AWD: The five types of Subaru systems
Driver Controlled Center Differential (DCCD) All-Wheel Drive:
The Impreza WRX STI uses Driver Controlled Center Differential (DCCD), the most performance-directed type of Symmetrical AWD. A limited-slip, planetary gear-type center differential provides a performanceoriented 35:65 front/rear power split. In automatic mode, the electronically managed continuously variable transfer clutch can vary the distribution ratio through the center differential as needed to suit driving and road-surface conditions. In manual mode, DCCD allows the driver to vary the front-to-rear torque distribution to optimize performance to suit specific driving conditions.
In 2006 and 2007 models, the front/rear default torque split, a function of the planetary-type differential, has been changed from 35:65 to 41:59 to reduce over-steer while accelerating out of corners. A mechanical limited-slip center differential was also added (to augment the electronically controlled differential) and a steering sensor input to the system to improve torque transfer response by more accurately relaying the car’s cornering situation.
In automatic mode, the center differential can vary the split ratio as needed to suit driving and road-surface conditions. Automatic mode provides the best all-around performance for most drivers and circumstances, varying the front/rear torque split automatically depending on driving conditions and driver input.The system responds to acceleration, deceleration, cornering force and wheel slippage to determine the ideal power distribution.
In manual mode, DCCD allows the driver to vary the front-to-rear power distribution to optimize performance to suit specific driving conditions. The driver first selects manual mode with a console-mounted button and then turns a thumbwheel on the center console to select from among six levels of center differential locking. Increasing the lock factor keeps more power at the front wheels, which the driver might want in certain driving conditions, on particular roads or to suit an individual driving style.