....Yes, Haldex aka Helldex isn't all that it's sold to be apparently (says me with very little direct experience of it). Plus, or rather Minus, the fact that 'AWD' will make a car somewhat heavier and probably less nimble - A characteristic we all like in our GTI's. No V6 soundtrack to make up for it either.
Whats the score with Haldex, is it not regarded as any good? I had 4WD in my S4 and found that fantastic, especially in the wet. Did that have a Haldex or is quattro a totally different thing? Pardon my lack of knowledge 
To put it simply, for all VAG four wheel drive cars (Audi quattro, Volkswagen 4motion, Å koda 4x4, Seat 4) - all the transverse mounted engined cars (Golf, Audi A3, Audi TT, Å koda Octavia, Seat whatever junk!) use Haldex, whereas all longitudinally mounted cars use Torsen, such as Audi A4, A5, A6 and A8 ranges, previous generation VW Passats, VW Phaeton, Å koda Superb.
The Torsen system is a permanent "always on" four wheel drive system. Prior to the latest Torsen diff, first seen in the current RS4, and eventually being incorporated in all Torsen Audis, there is a "default" 50:50 front/rear torque split. In laymans terms, under normal traction and road conditions, both the front and rear axles receive identical "drive" power. When road/traction conditions alter, the Torsen can dynamically (and steplessly) apportion torque (or drive) to either the front or rear axle. This split was initially up to 25:75 to 75:25 (meaning 75% of the drive going to one axle with the remaining 25% to the other). Later, this 50:50 diff was re-engineered to provide 20:80 to 80:20 on some models. For the latest Torsen, it is now set up with an asymmetric "default" torque distribution of "40:60" - which means that the front gets 40% driving force and the rear gets 60% under normal operating traction conditions. The window of operation of this latest diff is set to 0:100 to 80:20, though in practical terms, the front axle would never be reduced to an absolute zero torque distribution. The Torsen-based system is purely a mechanical system, requiring no electronic control, and NO maintenance. It works in a complete "fail-safe" operation, and is truely "stepless" in its torque transfer.
For the Haldex system, it works very, very differently. Firstly, the Haldex is NOT a "centre differential" like a conventional planetary-geared diff, or the Torsen helical-geared diff. It can NOT "apportion" torque, either "positively" like the Torsen, or negatively like a conventional diff. The Haldex is quite simply an electronically controlled "lock-up" clutch, and can operate fully "open" (transmitting zero drive), fully locked (transmitting 100% drive), or anywhere in between, just like "slipping" your foot pedal controlled clutch.
Now, on Haldex systems, under normal conditions, the car performs ONLY as a front wheel drive, with the rear axle operating under ZERO drive conditions. The Haldex will ONLY send drive to the rear axle AFTER the front axle has lost traction. The Haldex takes information from the roadwheel speed sensors (often incorrectly called ABS sensors), and compares the difference in rotational speed between the front and rear wheels, and then the Haldex controller instructs the Haldex clutch to start to "clamp", thereby sending
some drive to the rear axle. This method of operation is fine for straight line traction, but traction in corners can be quite "un-nervy", with some drivers being able to sense the "off-on-off-on" application of the Haldex unit. Naturally, if this happens in a long sweeping corner, under hard acceleration with inconsistent traction, then it can make the handling of the car very "twitchy" indeed.
Another significant disadvantage to Haldex systems, should any of the ABS/wheel speed sensors fail, then the Haldex wont work!
