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Old 11-01-2006, 11:22 PM   #7
Patrick Olsen
NASIOC Supporter
Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
1997 Legacy 2.5GT
1996 Impreza coupe


Originally Posted by Unabomber View Post
What is a differential? ... From there, the front and rear differentials distribute the power to their respective wheels.
A little fix for you.

Originally Posted by Unabomber
Open differential: Used as the front and rear differential in the WRX/RS.
As discussed elsewhere (I think you were party to that thread, and now I don't remember where it was...), the '02+ WRX has a viscous rear LSD, and it was optional (I think?) on the '99-01 2.5RSs.

Originally Posted by Unabomber
Torsen type differential: Used as the front (2005+) and rear differentials in the STi. Short for TORque SENsing differential. It’s worth mentioning that though Torsen is a brand name, it is the most commonly used name for this type of differential (Quaife, which also makes differentials for Subaru applications, is another torque biasing differential similar to the Torsen.) This type of unit is also known as a helical or mechanical type. It uses gears to split power between two axles. Once one wheel is off the ground or slips, it in essence, becomes an open diff or exhibits limited traction based on the torque bias of the unit.
Actually, the torque bias won't affect anything once one wheel is off the ground. The diff must be pre-loaded in order for it not to become an open diff in that situation. Torsen sells the T-2R for some applications, which in addition to having a higher torque biasing ratio (TBR) than the T-2, also (in some applications) is pre-loaded. The purpose of the pre-load is to keep the diff from becoming open if one wheel becomes unweighted.

Originally Posted by Unabomber
It has the added drawback of weight.
I'd be curious to see some weight numbers of, say, a Torsen/Quaife vs. a Cusco Type RS clutch-type rear diff. I'd be willing to bet the difference is just about nothing. Whatever difference there may be I don't think is responsible for any additional heat.

Originally Posted by Unabomber
The additional torque required to rotate a heavier differential will require more energy, energy is heat, heat is friction, friction and heat are wasted energy.
Heat is friction? Friction creates heat, but the opposite doesn't happen. And again, I don't think whatever marginal weight difference there may be between mechanical and clutch type diffs is going to create additional heat in the diff.

However, a mechanical diff does generate more heat. The way it works involves a bunch of gear interactions (which you can read about if you go to the Torsen FAQ), and that's what generates the heat. Again, going back to my Mustang, guys who open track with Torsen diffs have a lot more issues with the rear diff puking fluid (due to the fluid overheating) than with the stock clutch type Traction-Lok LSD. The guys with the Cobra IRS have even more issues, as they don't have the big, heavy live axle housing to dissipate all that heat.

Originally Posted by Unabomber
There are three different configurations, or “ways” for these LSDs. A 1 way differential means that the cam is shaped in such way as to have positive lock only when accelerating. The 2 way is constructed in a way to have positive lock motion in both acceleration and deceleration mode. The 1.5 way functions almost same as a 2 way but provides less lock when decelerating. The 1.5 way can provide more forgiving balance when braking than a full 2 way setup, although it is less effective for true racing applications, it provides easier operation for beginners in throttle off conditions.
You may have already seen this, but here's some good discussion of 1 vs 1.5 vs 2-way diffs from Dave @ Rallispec (via email to me).

Originally Posted by Unabomber
RS/WRX center: JDM STi 20kg center viscous unit. This increases the break away torque from the stock 4kg unit to hold rear traction longer.
As already mentioned, the Cusco Tarmac Gear, although expensive, is an option here, and a damn good one. 35% front, 65% rear fixed torque split - it really changes the way the car feels pretty drastically.

Originally Posted by Unabomber
I’m one of those guys that wants the “best” of upgrades, what differential should I go with? Clutch type differential. You can tune them for your car, you can rebuild them for better handling characteristics, etc.

Which is better, Torsen or Clutch type? There really is no better. The best way to explain the difference is to use an analogy. Torsen type diffs are like the popular spring/strut combos like STi take offs + Pinks or KYB AGX + Prodrive springs. They are what they are and they do a great job. Clutch type diffs are like coilovers. They have end user adjustability and require more set-up, maintenance, and alignment. In the racing world, in theory a coilover equipped car has the advantage if perfectly driven and set-up, but they can and have been beaten by the spring/strut guys. In the end, it can be simply stated as do you want more or less hassle, adjustability, rebuildability, expense, or OEM feel?
I guess tune-ability is nice, but I think it's a heckuva lot smarter to go with the most predictable, repeatable diff (that isn't going to wear out and should never need to be rebuilt/reshimmed/repacked) and tune the handling of the car with the much more readily altered components of the suspension (springs, struts, swaybars). In other words, go with a mechanical TBD at each end if you can. I just don't see the tune-ability aspect of the clutch type diff being a big selling point, as the hugely overwhelming majority of people here are not going to go through the trouble of removing a diff to change the % lock-up or the 1/1.5/2-way setting to change their car's handling.

That comes from a guy who has a Cusco RS clutch type in the rear right now. Hindsight being 20/20, I probably would go with the Quaife instead.

Last but not least, I think there are some good morsels of clutch type vs. mechanical diffs in my post here - (in case you hadn't found that one in your searching).

Pat Olsen
'97 Legacy 2.5GT sedan
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Last edited by Patrick Olsen; 11-01-2006 at 11:36 PM.
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