no (fawkin') ACD/AYC on gravel

I just got back from a strange drive. I was taking my daughter to camp, which means lots of gravel roads that we both know very well. I put the Braids on for the drive, because we both love a bit of craziness (and my wife and son weren't coming with us). The one "issue" - as it were - is that my ACD/AYC pump is now dead-dead (which is typical of Evo Xs), so we had an open center and open rear.

And, yet, the car was better. I felt so much more in control than usual that we were in 4th where we normally are in 3rd, and we took some turns in 3rd that I normally drop to 2nd for. I had no problem getting out of turns and just felt, all around, more in touch with the car. Most of all, while the tail wagged a bit under full power, I could wag it either way to set up for turns. I only used the hand-brake twice, and both were for 90* turns from one road to another. Otherwise, just a bit of a flick was needed for most turns.

My point - and I was trying to get to it - is that I was left thinking that the AYC is not a plus on gravel. It's a high-grip gizmo, maybe, and not at all useful on gravel. Now, I'm not saying that I'm not going to fix my ACD/AYC pump, because I can't imagine that I want to stay with an open center, but I'm seriously considering putting toggle-switches on the wires that activate the AYC solenoids, so that I can turn the AYC off from the driver's seat. (And, yes, the traction control and ASC was in OFF-OFF, as it always is for gravel.)

My question: have any of you driven an Evo X that has a Cusco RS rear diff, instead of an AYC? And/or do any of you have experience in an X with the AYC disabled for some reason (such as a dead pump)? Was this an odd experience or is the AYC not helpful on gravel?

Are my questions too stupid, technical, Evo-specific, or what?

If the active diff is not setup (which it isn't from the factory) to properly help on gravel, then it won't be much help. I never found diff setting on STi's very helpful during rally-x (I know it was worse the more I set it to the rear though).

Also, you need to know how to drive in order to get diffs to work for you. If you are a relative beginner, the diffs are going to hinder you (IMO) rather than help as they will be constantly changing what it happening, something that is never good on dirt. Consistancy is key.

Also also, depending on what "speeds" you are taking things at, open diffs are much easier to work with.

So yea, I can easily see the car being easier to drive with no rear active diff. The car "could" be more linear in it's handling, but as you noticed, open diff awd cars have a tendency to "wag" on corner exit.

Thanks for the reply. It never occurred to me that some (or all) of the difference would be the center. And now that you mention it, everyone gets their center reprogrammed; many replace the controller completely.

I will freely admit that my gravel experience is minimal. I have only been doing this for a year or so. My last toy ... er, project ... was pavement only.

I didn't look down (often), but we probable hit 80 or 90 at some point and around 50 in the fastest turn. In retrospect, thinking in terms of the gears, it was way faster than we should have been going, but the car was feeling really good.

I'm not claiming by any means to be an exspurt on it. There is some good reading to be had on specialstage in the construction section about diffs IIRC. You might want to take a look there!

Yep. What I've found so far (and this is true for some of the better-prepared Evo Xs out there) is that most people replace the rear diff with something like a Cusco MZ and reprogram the center from scratch using something like a KAPS. In other words, they turn the Evo X drive-line into that from an 8 or 9. Some go so far as to toss the Evo X rear diff case, replacing it with that from an 8 or 9, if the rules allow it.

"Evolution." I don't think that word means what you think it means ... at least when it comes to the X.

Remember the overseas cars have had AYC for what, a decade? The rally guys have been deleting the AYC and keeping the ACD. Factory ACD settings are pretty mild, but with a reflash or aftermarket box, they're certainly quicker.

Just a guess here - but with the center and rear not working, the car is probably less tail happy and more FWD'ish, which might be more comfortable for you, allowing you to push harder?

Dave

Regarding rally guys ditching the AYC and going with mechanical rear limited slips of some description... there's also the small matter that the rulebooks here at least say you can only have an active center. I haven't dug through FIA or whoever's rulebooks, but I can't imagine RA coming up with something like that from scratch.

Yep. I knew about the overseas cars with AYC since they are what made me antsy about waiting for one here. Funny how, now that I have it, I don't really like it, but that's typical for me (for all but marriage, Honey, I swear).

And, yep, they all seem to dump the AYC for anything but tarmac.

As to the car's behavior, it was actually a bit more squirrelly under power, but predictably so. Not much grip (snows on gravel) and a front-heavy pig, so the rears broke free first when I wasn't gentle, but, again, predictably so.

Thanks for joining in.

I've been searching and reading what I can off and on all day, but people (on the interwebs) are either too vague, demonstrably wrong, or too cagey to be of much use. What is it, exactly, that is wrong with the OE ACD controller (in gravel mode, for example)? Is it locking too much or too little under power? Not releasing fast enough on a lift? Or just has too simple a function relating inputs to locking?

I see that everyone who has a KAPS, GEMS, or Motec (or even a Gruppe-S) seems to love it (although some of that could be cognitive dissonance, given what these things cost); what I'd like to know what these controllers do that a reflashed OE controller doesn't or can't. Any insight welcome.

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Iowa999
I've been searching and reading what I can off and on all day, but people (on the interwebs) are either too vague, demonstrably wrong, or too cagey to be of much use. What is it, exactly, that is wrong with the OE ACD controller (in gravel mode, for example)? Is it locking too much or too little under power? Not releasing fast enough on a lift? Or just has too simple a function relating inputs to locking?

It isn't aggressive enough. It doesn't really have provisions for LFB. In the motec for example, any time you input a lock value over 60% it pops up a warning that says it's going to cause increased wear on components. I suspect the OEM lockup values are probably lower than the 60% but I don't know for sure.

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Iowa999
I see that everyone who has a KAPS, GEMS, or Motec (or even a Gruppe-S) seems to love it (although some of that could be cognitive dissonance, given what these things cost); what I'd like to know what these controllers do that a reflashed OE controller doesn't or can't. Any insight welcome.

I did some informal testing on ice tracks in steamboat a few years back. Drove a few laps with the stock ACD computer, then dropped in the motec one. Drove a few more laps and thought...shit, I wasted my money. Tossed back in the stock one and drove again and completely changed my mind. The motec had much more aggressive feeling when LFB.

I've been using the maps as supplied from motec but have seen other maps from teams in Ireland. Haven't had an opportunity to really test out the tarmac settings to see how they react, so haven't ever raced with them. Sorta hoping to have a chance to mess with the ACD in testing in a few weeks...but my car is so far removed from a rally car I'm not sure my results will really mean anything (big aero bits, 305 width slicks, etc.).

Dave

I would really appreciate it if you could explain what people mean by, e.g., "60% locking." (Feel free to stop reading here if my guesses about things are annoying to you.)

My understanding is that, for any given amount of clamping pressure, the amount of torque that is transferred from one output to the other depends on so many things that a simple number like "60%" isn't meaningful or can't be correct. The only way that I know of to read the "60%" as being accurate is by assuming that it really refers to the solenoid duty cycle, instead.

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Iowa999
I would really appreciate it if you could explain what people mean by, e.g., "60% locking." (Feel free to stop reading here if my guesses about things are annoying to you.)

My understanding is that, for any given amount of clamping pressure, the amount of torque that is transferred from one output to the other depends on so many things that a simple number like "60%" isn't meaningful or can't be correct. The only way that I know of to read the "60%" as being accurate is by assuming that it really refers to the solenoid duty cycle, instead.

I'm terrible at trying to explain or even understand torque vectoring. Here's my (probably wrong) guess:

0% lock is the diff acting fully open

60% lock the diff will be attempting to limit the slip, but still allows 40% to happen?

100% lock would be the diff acting like a 4x4 with chirpy tires around a corner.


The aftermarket systems also allow you to increase line pressure on the system, and I think that's what allows the higher lock percentages...and also burns out the pumps. With the Motec you can actually turn off the pump completely as well.

Dave

That's close to what I see in various other descriptions and I can't quite understand it. If we assume that "60%" means that the output speeds are only 40% different from each other compared to what they would have been if the diff was wide open, then that seems impossible to be true, since it all depends on some many other things, such as weight transfer and grip. But I must* be missing something, because what you wrote is close to what diff-makers say.

nb. * the use of "must" in this case must be taken with salt

ps. I'm aware that using "must" in a note defining "must" is circular

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Iowa999
That's close to what I see in various other descriptions and I can't quite understand it. If we assume that "60%" means that the output speeds are only 40% different from each other compared to what they would have been if the diff was wide open, then that seems impossible to be true, since it all depends on some many other things, such as weight transfer and grip. But I must* be missing something, because what you wrote is close to what diff-makers say.

nb. * the use of "must" in this case must be taken with salt

ps. I'm aware that using "must" in a note defining "must" is circular

Think of the center diff lockup just like the clutch pedal in your car. If your foot isn't on the clutch, it's 100% lockup, if you press the clutch pedal to the floor, it's 0% lockup. If you divide the range where the clutch is fully engaged and where it's completely disengaged into 100 increments, that is analogous to the percent lockup of the center diff.

That percent is going to correspond to some breakaway torque, as you know static friction has a higher mu than kinetic friction, so 50% lockup might correspond to 100 lb-ft before the center diff breaks away and the resistance of the plates diminishes quite a bit.. It's essentially a salisbury type diff where the ACD ECU can vary the breakaway torque in real time, really fast.