So at one of the Works Evo T/A tests we got to play around with the new Motec ACD controller... I don't know why everyone doesn't have one of these! The thing is so simple, yet super effective.

Basically the stock one just adjusts pump pressure going to the transfer case which adds or decreases the amount of lockup in the center diff. This does not mean more or less power goes to the back wheels, but it changes the amount of chance that the front and rear wheels work independently. Its quite a dynamic science, and super confusing (I'm not 100% clear on it myself).

The Motec unit allows the user to create custom maps to fine tune the power on/off dynamic of their Evo. You can set the lockup in the center diff based on throttle position, brake position (under throttle), and speed. I believe there also might be a slip angle compensation in there as well (I didn't play with it).

The basic idea is that less lockup on braking will cause the car to be a little less stable (usually in the rear) and thus (usually) helping turn in with trail braking. More lockup will stabilize the rear end (often on both acceleration and braking).

The other BIG thing this unit allows is to put more pump pressure into the ACD to compensate for power. Because the stock unit is limited (to around 27psi), more crank power changes the amount of lockup and therefore essentially creates a more open diff under power. The Motec unit can put up to 34psi of pressure [safely] to lock up the diff more effectively.

The one thing I'm still unclear on is the true dynamic of what openning the center diff does for an Evo. Someone was mentioning that because its a transaxle w/ a transfer case to the back wheels (v. a U.S./BMW style system where its an inline transmission with transfer case to the front wheels), the front set will always get 100% torque, and the locking is adjusting how much bias the rear wheels get at a maximum of 50%. I'm no expert on drivetrains, but looking at the Evo front/center diff, it looks like its one set of splines INSIDE the other, allowing EITHER set of wheels to get 100% of the torque. If this is the case, and the center diff was full open, would the set with least resistance get all the torque-- so with oversteer, would the car go all drifto and get all the power to the rear? If this is the case, this Motec unit is invaluable!...

Let me know if there are any drivetrain experts floating around!

 

wow this is interesting

 

I heard that it is pricey. Sounds like fun though.

 

Considering the functionality of the thing, its not too bad! Its a race car part, and for that, it isn't astonishing... $1700 at Works.

I come from the BMW racing world, where winning and losing often is based on the weight of your checkbook-- as in Moton shocks = $14000 to be competitive in a Modified class, so this Motec deal is fantastic. If they made like a rear active diff for a BMW, I guarranty people would be ALL OVER it!

It is pricey for a street car, but if performance is your goal, I would rate this higher than ANYTHING you can do for your drivetrain (once a clutch is in-- so your diffs in general). Coupled with a nice suspension setup, this thing has to be unbeatable.

 

Looking at the diagram, when the diff is unlocked, it works like an open diff would for a 2wd car. Assume you get the front wheels spinning (much more likely than the rear). The amount of power delivered to the rear wheels would be the difference between the total power output and the amount of power dissipated by the spinning front wheels, minus drivetrain losses. As the diff clamps down, this distribution of power approaches 50/50. At full lockup, you would have to spin all four wheels or none of them.

I'm not entirely sure, but I don't think the diff ever goes completely open. The torque delivered to each axle is a function of the state of "openness" of the diff, difference in speeds of the axles, and total power delivered, I think. As long as you're driving in a straight line and not spinning any tires, the power delivered to each wheel should (theoretically) be equal.

That being said, I can't imagine trying to tune this stuff. Its really complex. For instance, I've noticed that switching from tarmac to gravel to snow mode makes my car progressively looser on the AutoX course. I was talking to a guy at the last event (and not a hack driver either) that swore up and down that it made his car tighter to put it in gravel mode. I'm guessing that the different suspension and tire setups caused different reactions to the same change.

 

I can't claim to be an expert on these diff controllers, but the rally shops often have specific maps that pre-optimized for different conditions/setups. I've been chatting with Pete at Cascade Autosport about the GEMS unit that they sell and will probably use it on my PPIHC build.

Dave

 

is it possible to flash the acd ecu?

 

they have a few different rally designed acd controllers in Europe, all are very expensive, you are better off just running voltage direct to the pump controlled in car with a switch

 

Maybe for drag racing if you just want to lock the diff fully. But these guys are talking about using these controllers to maximize the handling of the car on a race course - Not something that full lockup will do.

 

Yes and no. Its possible, but no one has actually done the work to reverse engineer the stock ACD controller that I know of. Its a Renesas controller, just like the ECU, but its from a different line of controllers. First you would have to figure out the hardware interface, and then you'd have to disassemble the ROM, then start playing with numbers and see what happens.

 

A thing to remember with transmitting torque is that you can only supply as much torque as you have traction. If all 4 wheels have adequate traction it doesn't matter if the differential is locked or open all 4 wheels will get roughly equal torque. If the differential is locked, then most of the torque goes to wheels with traction. If those wheels don't have enough traction to accelerate the car without slipping then all 4 wheels will start spinning and some of the torque is lost to spinning the tires. Ultimately the behavior of the differential is heavily dependent upon available traction.

-Paul

 

Yikes, what a bad idea! The GEMS setups are ~$3000 from Cascade (no real data on their website), which includes the user tuneable ECU and three optimized maps for an "open class" us spec rally car. If you just want the computer, they're ~$1800 IIRC. http://gems.co.uk/products/diffcontrol/acd.php

If that's too pricey, take a look at the ones offered by Ralliart. They are not programmable, but you can select a unit for gravel or a unit for tarmac. I think they were around $600 each. http://www.rallispec.com/raparts_retail.pdf

I'm still torn on which to use for my application, but I'm leaning more towards the GEMS, as I'd be looking to use an aggressive tarmac map and an aggressive gravel map for PPIHC, and the ralliart won't allow me to do that.

Dave

 

can this unit be used by itself or do you have to have the motec ems?
i know that cosworth has something like this that they sell to authorized dealers, being that you guys are pretty good with cosworth- how come you did not try that unit?

 

If I remember the conversation correctly, I was told by a very reputable source that Cyber EVO in Japan offers a flash for the stock ACD unit. I believe it is being used by several road racing EVOs in the US.

EVOlutionary

 

That's interesting--is it user-programmable at all?