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      02-20-2011, 06:38 AM   #1
GreasyGinzo
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VLSD? Torsen? What?

What kind of diff do these cars have?
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      02-20-2011, 11:49 AM   #2
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Is the M-diff in the e9x any different than the e46?

I believe it's a viscous LSD, but some threads on here give the idea that it's a clutch type.
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      02-20-2011, 12:02 PM   #3
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It's essentially a variable lock up clutch type. It uses clutch packs but a shear pump determines the lockup by the differential in the individual wheel rate.
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      02-20-2011, 12:08 PM   #4
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It's a clutch type differential. It's basically the same concept and design as the E46 M3's differential. Here are two brief articles on it:

http://www.usautoparts.net/bmw/model...ig/m3_diff.htm

http://www.autos.ca/auto-tech/auto-t...ferential-lock
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      02-20-2011, 12:36 PM   #5
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Still viscous coupled, so won't be as durable or likely as effective as a true mechanical clutch type diff.

If the diff is the same as the e46 diff, there are some good threads on the other m3forum regarding this. However, I do believe the e9x has more lock than the e46 lsd?
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      02-20-2011, 01:30 PM   #6
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Quote:
Originally Posted by sensi09 View Post
Still viscous coupled, so won't be as durable or likely as effective as a true mechanical clutch type diff.

If the diff is the same as the e46 diff, there are some good threads on the other m3forum regarding this. However, I do believe the e9x has more lock than the e46 lsd?
It is a variable lock up it can be anywhere from 0 to 100% lockup. A normal clutch type diff cannot do that on the fly.

And unlike a torsen the m diff doesn't determine available lockup based entirely on coefficient of friction. For example if you lift a rear wheel with a torsen the diff is effectively a nomal open diff. The m diff engages lockup based on wheel speed like a clutch type diff under these conditions.

While the diff uses a viscous fluid it operates in a completely different fashion from a vlsd.

Fragility in a racing enviornment (track days aren't racing enviornments) seems to be the m diffs downfall; and I suspect like a viscous diff extreme operating temperatures hamper performance.

But for a street and hpde car it's a very good diff and I doubt you would see a performance improvement going to a torsen or normal clutch type diffs.
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      02-20-2011, 01:37 PM   #7
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On that note, an e-diff (ala F430/458) for the F30 M3 would be a nice addition. Proactive instead of reactive. And no, I'm not talking about a fake lsd that just applies the brakes to one side.
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      02-20-2011, 02:05 PM   #8
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Never said it was the same as a vLSD, but viscous coupled nonetheless.

I agree that for a primarily street driven car, there's really no reason to upgrade. If I were to choose though, I'd take a regular clutch type or torsen over the m-diff. I think wheel lift and thus open diff situation with a torsen/quaife is a non-issue for most. Even if one hits a curb or bump at speed, the car needs to be rather stiffly sprung to lift in most situations.

Off-topic, any opinions on the brake-steer on the mclaren. They didn't even bother with a LSD.
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      02-20-2011, 04:19 PM   #9
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Very complex...
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      02-20-2011, 11:08 PM   #10
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The two attached articles provide a little more information.
Attached Files
File Type: doc E46 M3 GNK New release - Viscodrive differential.doc (31.5 KB, 164 views)
File Type: doc E46 M3 Rear Axle Differential.doc (464.5 KB, 1217 views)
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      02-20-2011, 11:13 PM   #11
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Some additional information.
Attached Images
  
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      02-21-2011, 09:43 AM   #12
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I've taken apart several viscous diffs and the M diff doesn't operate anything like a viscous diff. There is no variable-viscosity fluid which physically provides the lockup on the M diff, the shear pump that is driven by wheel speed differential provides pressure to generate a clamping force to the clutch plates, and the clutch plates provide the lockup. The fact that the shear pump generates pressure based on wheel speed differential does not make it a viscous coupling since the shear fluid by itself has no effect on how the wheels are locked together. On a VLSD the fluid heats up with wheel speed differential, which changes its viscosity and makes it more difficult for the couple-dozen thin metal plates inside the diff to spin. The fluid itself provides the lockup. Completely different operating principle.

You can see why they'd blow up on the track eventually, especially without any additional cooling I bet that shear pump gets pretty hot and there's no way to relieve pressure since building pressure inside it is the whole point

There's a long history of not offering limited slips in fast mid-engine cars, the McLaren doesn't bother me, seems like a good solution to keep drivetrain weight and complexity down since most of the time (once up to speed anyway) the car will have enough traction on the inside wheel to keep it from turning over.
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      02-21-2011, 08:05 PM   #13
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So for dry roads, is there any advantage to the M-diff over a traditional mechanical diff?
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      02-21-2011, 10:15 PM   #14
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Quote:
Originally Posted by Richbot View Post
I've taken apart several viscous diffs and the M diff doesn't operate anything like a viscous diff. There is no variable-viscosity fluid which physically provides the lockup on the M diff, the shear pump that is driven by wheel speed differential provides pressure to generate a clamping force to the clutch plates, and the clutch plates provide the lockup. The fact that the shear pump generates pressure based on wheel speed differential does not make it a viscous coupling since the shear fluid by itself has no effect on how the wheels are locked together. On a VLSD the fluid heats up with wheel speed differential, which changes its viscosity and makes it more difficult for the couple-dozen thin metal plates inside the diff to spin. The fluid itself provides the lockup. Completely different operating principle.

You can see why they'd blow up on the track eventually, especially without any additional cooling I bet that shear pump gets pretty hot and there's no way to relieve pressure since building pressure inside it is the whole point

There's a long history of not offering limited slips in fast mid-engine cars, the McLaren doesn't bother me, seems like a good solution to keep drivetrain weight and complexity down since most of the time (once up to speed anyway) the car will have enough traction on the inside wheel to keep it from turning over.
makes sense to me

not entirely a complex system. If you want complex diffs, try the Evo's/STi's drivetrain
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      02-22-2011, 12:17 PM   #15
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Quote:
Originally Posted by sensi09 View Post
So for dry roads, is there any advantage to the M-diff over a traditional mechanical diff?

disadvantage

It sounds like the M-Diff is a viscous type
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      02-22-2011, 05:46 PM   #16
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Quote:
Originally Posted by sensi09 View Post
Never said it was the same as a vLSD, but viscous coupled nonetheless.
No, I would say it is clutch coupled. The viscous fluid is for sensing wheel speed differential, not for coupling.


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