Frequently asked questions
About the Evo 7 & 8 ACD system
I
am thinking of using an Evo 7 in competition but have heard that the new ACD system
is unreliable and 2 cars entered in the 2001 Network Q Rally both retired from
the event.
One
car did have ACD system problems, ours. Due to the need to fully understand the
system fully we added a data logging system and ran the pump on the ACD system
full time for some 10 minutes during a pre event test. This was done specifically
to collect data with the centre differential “fully locked” to establish
the efficiency of the unit. Unfortunately this overheated the pump – which
only failed at the start of the Network Q rally. The failure was entirely our
fault and not a Mitsubishi design issue. having said that, we achieved the goal
of collecting reference data files with the centre differential fully locked -
against which the effectiveness of the optional electronic control unit’s
software could be measured.
The second Evo 7 retired with a gearbox failure while testing a non-Ralliart manufactured
“dog” type gearbox. This type of problem could have happen on any
model of Evo
Is the Active Centre Differential system an improvement on Evo 4-6 VC type centre
differentials?
Absolutely, the system is far, far superior to the earlier systems, tyre wear
is reduced, traction improved and handling and braking are all more consistent.
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
What
do I need to be aware of when preparing an Evo 7 with the ACD system
The steering wheel if changed must be mounted on a boss with a drive ring feature to engage with the steering wheel sensor. We can supply a boss with the required feature
- The steering wheel sensor is orientation sensitive relative to the steering wheel boss. Three pegs drive the sensor, one of which is a master peg offset to the other pair. If using the special boss with a Momo or OMP steering wheel, ensure that the boss is assembled with the “TOP” mark aligned correctly relative to the steering wheel.
- The ACD pump assembly must be protected with a cover particularly for gravel events to stop damage. Under GpN regulations it cannot be moved from it’s original position – which means it and the reservoir are vulnerable to side impact damage. We produce a thick Kevlar protection box, which if multiple fixings are used to retain it to the body, is designed to protect the pump in side impacts.
- The ACD system is sensitive to dirt. To achieve a high efficiency level the clearances in the pressure control valve are relatively small. Dust and dirt must be kept out of the system. Parts removed during car build or for service requirements and Spare parts must have closure plugs fitted to hydraulic pipe connections. Pipes and voids must be clean prior to connection, and dust dirt must be cleaned from the reservoir cap and oil bottle before filling the system. Technicians that dare ignore good working practices during build and maintenance works will be the most likely reason for failure of the hydraulic side of the ACD system. We have a carbon enclosure available into which the ACD reservoir can be fitted, this has been produced specifically to keep dirt out of the system during dusty events as well as isolating the fluid from the trunk area to comply with GpN rules.
- If you propose to replace the standard oil line of the ACD system, the fitting of Quick release self sealing unions is recommended on all joints that may have to be broken for maintenance purposes. The initial expense would be easily outweighed if a single pump failure is avoided by stopping the ingress of dirt.
How
do I know if the ACD system is working?
All 3 mode lights on the dashboard will light if a fault were detected by the ACD electronic control unit on board diagnostics. On the road car a MUT2 dealer service computer is used for service works to “interrogate” the system. This is not available to non-Mitsubishi dealers and is an extremely high cost item even if you could purchase it. For road or Motorsport vehicles, a parts kit is now available to fit 2 pressure gauges into the production car system for monitoring purposes. This comprises 2 VDO 25 bar pressure sensors, pressure gauge a switch and an adaptors union kit as well as a wiring diagram. This allows the checking of the health of the oil pump, motor pressure limit switch and pressure relief valve as a “system pressure” The ACD differential itself is monitored as “ACD pressure”. The driver or technician can use a dashboard-mounted switch to check each via a single pressure gauge.
If
I fit the ACD monitoring kit can I tell what activity is happening while I drive.
No, the software strategies and the reaction time of the differential are so fast a log system is ultimately the only way to truly check and analyse the performance of the centre differential. Virtually all physical dashboard gauges of the VDO type are internally damped to stop the needles falling off during periods of intense activity. The pressure in the ACD system is altering in milliseconds so a manual gauge cannot reflect true activity within the system. BTR is working on production of a simple ACD analysis tool comprising an LED string, which will show the result of slip differentials between the front and rear axles. This will be in the form of a kit, the wiring for which will be “piggybacked” to the production wiring on the ACD controller wiring to receive and analyse wheelspeed information from all 4 wheels. Each LED will be to a % of slip value, so the more slip (error) the more LED’s will light. Monitoring will only be of value while full throttle is being used. As the 3 differentials lose efficiency through wear and tear more LEDS will light more regularly.
How
critical is bleeding the ACD system?
Very critical. As the system has a single hydraulic line from the pump/control unit to the ACD differential there is no oil flow as such. Bleeding the system correctly and thoroughly is key to the systems correct operation
How
do I bleed the ACD system?
A dealer would use the previously mentioned MUT2 service computer to conduct
a service routine, which “fools the system” and operates the pump
and proportion valve in the ACD system while the car is standing still. This develops
an oil pressure, which passes to the ACD differential, opening a bleed nipple
allows oil to flow freely until the technician can see the oil supply clear of
air, similar to a brake bleeding exercise. As stated before the MUT2 service computer
is not available to non-Mitsubishi dealers and is an extremely high cost item.
Some optional ACD electronic control units allow a small pressure to pass to the
differential unit to allow the differential to be bled if a 30kph wheel speed
is achieved and use of the handbrake forces a slip scenario. Pressures achieve
via this method are very low and care must be taken to ensure the system is fully
bled out. I can only recommend driving the car for a short road test and re-bleeding
immediately to ensure the system is clear of air.
Alternatively BTR has developed an on-board bleeding system in the form of a secondary
control unit which overrides the production ACD ECU and manually runs the pump
and controls the proportion valve. Via this method, pressures of up to the 11
bar maximum can be passed to the ACD differential without the car running and
ignition on thus ensuring reliable bleeding
If
I lose hydraulic pressure in the ACD system what are the effects?
Traction will be severely compromised, as there will be high levels of slip between the front and rear axles. The handling will be unpredictable due to the excessive slip, and braking distances will increase.
Can
I damage the ACD differential by driving at full speed without an operating pressure
for whatever reason?
It is unlikely to damage the ACD differential unit - as it would continue to operate as a conventional differential but without slip regulation.
My
car oversteers a lot and/or is unpredictable in corners
Drivers need to be aware an active differential system reacts faster and in a more complex manner than a conventional differential system. “Pumping” the throttle mid corner (rear wheel drive style) will cause the centre differential pressures to rise and fall quickly and thus the slip error across the differential will also change fast altering the torque split between front and rear axles. Smooth use of the throttle is essential to good corner lines – think like you know your throttle cable is frayed and about to break at any moment and drive accordingly.
Fuel system
Under
Group N rules can we remove all the pollution control piping, carbon filter etc
from the fuel tank?
No it should remain in place
Brake system
Under
Group N rules can we change the brake pipe layout?
No a cross linked pipework system must remain as per the production RS model with a front and rear brake pipe connected to each piston at the master cylinder. With the ACD system working, negative effects are unlikely to be felt due to the cross piping layout, this is because the ACD pressure locks the differential under braking, any imbalance between axles is thus redistributed via the transmission, similar to a WRC car system.
Can
the rear brake pressure limit valve be removed?
No it must remain in it’s original fitted position and connected
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