Mitsubishi Evo's FAQ's Pt1
How does the Active Centre Differential system work?
Mitsubishi ACD - “active centre differential system” ACD is the new major technical feature that distinguishes the previous Evo 4-6 models from the new Evo 7 ACD is an extension of the technology used in for the anti yaw control systems (AYC) employed in various previous Mitsubishi models in the rear axle position. The ACD system comprises an electric motor, driving an oil pump which pressures an oil reservoir to a peak 16-bar pressure. This reserve of “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 ECU (electronic control unit).
The control inputs for 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) and off centre position provided by 3 optical inputs generated by rotation of the steering wheel.
- Handbrake state – on or off
- System pressure – Hydraulic pressure state
- Mode switch state – to select Gravel, Snow or Asphalt software strategy (maps)
We fitted a Motorsport logging system to the E7 RS test car during our 90 mile shakedown run of the new car (in the well knownYorkshire Forest complex) prior to the Network Q Rally GB. The test data from the ACD system has provided a full understanding of how the system functions and it’s efficency.
The “Gravel” and “Snow” software strategies are similar; Gravel has the highest ultimate locking value of the two. In “Asphalt” mode a unique software strategy is employed
The system is technically far superior to the preceding Evo models VC centre differential systems. The centre differential performing well and able to limit front to rear axle “slip” on full throttle to a mean of around 0.6% on a slippery gravel surface. To put this in perspective, a good condition VC unit on a Evo 4,5 or 6 would struggle to be better than 30% aggregate slip value, which is traction going out of the window.
Pressure can be introduced and lost within the ACD system at speeds surprisingly close to WRC car standards, which has allowed Mitsubishi engineers to use a complex and sophisticated software control strategy.
Which ACD electronic control unit should I use?
The production Evo 7 comes fitted with a relatively “soft” control code within the production electronic control unit. This means the potential of the ACD system is not fully realised without fitting an electronic control unit.
The RA553681K1 has a much improved software strategy aimed primarily for high performance road use, which works the ACD system more effectively than standard. We can recommended this unit for track day and fast road applications.
The RA553681K2 has a software strategy which is very specifically for Motor Sport use, which works the ACD system more effectively and harder than standard and may have negative life and warranty implications for the mechanical parts in the ACD system. This should not be used on a road car. The FIA have been asked to clarify the legality of using this unit in Motor Sport events and for the moment the unit should be treated and is listed in the parts system as a non-GpN legal part.