Article about our LSD (GKN's Visco-Lok)

[Rassilon]

https://www.pistonheads.com/features...rentials/25275

Most relevant part:

PistonHeads:
What is the advantage of GKN's ETM [electronic torque manager] diff on the new M5 and M6 over the old viscous coupling system [Visco Lok]? How will drivers feel the difference?

Heinrich:
Visco Lok was purely passive. For BMW test drivers it was OK, but for the average M customer it will be much easier to drive the new M5 on the limit than with the older system. It's easier to accelerate out of bends closer to the limit, because the torque is more controllable. For example, with the Visco Lok if you step hard on the throttle, you can also the spin the outside wheel too much and that can bring it into a drift. If you don't want a drift you have to control it somewhere in between.
We can do that because we are looking at steering angle, what gear the automatic box is in, we have signals from the Canbus telling us how much torque the engine is currently delivering, we can look at the throttle position to see how much power the driver is requesting.

PistonHeads:
So what about drifting? Is this going to help or hamper those who want to?

Heinrich:
When BMW approached us before Visco Lok it said do you have a limited-slip differential that allows the driver who wants to to keep the vehicle in a constant drift on a circle. For drifting you need the differential almost closed, like the two wheels are connected to each other. Visco Lok is a speed-sensing system, so you need some slipping on the inner wheel before it can cut in. There's a delay. When you step on the throttle you feel that the inner wheel is spinning and the locking torque is building up. Before that there's a slight ramp in the torque in the outside wheel. And that reduces the side forces on this wheel relatively quickly.

With an electronically controlled system you can be pre-emptive. You know what the driver is doing, see what the engine torque is, see how fast they are applying the throttle. That means you can close the LSD even before the inner wheel is slipping. And then get it almost controlled by the throttle into a constant drift immediately.

The guys who were used to it could deal with Visco Lok. I personally am a reasonable driver, but rear-wheel drive with 500hp is not my special area, so when I drove the old M5 accelerating out of a bend on the handling track with Visco Lok I was almost afraid to push the throttle, the transition from neutral into oversteer is quite quick. With the new one the transition from one situation to another was softer and I felt much safer.

[bigjae1976]

The M3 visco lok of the E46 and E9X is OK. Same complaint - it locks up too slow. You can get pretty sideways but its controllable with throttle. The problem is the car won't want to come out of the drift easily because its always a step behind.

That's why a clutch diff is MUCH better. It works with you whereas the Visco-Lok is kind of doing whatever and you need to react to it.

[Rassilon]

Originally Posted by bigjae1976

The M3 visco lok of the E46 and E9X is OK. Same complaint - it locks up too slow. You can get pretty sideways but its controllable with throttle. The problem is the car won't want to come out of the drift easily because its always a step behind.

That's why a clutch diff is MUCH better. It works with you whereas the Visco-Lok is kind of doing whatever and you need to react to it.

Do you think getting rid of the friction modifiers in diff fluid can help?

[bigjae1976]

Originally Posted by edwardgtxy

Do you think getting rid of the friction modifiers in diff fluid can help?

I think the limitation with the OE unit is the design is just inherently not as responsive. Not sure if running no FM is good for the service life of the diff as well.

[roastbeef]

i'm not very knowledgeable on the subject, but i asked about using fluid with NO FM in a diff thread and it was a big no no for oem lsd life. might feel great for a short while, until it becomes an open diff.

[Richbot]

The factory fill didn't have enough/any FM on some of these. Changing to the FM-doped gear oil was a standard procedure to address customers complaining of low-speed chattering from the diff.

[dparm]

Originally Posted by edwardgtxy

Do you think getting rid of the friction modifiers in diff fluid can help?

I asked this very question on Bob is the Oil Guy. In short, the answer is that no, you don't want to run a non-FM fluid. It is there to control the LSD's response, but having too much is not a good thing.


https://www.bobistheoilguy.com/forum.../against_FM_in

[6ixSpd]

Thanks for the write-up OP. Pretty interesting tech.

Originally Posted by Richbot

The factory fill didn't have enough/any FM on some of these. Changing to the FM-doped gear oil was a standard procedure to address customers complaining of low-speed chattering from the diff.

Yup, it appears BMW now uses P/N 83222282583 "Rearaxle differential oil SAF-XJ +FM Booster" for rear diff service. At least that's what they put in my E92.

[Z K]

The non-FM fluid was OEM fill on the M3. They just added the newer FM fluid due to people complaing of diff noise.

The non-FM fluid is fine if you don't mind some noise. I'm sure the "non-FM" fluid has some FM in it, otherwise it'd have problems - BMW wouldn't spec it as OEM fill. (OEM fill for the Mercedes AMG LSD as well) It's just not as much FM as the newer formulation.

[dparm]

Originally Posted by Z K

The non-FM fluid was OEM fill on the M3. They just added the newer FM fluid due to people complaing of diff noise.

The non-FM fluid is fine if you don't mind some noise. I'm sure the "non-FM" fluid has some FM in it, otherwise it'd have problems - BMW wouldn't spec it as OEM fill. (OEM fill for the Mercedes AMG LSD as well) It's just not as much FM as the newer formulation.

The OEM fill was Castrol SAF-XJ which was already dosed with FM, but obviously, we don't know how much. It has been renamed to Syntrans LS.

It was simply revised for BMW with even more FM to address the customer complaints.