Audi has recently replacing the Quattro AWD system to the Quattro Ultra on their A6/A7 models. They claim the Quattro Ultra has better off-road capability(who want to off-road a A7 anyway) and save more fuel(a whooping 0.9mpg), compared to the Quattro. AFAIK, the quattro 4WD has three torque-sensing LSD.The center LSD has a base 40/60 ratio, which can change from 70/30 to 15/85 in case loss of traction or performance driving. The front and rear LSD have a max 3:1 torque bias ratio. This is a full-time AWD system. Lower-end models has a center LSD and two open diff. Electronic Differential Lock is used to control wheelspin. The so-called Electronic Differential Lock does not lock the differential, instead, they brake the slipping wheel to slow down to the non-slipping wheelspeed, wasting 50% power as heat. In contrast , diff-locks could transfer 100% torque to the non-slipping wheel. The quattro ultra has a center "split shaft" that connected to the front differential solidly, torque is transfered to the rear axle via a electronically controlled multi-plate clutch. Both the front and the rear differentials are open differentials. Two electronically controlled "locking hub" is available in the rear differential, which can disconnect the driveshaft to safe fuel. In case of off-road condition, the multi-plate clutch can from slip to fully lock up, effectively transfer 100% torque to the rear if the front has zero traction. In order to safe fuel when cruising, the multi-plate clutch and both of the locking hub are disconnected, making the car FWD. This is a part-time AWD system.