50/M52?) before you risk internal damage. A better way to get the best of both worlds is changing from a 5-speed to a 6-speed. This way you can maintain the final drive you'd like but have your 6th gear be an overdriven gear (ratio of less than 1) for freeway driving.