Originally Posted by NayBreak
I am a big fan of torsen diffs as well, but let's not overstate their functionality. A torsen diff can bias torque to a tire that is spinning faster, but it cannot send and hold 100% of the torque to that tire. The main benefit is that this is a gear driven system that is always engaged, so nothing sudden ever happens.
A torsen would shift torque away from a spinning tire, not to it. The gearing setup works as torque multiplication, so the tire that is spinning slower/not spinning would get a 2x 3x, etc (depending on the torque bias ratio designed into the diff) of the torque.
The key problem is that zero multiplied by anything is still zero. If one wheel is on glare ice and one wheel is on dry pavement, you still only get a multiple of the very small amount of traction available on ice, where a locked diff would get all of the traction available by the tire on dry pavement.
Still, not a lot of vehicles are equipped with locking diffs (and driving at highway speeds on ice with a locked diff is asking for serious control problems) and given the choice of a torsen or clutch based limited slip, I will choose the Torsen each time.
A clutch driven limited slip engages the clutches upon slip to also bias torque - it cannot lock 50/50 or lock at all, but the engagement can be sudden and cause that rear kick out.
Yes. A clutch based limited slip cannot lock. The most it can do it split torque 50/50 as long as the difference in traction between both tires is under a certain limit. If one tire is on ice and the other on dry ground, the tire on dry ground gets torque only up to the holding power of the clutch packs. And of course, this is a wearable part and you only find out the clutch pack in your diff is fried when you really, really need it.
I am in agreement with everything else you said. I also think it is pretty important for drivers to understand the specifics of how their AWD/4WD system works and what type of diffs they have to best understand how to put the system to work for them.
Finally, is there anyone that is really terribly pleased with the traction control system on their vehicle? I haven't found a system that I found worth a damn. The stability control stuff can be useful, but traction control, especially in the Turbo Subaru implementation (hey, a wheel slipped a tiny bit! Lets dump open the wastegate and unspool the turbo while slamming on the brakes to make sure the vehicle stops!) just seems terrible.