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Discussion Starter · #1 ·
has anyone read the last issue of the mlr mag, issue 25?

BTR have explained the how the acd works but they seem to mentioned that gravel and snow strategies do not appear to be sensible for track day use.

did anyone understand why?

i always have the car in gravel mod even on track and it seem to be the best setting that suits my driving.

any comments


Discussion Starter · #4 ·
I haven't read it as I don't have WC yet but I guess it's because the FR torque split is bias towards the front on gravel and snow settings but more towards the rear on the tarmac setting.

Discussion Starter · #5 ·
someone must have the acd article in digital formate?

to summarise though
the ACD system comprises an electric motor, driving an oil pump, which pressures an oil reservoir to a peak 16bar pressure at which point the electric motor is turned off. if the pressure drops below 10bar it cycle repeats. there is 8 sensors that control it and it could be set in 3 modes tarmac, gravel and snow.

the gravel and snow sfotware strategies are similarly complex. gravel has the highest ultimate locking value of the two.

the tarmac mode the centre dif is generally running low pressure (low lock) unless the car is breaking when the pressure increases or cornering when it reduces. this is the basic mode that most owners should use for normal day to day driving.

the software strategies in the production acd unit is relatively soft and looks to be baised towards safety and long component life. for track use the ralliart fast road ACD is better suited and compliment the chassis better than the standard.

the system technically is very good. the centre diff performing well and able to limit the front to rear axle slip to a mean of around 2% witht the production unit and 0.6% with the rally unit on gravel surfaces.

the pressure can be introduced and lost within hte acd system at close to WRC standard speeds, which has allowed mitsi engineers to use a complex and sophisticated control stragtegy.

the acd is not a gizmo and is far superior to the EVO 4-6 vc centre diff. the system has more benefits on slippery surfaces than dry surfaces but is better on all surfaces overall.

Discussion Starter · #6 ·

I didn't understand that part either. I have found that gravel is the best setting and I believe that SuperL thinks the same, even with the K1 unit.


Is that right? Do you still prefer the gravel stting with the K1 ECU?

Discussion Starter · #7 ·
Hi there,

Although I didn't read that mag (it is kind of hard to get it over here), the basic thing about the way the ACD works explains why it might not be as sensible at the track at the gravel or snow settings. Here is the thing.

When the center diff is in the full lock mode (50/50), it creates a lot of push on both sides of the car. Car basically goes wider than it should, since both ends are pushing, but it is very stable and it is hard to spin the car.

With a center diff fully open, power goes wherever is easier for it to go (like regular open diff). In the case of the full throttle application, when car is entering the turn, most of the power will go to the rear end and spin is most likely going to happen. The fact that the most of the weight is in the front anyway, helps as well.

So, what they try to do with the ACD is to control how much of the oversteer the car is going to need. In other words, the question is how much (or how long) the center diff will be in the open mode (which will induce oversteer). I am purposely ignoring the fact that there is also AYC in there and it helps a lot, but just for the simplicity, it is not mentioned.

Now, when comparing traction between the dry tarmac vs. gravel or snow, it is obvious that slippery surface require less open diff than the more gripy one. So, car will have certain characteristics in the tarmac mode (and this is usually the most aggressive oversteer mode) and it will allow drivers to rotate the rear end let say quite a lot. Now by switching to the more slippery settings, center diff will stay more in the locked mode and in the case of the dry tarmac, it will be more difficult to rotate the rear end.

I hope this was helpful!

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