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Old 03-13-2012, 08:37 AM   #1
BMWDavid
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Torsen limited slip

Not sure if this subject has been talked about, but as you know the BRZ comes with a Torsen LS diff. Should be a much better unit than the typical clutch pack type LS. I'm not fully versed on the operational differences between the two types of Limited slip's, perhaps others with a better knowledge of these two could chime in.

I know the clutch pack types seem to have limited life and require special lubes.
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Old 03-13-2012, 08:52 AM   #2
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http://auto.howstuffworks.com/differential6.htm

My Boss comes with a Torsen differential as well.
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Old 03-13-2012, 09:01 AM   #3
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Originally Posted by BMWDavid View Post
Not sure if this subject has been talked about, but as you know the BRZ comes with a Torsen LS diff. Should be a much better unit than the typical clutch pack type LS. I'm not fully versed on the operational differences between the two types of Limited slip's, perhaps others with a better knowledge of these two could chime in.

I know the clutch pack types seem to have limited life and require special lubes.

if you do not know, then do not make such statements as "should be much better."

to limit slip, a limited slip differential has an internal mechanism that uses pressure in one form or another to force the slipping wheel to keep on turning.

torsen is a self contained unit and needs no replacement, however it CAN wear out if you overpower it. Just like everything else! However, it is non adjustable, so you can't upgrade it to handle more power or change its engagement and disengagement characteristics.

clutch packs also come in many different forms, OEM is one thing, after market clutch packs of 1.0, 1.5 and 2.0 slip variety are an entirely differnt ball game

to say they don't last is to say that the clutch in your car doesnt last, so you should buy an automatic. If you add a turbo kit and do hard launches for days on end, you'll fry your clutch. That's not the clutch's fault, that's your own foolishness.

same goes for clutch type lsd's.
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Old 03-13-2012, 09:07 AM   #4
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A clutchpack type LSD has a faster and more predictable lockup. They do need friction modifiers in the oil, and occasional rebuilding if you race or abuse them. But, they can be adjusted for lockup speed and force, as well as the ability to do so separately for on- or off-throttle behavior. Clutchpack LSDs are preferable for any form of racing.

Torsen (TORque-SENsing) LSDs are simpler in that they have a set lockup speed and force. However, they are dependant on the slipping tire to have SOME grip. If the slipping tire has no grip at all (i.e. it's in the air) the torsen will not lock and will act like an open diff. So they are not the choice for off-road or competition where the car may lift a wheel. However, the nice thing, as stated above, is that they never need a rebuild and can work just fine in straight gear oil.

So while the torsen LSD will work perfectly fine for the vast majority of FT owners, an upgrade to a clutchpack type LSD will be a popular mod for racers looking for 10ths.
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Old 03-13-2012, 09:08 AM   #5
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I tend to agree with you that the torsen units are better than clutch packs. Both have their ups and downs but a properly tuned setup can make great use of a torsen diff.

The primary difference between the two is that at a clutch type differential locks the rear wheels together and delivers an amount of power to the wheel with traction based on the breakaway torque of the clutch pack. In a torsen diff on the other hand, as torque between the two wheels becomes uneven the diff mechanically provides more torque to the wheel with more traction, and thus more potential to use the increased torque delivered to it. How much torque it can transfer to one side is dependant on the torque transfer ratio built into the differential.

The downside of the clutch pack is along the lines of what you said. The clutch plates in the differential are a wear item and typically produce quite a bit of noise. The operation of the differential also changes substantially as the clutch plates wear, meaning unless they are kept in top condition between heavy use (i.e. track days) their operation will become inconsistent. This has to be adjusted for in your chassis setup if you want to preserve optimum performance meaning clutch diffs require a lot of maintanence to remain competitive. They're also reputed to have inconsistent or on-off-like engagement although this is generally only a problem in cheap or poorly tuned clutch diffs. In a decent setup lock up is very smooth and predictable.

Torsen diffs, unlike clutch packs, are virtually silent and require no maintanence making them ideal for OEM applications. The biggest drawback to them is they're generally more expensive than a clutch pack and, like an open differential, if one of the wheels loses contact with the road the diff will route all of the power to that wheel. In other words if you lift a tire you can't put any power at all down until it comes back into contact with the road. Some torsen type diffs now use a preload spring (like clutch diffs do to combat the same problem) to allow the diff to keep some power to the wheel with traction in the event a tire is lifted but I don't know if this is the case with the one in the BRZ. However, with proper chassis and suspension tuning you ideally shouldn't be lifting wheels anyway.

Here is a good link for further reading: http://www.taylor-race.com/pdf/under...ferentials.pdf

Last edited by Corey; 03-13-2012 at 09:36 AM.
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Old 03-13-2012, 09:16 AM   #6
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Originally Posted by MrBonus View Post
http://auto.howstuffworks.com/differential6.htm

My Boss comes with a Torsen differential as well.
Not sure if I mentioned it before, but awesome car choice

If I weren't looking for a DD replacement (and didn't have a track car already) I'd have a Boss LS sitting in the garage.
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Old 03-13-2012, 09:37 AM   #7
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Originally Posted by Dave-ROR View Post
Not sure if I mentioned it before, but awesome car choice

If I weren't looking for a DD replacement (and didn't have a track car already) I'd have a Boss LS sitting in the garage.
Thanks!

It's a large car but I can appreciate its relative mechanical simplicity. Only a few months before track season starts up so I can't wait to get it out there.
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Old 03-13-2012, 10:07 AM   #8
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Originally Posted by Dave-ROR View Post
Not sure if I mentioned it before, but awesome car choice

If I weren't looking for a DD replacement (and didn't have a track car already) I'd have a Boss LS sitting in the garage.
I looked into a Boss Laguna Seca too and it has massive pace and potential, but just my preference, it's too big for me. Nevertheless, a very capable car.


Back to the OT

Different LSD's work best for different applications. In my experience, robust clutch-pack LSD's [i.e. OS Giken] are quickest on the track in a rear axle, however ATB's [i.e. Quaife, Torsen] are quickest on the track in a front axle.

ATB's require zero maintenance and do not change performance over time, whereas clutch-pack LSD's require a friction-modifier gear lube and the discs do wear over time. That said, the rate of disc wear is proportional to usage and individual LSD model.

In one of my MINI's [an '07 S w/factory 20% lock clutch-pack], I fried the discs in the LSD in one day at an HPDE. The diff wasn't durable enough to handle stock torque in a racetrack setting with Hoosiers. In that application, an OS Giken would've been a good clutch-pack choice, but an ATB works even better in front-drive because it distributes torque to the tire with the most grip [mechanically] and pulls the car around the corner. This of course isn't directly applicable to the BRZ, it's just perspective how there are different solutions for different applications.

I for one don't mind the car comes with an ATB. It's not "ideal" for rear drive in terms of lap-times, giving up a few tenths for reliability and consistency, but then again, this car has never been about that. It's also a plus over an open-diff. Lastly, I like the hands-off approach to maintenance.

That's my $0.02
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Old 03-13-2012, 10:55 AM   #9
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Hey guys...thanks for all the reply's. I see that both types have their pro's and con's. I had a clutch type on my '95 E36 M3 and it was toast at about 15,000 miles.

I like that the Torsen basically won't wear out and for street use looks to be a good choice.
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Old 03-13-2012, 11:22 AM   #10
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Hey guys...thanks for all the reply's. I see that both types have their pro's and con's. I had a clutch type on my '95 E36 M3 and it was toast at about 15,000 miles.

I like that the Torsen basically won't wear out and for street use looks to be a good choice.
a Torsen will toast just as much as a clutch pack if you pound on it.

if your OEM M3 diff blew after 15000 miles then maybe it was driver error.
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Old 03-13-2012, 11:30 AM   #11
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Originally Posted by 7thgear View Post
a Torsen will toast just as much as a clutch pack if you pound on it.

if your OEM M3 diff blew after 15000 miles then maybe it was driver error.
Torsen's don't have wear items other than gears. They'll only break if one of your drive wheels leaves the ground and you shock the assembly upon landing [i.e. keeping your foot in it]. Then again, doing that is just as much risk to axles, pigs, trans gears...everything else in the drivetrain. In your situation, the Torsen was the fuse.

Clutch-packs can be destroyed in minutes if they're ill-suited to the application...like the OEM clutch pack in the R56 MINI Cooper S in the example I gave above. They don't "blow", the discs simply slip and turn it into an open-diff.
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Old 03-13-2012, 11:38 AM   #12
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if it's mechanical, it will break
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Old 03-13-2012, 11:38 AM   #13
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I have never heard of a Torsen (or helical as they are often referred) "toasting" in any reasonable conditions, 500hp autocross on R-comps maybe, but in their intended use the units are basically indestructible.

I have a stock S15 helical LSD (torsen, whatever) in my 350whp S13 track slut and it has no issues as long as I keep both rear wheels on the ground.

Every mechanical device has a limit, you won't find the breaking point of the stock LSD portion of the Diff without R-comps or drag slicks. It is not an item of concern and is a god send for this car given how well the units works on the street, power over is predictable and smooth, there is almost no ramp to the torque vectoring, is is unimaginably smooth, even more so than the stock Viscous coupled units.
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Old 03-13-2012, 11:47 AM   #14
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if it's mechanical, it will break
Sure, but it's less likely to break under most track conditions where you aren't shocking the unit, whereas many OEM clutch-type differentials can be fried by something as simple as repeatedly lifting an inside rear tire.
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