4.10 LSD now slipping

[kaiser16]

Hey there, my 318is came with a LSD 4.10 and I tested it to see if it still works and it is completely useless. I have the correct diff fluid in it with the friction modifiing additives but it seems that the viscous couplings are worn out. I was wondering if anybody tried to just fix the innards of the diff or if someone has ever tried to swap it with a torque sensitive diff like they have in miatas. Has this happened to anyone else? I have 280 000km

[DesktopDave]

The LSD pack is sealed IIRC.  It's filled at the factory.  The gear oil is just to lube & cool the gears in BMW diffs.  Your best results might be from a rebuilder, usually a clutch pack refresh goes for about $250 or so.  Take that opportunity to change the diff ratio or the stock 25% lockup rate.

Alternatively, you could use a Z3 3.0 torsen LS medium case diff.  IIRC it was an option on only the six cylinders from 96-99...just like the Quaife or KAAZ units.  You'd need a rear cover from an e30 medium case diff, but other than that I believe the swap is very straightforward.  My medium case 3.73 LSD went into my e30 with very few issues.  Come to think of it, my LSD has never worked all that well either.

BTW, I'm told that all small-case diffs have the same LSD clutch pack.  If you know how to set pre-load on the diff nose nut you can grab one from many four-cyl BMWs.

[gearheadE30]

They do tend to wear after awhile-my LSD 3.73 can't hold any torque from the turbo, really.

Your differential is a clutch plate differential, not a visous diff. That right there could be your issue, if you put viscous-type fluid in it. You can buy clutch plates for the diff as part of a rebuild kit, but I don't recommend doing it yourself unless you know how to set pinion lash. That said, rumor has it that you can also fit an extra plate in the factory stack to increase torque capacity.

the factory 25% is typically plenty, IF it is in good condition. Most are less than 20%. Some E30 M3's (along with e34 M5's, but that is a different story) came with 40% limited slip rears, but those are hard to find, and are not marked in any unique manner.

Z3 Torsen is great, and with the cover will bolt in, as Dave said. The nose of the diff is a little longer, so you will have to loosen the compression fitting on the driveshaft and slide it in slightly.

[harvey2]

Guys, I'm puzzled by your mention of changing the rear cover on the Z3 Torsen diff.  I've got a stock Z3 torsen diff in my '91 318is and it was a straight bolt-in, with no fit problems.  Why change the rear cover?

[DesktopDave]

Guys, I'm puzzled by your mention of changing the rear cover on the Z3 Torsen diff.  I've got a stock Z3 torsen diff in my '91 318is and it was a straight bolt-in, with no fit problems.  Why change the rear cover?

You're right...I didn't finish before I posted. My kids were needier than usual today...:mad:

I was going to add that e28 and later e24 diffs are medium case as well, but usually have a different rear cover.  Of course, they'll fit right in with the matching size e30/318Ti/Z3M cover.  And I've heard that even e34 diffs, despite the different case, have the same internals.  BMWs are like Legos that way.

[harvey2]

Thanks for clarifying Dave.  I also wanted to mention that the improvement of going from an open diff to the Torsen was, shall we say, "Mind-Blowing!", in autocross competition.  Its also nice to know that a Torsen doesn't wear out cause it has no clutch plates, and it doesn't cause any sudden push or pull of any sort at any time going into, apexing or coming out of a corner.  Very smooth.  

The only thing that it does cause is confusion.  My mechanic had the car up on a hoist and did the classic check for an LSD by spinning one wheel by hand.  That test doesn't work with a Torsen, so he said "hey, you got ripped off, that's not an LSD".  Driving is the real proof, and yes it really is an LSD.

[DesktopDave]

Harv - how does the Torsen do in everyday driving?  Does it help much with snow?

[kaiser16]

Isn't clutch plate and viscous diff the same thing

When one wheel slips a lot more then the other the clutch plates create friction and the silicone based fluid expands when the temperature rises and presses the clutches together. The diff then acts like a set of planetary gears and transfers torque to the opposite wheel. This system is only good for a certain amount of torque and then it starts to slip again, that's why when you have a turbo your diff will act like a open diff.
And where did you guys see rebuild kits, I can't seem to find any.

[harvey2]

Harv - how does the Torsen do in everyday driving?  Does it help much with snow?

Sorry, Dave but I couldn't say.  This car isn't a daily driver so It doesn't go out in the snow, and I'm pretty easy going on the street especially in the rain so it never really gets pushed enough to notice.

[DesktopDave]

Isn't clutch plate and viscous diff the same thing?

I'm no expert, so take all this with a grain of salt, but they aren't.  A viscous diff should always be labelled as a VLSD.  It works a lot like an automatic transmission's torque converter.

The BMW (ZF?) design works more like a transmission clutch pack.  It uses a shaft, cut to look like a wedge, connected to the spider gears.  If one output shaft starts turning at a different speed, that wedge drives two plates apart against strong springs, engaging a little clutch pack and locking the diff.  They're normally more durable than the viscous type.

The Torsen uses cleverly designed gears and friction to send torque to the wheel that has traction.

There are also electronically activated LSDs and the various types of differential braking traction control systems.

[bflan2001]

I'd also like to know where to find these rebuild kits. I've found all the individual parts for sale but its a lot more than 250