Questions about 3.91 differential...

[Julien]

Ix cars came with viscous diffs.. unlike LSD varients they act like a regular diff, as in one wheel will spin if it loses traction BUT after three or four rotations of the wheel the viscous substance inside the diff will stiffen and effectively "lock" both wheels together. Great for snow/mud conditions, not so much for drifting.

- Mike: 3,91 came in british 325i sport (with factory mtech kit)

Julien

[Funkmasta]

Some E24 autos had 3.91s... I have a very nice, low mileage 3.91 open if anyone is interested...

Edit: This would be a medium case diff.

Correctomundo... Especially the 88-89 variants.

[Jon325i]

Correct me if I'm wrong here guys - but didn't some E34s come with a 3.91 ratio diff? Several variations on this platform.....M20, M30, M50, S38 & M60 V8s. Slushy & shifty gearboxes too.

Jon

[Funkmasta]

yes, 391 is a common ratio for E34s, but the E24 case is a direct bolt-in. :D

[Jon325i]

yes, 391 is a common ratio for E34s, but the E24 case is a direct bolt-in. :D

Ahhhh....thanks for the clarification Funky

[TDE30]

Correct me if I'm wrong here guys - but didn't some E34s come with a 3.91 ratio diff? Several variations on this platform.....M20, M30, M50, S38 & M60 V8s. Slushy & shifty gearboxes too.

Jon

North American E28 M5's, various E34 M5's (primarily non-North American examples,) and North American E24 M6's came with the 3.91 LSD (all 25% lockup IIRC.)

[T3rd Ferguson]

I miss my 3,64 LSD

[Stu]

11/83-9/84 318i U.S. cars had a 3.91 with limited slip as an option (Typ168 )

[Funkmasta]

North American E28 M5's, various E34 M5's (primarily non-North American examples,) and North American E24 M6's came with the 3.91 LSD (all 25% lockup IIRC.)

Right, but all of those are large case differentials, and will not bolt into an E30 without serious modification.

[Funkmasta]

11/83-9/84 318i U.S. cars had a 3.91 with limited slip as an option (Typ168 )

All small case. :( No bueno for any car that Matty would drive :mrgreen:

[TDE30]

Right, but all of those are large case differentials, and will not bolt into an E30 without serious modification.

Ah, okay, I guess that does make sense. Any advantages to having a larger case LSD?

[Mot]

i wanna 3.46 lsd...

[Funkmasta]

Ah, okay, I guess that does make sense. Any advantages to having a larger case LSD?

Just strength... a medium case diff is plenty strong enough for the weight of an E30, no matter how much power you throw at it.

[Stu]

All small case. :( No bueno for any car that Matty would drive :mrgreen:

Yea, 168mm unfortunately. I didn't know what Matt was intending; just thought I'd point it out. We're on the hunt for a small-case s3.91 for my dads 318is so if you ever get a car with one, lemme know. :mrgreen:

[MR 325]

Thanks for all of the info everyone. Looks like I'll probably just stick with my 3.73, lol. If anybody does come across an E24 3.91 LSD, feel free to let me know!

Oh and Joe, I would definitely break that 318i diff, you're right! Hell, a friend of mine has one in an M20 car and blew it up somehow!