OK, I'll try to answer all the questions. First off, let me acknowledge that a Torsen-type front isn't for everyone. But it does have its place, just like a locker does.
Regarding torque bias ratio (TBR), for those that don't know, it is what is used to define how much torque transfer the differential is capable of. In its simplest form, its torque on the high traction side divided by torque on the low side. If its 2:1, then 2/3's go to the good side, 1/3 to the slipping side. With 3:1, 3/4's to the good, 1/4 to the other. See the pattern? With that in mind, and open diff is basically 1:1, and a spool or locker is infinate:1.
To answer the first question, well, it depends. It depends on what specific type of competitive LSD you are comparing to, and it depends on what point during the diff's life you are considering as well. Plate clutch type LSD's (Like the Ford Traction Lok) tend to have similar or slightly lower TBR's when brand new. But they loose TBR rather readily as they are worn (this is one reason why mini-spare tires have speed limitations). I've seen some that had little more performance then an open diff after 30-40K miles of use. Cone-clutch LSD's, such as Auburn, often start out quite high, but it depends on the model. But they too loose bias rapidly. A typical Torsen unit may only be at 2:1 or 2.5:1, but they bias is stable over life. Another thing to consider is that clutch type LSD's usually (but not always) have a spring preload. That might be beneficial in the rear, but in the front it will cause steering issues similar to, but not as severe as, a locker. The other popular brand to compare to is Tru Trac. What can I say? The Tru Trac is the same class of diff as the Torsen. They often have a bias that's greater then 3:1, but that's high enough that it could cause steering issues too. Our experience is that 2.5-2.8:1 is the limit for driven front axles before there are problems. Even that might be pushing it, depending on the vehicle's steering and suspension design.
GJarrett's concerns are valid. In order for a biasing diff to work, there has to be some reaction torque at the wheels. The other way to look at the TBR is as a multiplier. In this case, what it means is that amount of torque that the good traction wheel will get is equal to the amount the low traction wheel can use (before it slips) multiplied by the TBR. If the TBR is 2:1, and the low traction wheel can use 100 ft-lbs, then the high traction wheel will then receive 200 ft-lbs, for a total of 300. Any extra beyond that which may come from the driveline would be lost. If a wheel is in the air, then the amount of torque it can use is zero. The TBR x zero is always equal to zero. In this situation, yes, brake modulation will work. Like I said, the diff needs reaction torque on each side to work. But it doesn't care if that comes from the tire's contact with the ground, or if it comes from brake application. As such, the amount of torque that the wheel on the ground would get would be the TBR x whatever amount of brake torque is applied.
I should mention, though, that in a 4WD vehicle that's locked across the center (like a traditional truck system), if just one wheel is in the air, it will not spin. In order for the differential to allow cornering, one side speeds up relative to the diff carrier body and the other slows. Selectable lockers work because they lock the output to one wheel to the carrier body. If one side gear cannot turn inside the differential, the entire gearing package in the diff is effectively locked up. I mention this because when just one wheel is in the air, the other wheel is on the ground and can't slow in relation to the rest of the vehicle, and the carrier body of the diff can't speed up, because it is locked thru the t-case to the rear axle, which also has traction. So, by default, the diff becomes locked via the 4WD system. This will hold true until one rear wheel also looses traction. If you used a rear locker, then that won't happen until both rears spin. Also, if you get into a situation where you lift diagonal wheels off the ground, then gravity will pretty quickly adjust the situation (hopefully in a non-detrimental way). Granted, this applies to an open diff too, but the Torsen does something an open can't, and that's bias torque continuously to which ever side can use it best. This becomes significant on loose roads that run a little faster, and it also allows it to be benecial for on-road use (you can't say that for a locker, can you?).
What it comes down to is this: if you're a hard-core rock crawler, then don't waste you time with this. You will better served with a locking differential. But if your thing is mud, or driving trails, fireroads, logging roads, or loose dirt roads, then this is a great thing to have. Also, if you live up near me (in the northeast), then winter service becomes an issue. A 4WD truck with a Powertrax would be unsteerable on a snowy, icy road. But a Torsen-equipped truck will work very well. I've run road rallies in the FX4 when my Mazda 323GTX was out of service. And on the tight, narrow, loose dirt roads of the lower Adirondack region of NY, the truck did very well, even up to highway speeds. Also, we've been supporting Rob MacCachren's BITD Ranger (as mentioned before) with this unit for the last 2 seasons. He's subsequently become a big Torsen supporter. The LASD race team are also recent converts to Torsen supporters. So it becomes an issue of what you need and what you intend to do...
Sorry for the length of the reply...
Thanks.
