Our diff is not a Viscous Clutch Limited Slip

[Arith2]

So throughout the years, many bash on our diff for being a viscous clutch when it is simply not. It doesn't matter how knowledgeable someone is on differentials, wrong information is wrong information. Here is the right information.

There is actually a piston that can create up to 38 bar on speed difference between wheels. This is not due to fluid warming up causing clearances to tighten which is how VCLD works. It has nothing to do with temperature or fluid in the clutches. There is a shear pump simply activated from wheel spin difference. The system holds about 46 grams of fluid. The chart shows it activating at a 2 rpm difference at a force of 180nm and at 5 it is a little above 200. The line is linear from 2 to 50 difference of rpm.

I can't transfer the chart but the slope shows about a 90nm increase every 5 rpms.

Now some speculation begins. I found some info on silicone based fluid viscous clutch diffs solely for the fluid and there's a general consensus with other manufacturers that the fluid starts to go bad around 60k miles when used in a VCLD. The pump is a sealed unit and can't be service. The housing must be replaced. That's still a huge downside. The fluid is still not subjected to the same heat and forces as fluid in a VCLD so it probably lasts much longer. Age is also a factor with our cars getting old. Who knows the last time a new carrier for our cars was manufactured by GKN... so that's really the major downside. If the fluid thins out, there is less pressure and less locking. Ultimately, many of us have diffs the still lock well 15 years later so the design is pretty darn good for road use. New, I'd argue that it's also great for the track, however, none of them are new.

I need to verify but I think they may have used the design up until the electronic diffs.

[Albino09]

Some pictures would help if you're able to share them. The problem is that differentials are named by the nature of their method of transferring energy. If our diffs are transmitting energy through fluid friction only, it still sounds like a VLSD in practice. To me, it sounds like you are saying the "M-Variable" differential means they have used a VLSD and applied a pump to it which drives the amount of lock (via fluid pressure rather than purely through temperature). If that is what you are saying, it is still a viscous differential, even if it is slightly more advanced than the typical unregulated VLSD.

[Nuclear Rabbit]

My '05 came with a technology brochure which had some kind of graph on how the lockup rate of the LSD worked.(It had some interesting engineering chart I haven't seen anywhere else / online) I can't seem to find it anywhere on the internet. (I don't have the car any more otherwise I would post it).

[Arith2]

My '05 came with a technology brochure which had some kind of graph on how the lockup rate of the LSD worked.(It had some interesting engineering chart I haven't seen anywhere else / online) I can't seem to find it anywhere on the internet. (I don't have the car any more otherwise I would post it).

It's not public information. It's in the functional description of the E46 M3. There's a whole document on it and this is part of it.

[Arith2]

Some pictures would help if you're able to share them. The problem is that differentials are named by the nature of their method of transferring energy. If our diffs are transmitting energy through fluid friction only, it still sounds like a VLSD in practice. To me, it sounds like you are saying the "M-Variable" differential means they have used a VLSD and applied a pump to it which drives the amount of lock (via fluid pressure rather than purely through temperature). If that is what you are saying, it is still a viscous differential, even if it is slightly more advanced than the typical unregulated VLSD.

It'll still have to the characteristics because the clutches are wet and you can't escape physics but the pump is the main mode of operation. It's a cool design in my opinion. Obviously, it was not designed for a track car. Once again, it's that compromise between drivability and performance.

[thegenius46m]

It's viscous by definition. The clutches are indeed activated by fluid pressure via a pump. Regardless its a garbage design and a huge downgrade from E36 M3 and E39 M5 mechanical clutch based diffs of prior.

[Albino09]

It'll still have to the characteristics because the clutches are wet and you can't escape physics but the pump is the main mode of operation. It's a cool design in my opinion. Obviously, it was not designed for a track car. Once again, it's that compromise between drivability and performance.

Basically, it is better than what you might find in the back of a 370Z which severely suffers from overheating un-pressurized differential fluid, but it is not as good as a ramped clutch-type or torsen/geared LSD.

[Arith2]

Basically, it is better than what you might find in the back of a 370Z which severely suffers from overheating un-pressurized differential fluid, but it is not as good as a ramped clutch-type or torsen/geared LSD.

I agree the design is definitely below mechanical designs. Most mechanical designs are just the most reliable in practice. You can't beat mechanically simple consistency. For street use, the M variable just suits the majority, not someone like yourself tracking the car.