The Active Centre Differential, first introduced in the Evo VII, is an electronically controlled hydraulic multi-plate clutch which distributes torque between the front and rear to improve traction under acceleration out of a corner. It works in conjunction with Active Yaw Control which enhances grip and steering response whilst driving through the bend itself. Using sensors, ACD regulates slippage in the 50:50 torque-split diff from free to lock-up according to speed and load. So under hard acceleration the ACD moves towards lock-up to put more torque down on the road for stronger traction, but with rapid steering inputs it operates virtually like an open differential to improve steering feel and response.

This revolutionary 4WD system regulates differential limiting force. The ACD replaces the viscous coupling-type differential used in the past with a hydraulically actuated multi-plate clutch. Developed with rally use in mind, the ACD uses a center differential to distribute drive torque equally between front and rear wheels and thereby improve steering response at the same time as enhancing traction.

The multi-plate clutch employs the same kind of steel plates as mechanical limited-slip differentials, Using sensors, the system electronically optimizes the cover clamp load to match driver input and vehicle operating status. Thus able to regulate center differential limiting action from free to locked, as conditions require. ACD's multi-plate clutch delivers up to three times the differential limiting force of a viscous couple.

Under hard acceleration, the ACD reduces slippage and approaches a locked state, thereby transmitting more torque to the road surface for better traction and acceleration. When the driver makes rapid steering inputs, meanwhile, the ACD operates virtually as an open differential to improve steering response and feel through corners while retaining outstanding 4WD stability. The ACD also enables the driver to manually select one of three modes - Tarmac, Gravel, Snow. Last but not least the ACD operates in a virtually open state when the parking brake is used, thereby enabling rapid and effective side brake turns.

Mitsubishi's Active Yaw Control traction enhancement system uses a computer to optimally regulate torque transfer in the rear differential on 4WD models and thereby tailor rear wheel differentials to match driver operation and vehicle operating status. In this way, MMC's proprietary system both equalizes the load on the four tires by adaptively regulating the yaw moment that acts on the body and improves cornering performance without inducing any sense of deceleration.

When accelerating through a corner, AYC reduces understeer by transferring torque to the outer wheel; when decelerating in a corner, AYC enhances stability by transferring torque to the inner wheel. AYC also improves traction on surfaces with low or split friction coefficients and has fully proven its worth since it was first introduced in Evolution IV.

For Evolution VII, all parts of the torque transfer mechanism of rear differential have been uprated to match the increase in engine torque, while breather and clutch operating durability have been improved. The system shares the same computer, hydraulic actuator unit and sensors as the ACD, thus reducing weight and improving reliability.

On Evolution VII, control of the ACD and AYC systems is integrated by computer. ACD control is based on: (1) A feedback control strategy to improve vehicle stability by keeping actual body attitude as close as possible to pre-determined attitudes as derived from steering angle and vehicle speed and, (2) A feedforward control strategy that responds rapidly to driver acceleration and deceleration actions. By combining these strategies in an optimal manner, ACD achieves the outstanding stability of a full-time 4WD vehicle and enhances steering response while realizing the superior traction of locked up 4-wheel drive. In the integrated system, ACD feedback and feedforward information is transmitted to the AYC control system using parameters in such a way that the larger the ACD differential limiting force is, the larger the yaw moment generated by the AYC system.

This precise and integrated control operates so that, for example when accelerating out of a corner, the ACD enhances traction and the AYC enhances steering response and cornering performance. And because of its seamless nature, the driver is unaware of the integrated system as it operates to improve acceleration and handling more than the ACD and AYC systems would if they were operating independently.

ACD+AYC control schematic Reduces slippage in proportion to deceleration to improve stability.

Increases slippage in proportion to steering angle and steering input speed to improve steering through corner Reduces slippage in proportion to throttle opening to boost traction Transfers torque to outer wheel to match steering angle and steering input speed, to improve steering through corner.

Transfers torque to outer wheel in proportion to throttle opening to reduce understeer and improve cornering performance (In cornering under deceleration, reduces oversteer by transferring torque to inside wheel)