Riddle me this: small-case 3.91 LSD diff - what the hell did it come on?

[irish44j]

So my '85 318i has a 3.91 LSD in the small case. It has the tag with code S391 590 and both an "X" and an "S" stamp on the top in white.

All of my research everywhere shows that the only LSD that came in the e30 small case was a 4.10, in the later 318is.

Not that it really matters, but I'm interested in what this diff came off of? I saw one reference to Canadian-spec early e30s getting a 3.91 LSD (would make sense with all the snow), but can't find much confirmation on that.

Is it off of a 325x? I thought those were medium-case though....

Anyone know?

EDIT: Some pics:

[Holland]

I don't know, but that is really cool if it's true.

[irish44j]

just did a quick edit....it has an "X" stamped on the top as well as the "S"

[trackjunkie21]

not 325ix, it wouldn't have an S on the top.

[Pantless Spency]

e21

[irish44j]

e21

I was thinking that, but didn't the e21 have a different backplate/fin arrangement?

this is a photo I found of an e21 3.91 LSD for sale


mine looks like this, with the offset plugs

[TimKninja]

My 85 325E has a LSD in it. I cant make out the tag, the thing is rusted to oblivion. but its still an LSD... car WAS an automatic when I bought it.(Not any more obviously)

I think the PO used my car as his winter beater...might be why it has an LSD. but it doesnt look like its ever been out of the car before...

IDK just chiming in, on an Off car that shouldnt have an LSD but does.

[Pantless Spency]

I was thinking that, but didn't the e21 have a different backplate/fin arrangement?

this is a photo I found of an e21 3.91 LSD for sale


mine looks like this, with the offset plugs

the second one doesn't look out of an e21.... lol wtf else has a small case lsd?

[Kershaw]

not 325ix, it wouldn't have an S on the top.

why not? the rear is a LSD. 3.91 is the rear diff ratio for a late model e30ix.

[irish44j]

the second one doesn't look out of an e21.... lol wtf else has a small case lsd?

only thing I can find is the late e30 318is, but that had a 4.10 LSD.

why not? the rear is a LSD. 3.91 is the rear diff ratio for a late model e30ix.

the ix had a 3.91 LSD, but it is was a medium case



maybe mine came out of the Yugoslavian-market e30 or something wierd :crazy:

[irish44j]

on the side of the case, here is what is on the metal itself, in extruded characters:

1210 421 C

[trackjunkie21]

why not? the rear is a LSD. 3.91 is the rear diff ratio for a late model e30ix.

Was it not a Viscous type differential? and this one is definitely stock equipment from that 318i. In the rustbelt, a decent amount of e30's came with LSD stock.

[Wanganstyle]

I have a 3.91 168mm LSD. it actually had a broken top cap and is in dissection right now.


some euro model e30's had these (168mm 3.91). I'm sure there were LSD options.

some M10 318's also had these (3.91 in 168mm+LSD), I know some had 3.64 in 168mm
one could easily custom build a 3.91 168mm LSD also, just fit the proper LSD/ring and pinon to a 168m case.

[MR 325]

Definitely not an E21 diff, they don't interchange.

Definitely not an iX diff, they had medium case.

Sounds like it's the factory diff from your car to me.

[jlevie]

According to realoem.com, a 3.91 limited slip was used on 318i cars with manual transmissions from 1983-1988 (part number 33101211466).