Some info I got whilst looking at the EVO for competition...
AYC & ACD systems
The new ACD system is an extension of the technology used in the anti yaw control system employed in various previous Mitsubishi models. Recently introduced, the ACD system comprises an electric motor, driving an oil pump, which pressures an oil reservoir to a peak 16-bar pressure then the motor is turned off. If the reservoir pressure drops to 10 bar the cycle repeats. This system pressure is fed to the piston of the ACD plate pack via a modulation (proportion) valve. The modulation valve is software controlled by the ACD electronic control unit. The ACD electronic control unit is a completely separate item to the engine electronic control unit. The control inputs to the ACD electronic control unit are
4 wheel speeds
G force both lateral and longitudinal
Throttle position a variable value
Braking state - on or off
Steering angle neutral position (straight ahead)
Steering angle rotation from straight ahead provided by 3 optical inputs generated by rotation of the steering wheel.
Handbrake state on or off
System pressure Hydraulic pressure state a returned voltage from the pump pressure switch in the 0-5V range
Mode switch state to select Gravel, Snow or Asphalt software strategy
Owners should be aware that in order to change the steering wheel, provision must be made to drive the steering angle sensor. OMP has produced a unique steering wheel mounting boss to engage and drive the ACD steering angle sensor. The part is available from OMP dealers part number OD/1960M1290. The steering angle sensor ring has 3 offset square drive slots, which can easily be seen as a new detail to the rear of the standard steering wheel - just outside of the centre-mounting hole. The sensor drive slot alignment is essential for the correct functioning of the anti yaw control and ACD systems. One of the three is a master slot and is aligned at the 3 o clock position as viewed when seated in the car, in the straight ahead steering position. Correct assembly of the steering wheel and boss are therefore critical. The boss features a Top mark to aid correct assembly.
The anti yaw control and ACD systems have some sensitive components which are not particularly dirt tolerant. It is therefore important that dirt is not introduced to the system during routine oil changes or topping up of the AYC / ACD fluid reservoir. The reservoir neck and cap should be cleaned prior to removing the cap and topping up.
Three variants of ACD electronic control unit exist. The production version and two Ralliart variants - RA553831K1 being listed as a fast road unit and RA553831K2 a Rally unit.
BTR added a Motecฎ logging system to their Evo 7 RS test car for a 90 mile shakedown run in the Yorkshire Forest prior to the Network Q Rally GB. This produced some interesting data on the ACD system and a good understanding of how well the system functions. Of general interest to the average owner is
That Gravel and Snow software strategies are similarly complex. Gravel has the highest ultimate locking value of the two. This is true of all the electronic control unit variants. The aggregate locking value in Gravel mode being 4 bar peaking at 10 bar on the production electronic control unit and 6 and 10 bar on the rally version of the ACD electronic control unit.
In Asphalt mode, the centre differential is generally running low pressures (and therefore ultimate lock value) unless the car is braking, where the pressure increases on braking and reduces as steering angle increases during turn into the corner. This is the basic mode that most owners should use for normal day-to-day driving.
The software strategy in the production ACD electronic control unit is relatively soft and looks to be biased towards safety and long component life. For performance and track day use the Ralliart fast road ACD electronic control unit will compliment the chassis better than the standard unit. The Rally unit has the most aggressive software strategy, working the pump assembly and differential hard. To get the best of both worlds, long system life and track performance, fitting the K2 Rally specification performance electronic control unit for track days only could be considered while fitting the road unit for off track use. If this is considered too much trouble do not fit more than the K1 electronic control unit. In all cases run Asphalt in normal use.
The Gravel and Snow strategies do not appear to be sensible for track day use.
Misalignment of the suspension resulting in the steering wheel being offset while driving in a straight line will screw up the functioning of AYC and ACD systems.
The system is technically very good. The centre differential performing well and able to limit front to rear axle slip to a mean of around 2% with the production ACD ECU and 0.6% with the Rally ECU on a gravel surface.
Pressure can be introduced and lost within the ACD system at close to WRC standard speeds, which has allowed Mitsubishi engineers to use a complex and sophisticated control strategy.
To summarize - ACD is not a gizmo and is far superior to the Evo 4-6 VC centre differential control system. The system has more benefits on slippery surfaces than dry surfaces, but is better on all surfaces overall.