Subaru uses several variations on their all wheel drive system, each of which features improvements over previous versions. As the years have passed, the vehicles have become more sophisticated, especially the WRX and WRX STi models. Subaru has been making AWD vehicles since 1972, and has learned a little something about it in the interim. Starting in 1996, every non-minicar Subaru features an all-wheel-drive system. The differences basically boil down into three areas: Differentials, the center differential, and computer control.
Before talking about that, though, it's necessary to bring up just what makes Subaru's AWD layout so effective. The key to the system is the Boxer-layout flat engine. Using horizontally opposed cylinders makes the engine both shorter and flatter, which means it can be mounted lower, and farther back. This improves weight distribution and decreases the polar moment of inertia, meaning that there is less to overcome in order to turn the vehicle. In addition, all of the powertrain mass is mounted down the center line of the vehicle, which keeps left to right balance, improving stability.
The very first generation of Subaru AWD (back in the seventies) used a system with two states; the vehicle was normally FWD, but the rear wheels were engaged (at a speed different from that of the front) in situations involving a loss of traction.
Basic, more modern Subaru AWD (as opposed to 4WD) utilizes a center differential with three states. In the first state, the differential does not drive the rear wheels, placing the vehicle into a FWD mode. This is the normal state while cruising (such as on the freeway.) In the second state, the center diff is engaged, allowing front and rear wheels to be driven. Any time you perform significant acceleration or deceleration, the diff is put into this state. In the third state, the center diff is locked (spooled) and the front and rear differentials are driven at the same speed at all times. In the 1993 Impreza, this occurs on the automatic trans only when shifted down into first gear, with the manual button engaged.
Introduced for the legacy was the VCD (Variable Center Differential) system. This is a full-time AWD system in which the torque split varies from 0 to 50% rear automatically.
Latest-model Subaru AWD (for 2006) in the WRX and WRX STi only features the "DCCD" or "Driver Controlled Center Differential". This system allows the driver to (optionally) control the torque split between the front and rear of the vehicle. The user can select a locked-in 35% front/65% rear split, or manually select anything from 0 to 50% rear. In automatic mode, the vehicle will automatically split torque up to 50% rear. Earlier versions of WRX STi with DCCD allowed the user to lock in up to only a 55% rear bias, or select up to 50%.
2006's AWD update also includes a sensor for steering position, which assists the vehicle with yaw control, which ensures that the vehicle does not spin out, by automatically reducing power output, and by utilizing computer-controlled ABS at each of the four wheels independently.
Various models of Subaru have different front and rear differentials. In the non-WRX imprezas (Except for some outback sport wagons) there are open differentials front and rear, which WRX imprezas and the Legacy get a viscous front differential and an open rear. STis have a viscous front, and a mechanical limited slip diff rear.