Longitudinal quattro Systems

In the original quattro system, the engine and transmission are situated in a longitudinal position. Power is sent through the transmission to a mechanical center differential which splits it between the front and rear axles.

After 1987, the inclusion of a Torsen (torque-sensing) center differential, instead of a manually-locking one, allowed engine power to be redirected to individual axles as driving conditions warranted. Under ideal conditions (i.e., grip is equal in both front and rear axles) power is split 50/50 front to rear. In extreme conditions (i.e., when grip varies from front to rear), a maximum of 66-80% (depending on the transmission or model) of the engine's power can be directed to the front or rear axles. The mechanical nature of the Torsen centre differential helps prevent wheel slippage from ocurring by diverting power to the axle that has more grip. In comparison, viscous coupling and electronically controlled differentials that are used in other all-wheel drive systems are reactive since they only redirect power once wheel slippage has occurred. The advantage is felt under acceleration while turning since the power transfer between axles is less sudden, reducing the chance of spinning.

The Torsen-based quattro system also offers an advantage in the opposite function of distributing power to the wheels: engine-braking. When engine-braking is used to slow the car down, the resulting loads on the front and rear axle are stabilized by the Torsen system in exactly the same way that engine power is diverted - mechanically. This allows the spreading of the engine-braking load on the tires among the four wheels. The Torsen-based quattro-equipped vehicle is able to execute a more stable high-speed turn under deceleration with less risk of spinning due to loss of grip in the front or rear axles.

Additionally, this configuration greatly reduces torque-steer due to the equal length drive-shafts on the front axle. However this configuration of the quattro system does have some limitations. With placement of the engine-transmission assembly in a fore/aft position (longitudinally) there arises the need to place the front axle well behind the engine, leading to a common criticism of Audi vehicles: being nose-heavy. In other words, the ideal 50/50 weight distribution cherished by many driving enthusiasts is not possible. Recently the centre differential has been adapted to a 40/60 front/rear static split (i.e. when grip is equal on both front and rear axles), allowing more rear-wheel-drive-like handling characteristics. This was first introduced in the 2006-model RS4 and shortly thereafter in the Q7 SUV. It is planned to be incorporated in all future quattro-equipped cars which use longtitudinal-mount engines.