I got my new car and driver and they have mention that the evo power split varies from 100 front 0 rear to 50 front 50 rear I thought the evo varies from 65:35 snow, 50:50 gravel, and 35/65 tarmac. They didnt' even mention anything about the programs on the evo either.

My question now is that does Car and driver know's what they are talking about or is the US version of the ACD different for the JDM counterpart?

 

It was my understanding that the ACD system continually varies the torque split (i.e. ACTIVE center differential). Thus there is no hard and fast number as to how much torque is going to each axel.


...and yes i believe that 50% is the most torque that the system can send to the rear.

d

 

What is the point of having ACD if you can't send a little bit more to the rear? ACD doesn't help you handle better on Tarmac right? That's what AYC is for right?

 

It seems reasonable that %50 to the rear is max because I understand the center diff. is still viscose coupled. If true, then it can slip or lockup one to one which means < 50 or 50 since the front is always direct. Does anyone know any better here?

 

The point of ACD and AYC is to improve handling across the gamut of situations. The ACD controls the power split between the front and rear axels. If the front starts slipping, more power will go to the back. If the back slips, it goes to the front. From all I can gather -- I'm not expert on the ACD system -- a 50% rear bias is the max.

AYC, on the other hand, electronically controls the rear differential. Therefore it can send power to the left or right depending on which has better grip. I'm not sure of SAYC includes the front wheels as well...It's not like the US gets it anyway.

d

 

Oh okay I see then. I just thought based on what I read that there are just snow, gravel, and tarmac as fixed settings. I didn't know that it will vary the torque split based on wheel slippage.

 

Oh okay I see then. I just thought based on what I read that there are just snow, gravel, and tarmac as fixed settings. I didn't know that it will vary the torque split based on wheel slippage.

 

The settings are just the starting point. The ACD can send all power to the front if it wants to, but no more than 50% to the rear. So when you select one of the settings, it uses that as the base and then can adjust based on slippage.

 

The setting you put the ACD on will give the computer a base map to work from. My guess is that Tarmac is a 50/50 split, approx 60/40 for rain, and 65-70/35-30 split for snow. The computer then does not allow more that that given percentage to that part of the car(being front or rear). If the computer did whatever it wanted there would be no use for the selector switch. Also the Evo does not send more than 50% to the rear.



Note: I say RAIN setting for ACD, because that is what I use the gravel setting for. Cause I never plan to take the MR on a gravel road. So that is why I said that.

 

You are all correct that no more than 50% of the power can go to the rear, it's just not possible from a mechanical design standpoint.

 

not that I am saying you are wrong....but care to expand on that......?

 

The front differential housing is directly driven by the final drive output shaft in the transmission. The front differential housing has a ring gear bolted to it that drives a pinion gear attached to the center differential. If the center differential is decoupled the front differential is still driven directly, but if the center diff is locked, you get a 50/50 split. You can't decouple the front dif from the tranny output shaft, so power is always going there. At a minimum, 50%. Hard to explain, but if you look at the design it becomes easier to understand.

 

I understand the concept behind the evo's drivetrain, so yes you are right about that particular design. But to say that "it is not possible from a mechanical standpoint". Is not only a very stout claim, but it is also a claim that could be proven otherswise most likely. Nothing is impossible. You just have to open your mind and think outside the box. I am not trying to argue with you by any means, just trying to say someone somewhere will figure something out that was previously thought impossible.

 

the torque split should allways be 50/50 the amount of lock is controlled by the ACD ecu and a multitude of sensors. I think C&D is wrong.
http://www.btrprep.com/e7_qa.htm

 

I should have been more clear.

"From a mechanical design standpoint the Evo's drivetrain cannot send more than 50% to the rear."

The fact is, yes, more can be sent to the rear with other designs, a prime example is the Skyline. It is 100% RWD unless power is needed at the front.

- Steve