Section II – Technical specifications of the ACD

A. Historical overview

Mitsubishi’s Active Center Differential (ACD) was first employed in 2001 on the Evolution (Evo) VII model. The ACD was to be used in support of the Active Yaw Control (AYC) carried over from the previous Evo models which controls the yaw of the car using the difference in driving and breaking forces between the two rear wheels. (1)



When the Lancer Evolution VIII model was released in the US in the spring of 2003, the Evo was outfitted without the ACD or AYC found in other markets. Instead, it was outfitted with the Viscous Coupling Unit (VCU) which distributes the torque evenly (50:50) to the front and rear wheels. This VCU was also equipped on the 2004 model year Evo in both the standard and RS models. (2)

Beginning with the 2005 model year Evo VIII, Mitsubishi decided to outfit the car with the ACD unit instead of the VCU, yet still left the AYC unit out. Bastards. However, the ACD was available to all variants of the model: RS, Standard, and MR. (3)

In October 2005, the 2006 model year Evo IX was released to the US market. Again, as it was with the previous model year, the ACD unit was standard equipment on all variants: RS, IX, and MR. And again, the AYC was not included. Still bastards. (4)

B. What is the ACD?

*NOTE* If you are unfamiliar with how a Limited Slip Differential works, then please look that up first. Howstuffworks.com, answer.com, and even EvolutionM.net have many good descriptions and diagrams available. */NOTE*

The ACD is a bevel-gear type center differential with a front/rear torque distribution of 50:50. The differential is controlled by an electrically controlled hydraulic multi-plate clutch. For greater dependability, the clutches were made from steel. (1)

The differential itself is located inside the transmission housing while the ACD clutch pack is located inside the transfer-case. (https://www.evolutionm.net/forums/at...chmentid=70736 Light green and dark blue sections in this PDF by SuperHatch (5))

The ACD ECU is located to the right side of the glove box, behind the passenger side kick-panel. (6)

The ACD hydraulic unit is housed in the engine compartment and regulates the hydraulic pressure of the multi-plate clutch within the range of 0 to 145 psi. The maximum limited-slip torque of the multi-plate clutch is about three times that of a conventional VCU. Translation: 3 times the grip transferred to the wheels that need torque. (3)

There are many different sensors used by the ACD when determining how it will control the center differential. ABS input, steering wheel angle, throttle opening, wheel speeds, and longitudinal and lateral movements of the vehicle are constantly measured by the ACD. Additionally, the driver can select how the ACD will perform by selecting one of three different modes from a switch in the cabin: Tarmac, Gravel, or Snow. (7) (1) (3)





When comparing the USDM version of the ACD to that of other markets Evo’s, the difference lies with the USDM ACD not connecting to an AYC control unit & hydraulic system since the car does not have them. (1)

C. How the ACD works

In short, the ACD unit hydraulically controls the limited-slip locking state of the center differential which ranges between full locked, full open, and every point in between these two states. It calculates what the optimum locking amount would be using real-time input from the previously mentioned sensors and which ACD mode is selected. The ACD is extremely fast in operation, bettering the performance of the standard VCU and Viscous units. (3) (7)
When the car is accelerating or decelerating rapidly, the ACD begins to engage the limited-slip locking on the differential. The harder you accelerate or decelerate, the more it locks. This is done to provide maximum lateral stability and wheel traction. (3) (8)

The ACD allows the differential to operate in more of a free state when steering movements are made. When turning, the wheels in the front have to travel a further distance than the wheels in the rear. This is because the wheels in the front travel using a larger turning radius than the rear wheels as you can see from this picture. (turning radius.gif) So in order for smooth turning, the center differential needs to become open to allow for the front set to turn at a different speed. If you were to leave the center differential locked, the wheels would skip and skid badly. (1) (3)



If the front wheels are spinning faster than the rear wheels, then the ACD begins to lock the clutches up. If the amount of force exerted by the front wheels is less than the amount the clutches resist, then the slip is stopped and both front and rear spin at the same speed. However, if the force the clutches can resist is less than the force exerted by the slipping set of wheels, then the wheels will slip, but only by how much remaining force the slipping wheels overpowered the clutches with. (1) (9)

D. How the Tarmac/Gravel/Snow switch influences the ACD

Despite popular belief, this switch DOES NOT change the torque split. The differential is geared at 50:50 and cannot be changed by the push of a button. What this switch actually does is quite simple. Each setting determines how long the ACD will delay in freeing the center differential after a steering input is made. In addition, it will determine how much locking force the ACD will exhibit on the clutch pack and center differential. (10) (11)

Tarmac is the setting to be used in dry, paved conditions. In this setting, the ACD will almost immediately allow the center differential to go into a free state upon detecting a steering input. Additionally, this mode provides the strongest limited-slip clamping force of the three modes. (7) (12)

Gravel is the setting designed for wet or loose roads. In this setting