Yeah we are pretty much saying the same thing. Remember that the ACD not only varies the maximum sustainable torque difference you describe, but also the time it takes to go back to open diff status when a steering input is introduced(in an inverse relationship, the SNOW setting holds the longest after steering input with the least amount of pressure while TARMAC holds with the most pressure and disengages almost immediately after steering input). If it didn't, say if the relationship wasn't inverse and SNOW held for the longest and ALSO held the strongest, you'd have all your available engine torque going to the wheels in the snow, which wouldn't do you any good. The important thing to remember is that the less force the clutches have, the less torque is available to the wheels up to the clutch slippage point. At that point, the remaining available torque is limited via open diff characteristics which is a good thing in the snow. You don't want all your power trying to push you forward in the snow, it does you no good and actually hiders traction.


Gotta love the ACD.

 

very helpful thread...

 

Describe that line a little better.

I would say the diff a torque bias towards the axle with more grip with slipping clutches, just not 100%. But much more then what an open diff would do.

 

Right. All things being equal, lets say you have 100 "units" of torque to be distributed through the center diff via the engine and in SNOW mode it takes 25 units to make the ACD clutches start slipping. Where do the remaining 75 units go after slippage starts? It goes through the open diff mechanism. So 25 units of torque are being placed wherever traction is "best" and is not traction limited (in other words even if all 4 wheels are spinning that 25 units is there), while the remaining 75 units are being limited by the end with the least traction via basic open diff operations, and split 50/50. So now you are putting down 25 units that can go wherever traction is most abundant, and lets say of that remaining 75 units it only takes 10 more units to get an end slipping. So you are putting down 35 total units of torque total out of a possible 100 input to the center diff, with 25 of those going wherever traction is best, plus 5 more to the front and 5 to the rear.

What is important is when you compare this operation to TARMAC mode. Let's use the same scenario as above but put the car in TARMAC mode and let's say it takes 75 units of our 100 available to overpower the ACD clutches. Immediately you can see that before the clutches ever slip, we are going to have 75 units of power available! Not good at all if we are on ice. Remember is only took 35 units to get an end slipping in the above scenario. Now we have 75 units shredding snow and tires and we aren't going anywhere. Think of the principle of taking off in 2nd or 3rd gear on ice instead of 1st. You want to LIMIT torque in this situation (via a higher gear) so you can get moving. So TARMAC isn't ideal for driving on snow. SNOW mode lowers the torque output threshold up to the clutch slipping point. TARMAC sets this threshold much higher, which does you no good on snow and ice.

 

man that makes my brain hurt.

clutch slip happens at the end of the "distribution" not at the begining. If you have 1 axle that can support 10 "units then each axle gets 10 units due to the open diff. if the acd can transfer an additional 25 units then the axle with good traction gets 35 total and the slippery axle gets 10 so you get a total of 45 units of combined output not 35 as in your example. The remainder of the engine output goes into crazy wheel spin on the traction limited axle and you end up banging off the revlimiter and burning up the ACD.

I don't think (i'm speculating) that the snow mode is supposed to limit torque output. The whole point of awd is to get the maximum torque to the ground regardless of which wheels have the traction. What it does do is keep the front and rear locked together which increases the stability (aka increases understeer and reduces turn in ability) of the car. That is good for slippery conditions. It lowers the clamp load on the diffs for two reasons. the first is if all 4 wheels are on snow you won't have a large traction differential. the second is if the diff is locked almost all the time. if you are on slightly grippy ground and make a turn the diff has to allow wheel speed differences, but because the acd isn't unlocking the diff the wheel speed differences come in the form of over powering the acd. If the locking limits of the acd in snow mode were higher it would put more stress on the acd or cause wheels to slip/skip.

 

You are correct that the number in my example should have been 45, because if it takes an additional 10 units to make one end slip, then the other end sees 10 units also, BECAUSE of the 50/50 split open diff. The other 55 units are "lost" through the open diff. That's just the way an open diff works. Can't get around that. Be very careful. Do not associate the speed at which the wheels are spinning with torque. Fast spinning wheels don't mean more torque.

So that's 25 units being sent via clutch packs (which is distributed just like a locked axle), and 20 units being sent via the open diff(distributed 50/50). This is just basic differential operation.

As for your second assertion about snow mode limiting torque. That is EXACTLY what it does, quite simply because more of the power is being routed through the open diff side vs in TARMAC mode because the clutches start slipping sooner. It's a function of the open diff, nothing else( Again, wheel spin has nothing to do with torque). Lots of torque on snow does you no good, so this is exactly what you want to do. But you don't want ALL your power routed this way, else you'll not move at all. That's where the clutch pack comes in with it's lower torque threshold.

If the locking limits in SNOW mode were higher, it would actually put LESS stress on the ACD with both ends in the snow because you won't be generating much torque because you've overpowered the tires by a large factor. So the clutches don't slip due to lack of torque and there is no difference in wheel spin speed front to back so the open diff portion isn't getting stressed either.

 

the torque isn't "lost". It's put into accelerating the "slipping" drivetrain components, ie. the axle with less traction. The diff itself can't limit torque output, only change the distribution. If you put in 100ft-lbs, 100ft-lbs come out(ignoring losses of course). Weather it goes to to the ground is a different question.

The clutches only start slipping if the max torque difference(for the specific clutch pressure) is exceeded. all 4 wheels on the same surface is going to yeild a minimal torque diffreence and the diff will remain locked. I agree that "lots of torque on snow" is not good but you achieve that by not flooring the gas or running in a higher gear, not by the diff destroying energy. Also the torque and the rotation speed are related, just not in a more speed=more torque way. Conservation of energy is still in effect

 

snuck an edit in there while i was replying.

I agree that on snow there would be minimal stress on the drive train. That's not what i said. I said that if you ran in snow mode on grippy road and then forced the front and rear wheels to spin at different speeds (turning), something is going to give. On snow the tires will willingly slip with minimal force so it's not very dramatic. On dry ground the forces requried to make the tires slip are much higher and doing so is much more dramatic. It's easier on the drivetrain to not force it to deal with such high force levels and let the acd run at a lower pressure.



ok that's it I'm out

 

Oldie but still a goodie.

 

Yeah at this point I think we are getting into the definition of what "is" is, you know. The torque IS lost. I think we are again saying the same thing 2 different ways. No traction = no torque. Great conversation.

Direct from HowStuffWorks.com:

"The open differential always applies the same amount of torque to each wheel. There are two factors that determine how much torque can be applied to the wheels: equipment and traction. In dry conditions, when there is plenty of traction, the amount of torque applied to the wheels is limited by the engine and gearing; in a low traction situation, such as when driving on ice, the amount of torque is limited to the greatest amount that will not cause a wheel to slip under those conditions."

Perhaps this quote even needs some adjusting. Instead of saying "applied to the wheels" we could say "applied to the ground THROUGH the wheels" and we'd both be on target.

 

The fact we didn't get the SAYC sucks. Seeing the likely reason in black in white still chaps my azz to no end.

Our car is nearly 200 lbs heavier than the Japanese cars because of this extra emissions equipment and the heavy-as-lead bumper beams too.

A guy nearby has an Evo VII RS2, and it has super narrow Recaros. I would love to drive that car just to see what early AYC in the VII felt like.

I realize that we can upgrade way beyond what SAYC will offer through suspension mods, but Evos overseas are faster than ours are (Top Gear, etc).

 

Terry S - where did you find the technical article? do you have more articles of that ilk about other systems topics on the evo? shoot, other articles like that, technical bulletins, maintenance manuals, etc would be awesome if you have them.

 

I made it based off of my own research and understanding of the topic.

I was working on a MIVEC one... Maybe i'll finish it up someday.

I'm also working on something else (it's kinda hush-hush) but for another forum.

Terry S

 

Another question, is it safe for the ACD to make a hand brake turn or is it going to cause premature wear on the ACD.

The article says the hand brake opens de ACD, but do you recomend not to lock the rear tires on the autocross.

 

YES

YES - you will be faster NOT doing a handbrake turn.

EVOlutionary