Here’s What Makes Audi’s Legendary Quattro AWD System So Special – SlashGear

The first version of Audi’s quattro featured three mechanical differentials — front, center, and rear — with the option to lock the center differential with a vacuum system. This resulted in a 50-50 torque split between the front and rear. In the 1980s, Audi replaced the center differential with a Torsen (or torque-sensing) differential, allowing torque to be directed to the wheel with traction instead of spinning the wheel without.

A regular open differential allows the wheels in an axle to spin at different rates, which is great if you need to make tight turns, but for situations where you don’t have traction, it results in the wheel spinning aimlessly. To improve grip, Audi kept a rear locking differential when it added the Torsen to its quattro system. The change to a Torsen and rear locking differential meant that up to two thirds of torque could be sent to either axle when needed. 

Audi really began to modernize its AWD system with the introduction of the transverse-engine Audi TT in the year 2000. The company switched out the Torsen center differential with a hydraulically-controlled, clutch-operated Haldex coupling, splitting power between the front and the rear. 

This meant that the Audi TT was front-wheel-biased until it lost traction, distributing more of the torque to the rear axle to compensate. In addition, the TT also introduced EDL (electronic differential lock) that used the ABS system to distribute torque on a per-wheel basis.