Understanding the Porsche AWD system

[Malakas]

RennTechers,

So I've searched (largely in vain) to read an in-depth description of the P-car AWD system and how it works with regard to traction and PSM.

I get the whole viscous fluid thing, but I don't really get the sequence of events... for example... let's say you drop the clutch from a start: the rear wheels spin, which (simultaneously) thickens the "fluid" driving more power to the front wheels AND activates the PSM to control the wheelspin?? If this is true then, you should be able to turn off the PSM and do a smoking launch, because it should take at least a second for the front wheels to "get power" - meanwhile the back ones have laid a little rubber. However, I have been told doing such an action in my 4S will result in a clutch that explodes.

Seeing that the center differential is not mechanically controlled, there should be a little lag to the front wheels getting power, no?

If on the track, you begin to slide the car... the PSM kicks in AND the front wheels get more power... but doesn't the PSM cut the power... confusing...

I ask because Im nearing my first autocross (in the c4S) and want to understand what will happen if the car goes into a drift, and/or how to launch the car at the start.

Thanks to all!!

[JeTexas]

Is the autocross here in Houston? If so, where?

And I have zero understanding of the AWD system.

[Malakas]

Is the autocross here in Houston? If so, where?

And I have zero understanding of the AWD system.

Check out this site http://www.lsrpca.com/Calendar/calendar.htm.

I've run before at GGP in my Honda S2000. Pretty large parking lot - the last course was nice... if its not crazy hot / raining / the baby is not screaming (aka wife is happy), Im going to try and make it.

[JeTexas]

Is the autocross here in Houston? If so, where?

And I have zero understanding of the AWD system.

Check out this site http://www.lsrpca.com/Calendar/calendar.htm.

I've run before at GGP in my Honda S2000. Pretty large parking lot - the last course was nice... if its not crazy hot / raining / the baby is not screaming (aka wife is happy), Im going to try and make it.

We used to X at the old airport Riverside Annex outside of College Station when I was in the Texas A&M Sports Car Club. Tons of pavement out there, but I haven't kept up with any of that stuff.

[judgejon]

RennTechers,

So I've searched (largely in vain) to read an in-depth description of the P-car AWD system and how it works with regard to traction and PSM.

I get the whole viscous fluid thing, but I don't really get the sequence of events... for example... let's say you drop the clutch from a start: the rear wheels spin, which (simultaneously) thickens the "fluid" driving more power to the front wheels AND activates the PSM to control the wheelspin?? If this is true then, you should be able to turn off the PSM and do a smoking launch, because it should take at least a second for the front wheels to "get power" - meanwhile the back ones have laid a little rubber. However, I have been told doing such an action in my 4S will result in a clutch that explodes.

Seeing that the center differential is not mechanically controlled, there should be a little lag to the front wheels getting power, no?

If on the track, you begin to slide the car... the PSM kicks in AND the front wheels get more power... but doesn't the PSM cut the power... confusing...

I ask because Im nearing my first autocross (in the c4S) and want to understand what will happen if the car goes into a drift, and/or how to launch the car at the start.

Thanks to all!!

I think I understand the question, but I have no clue as to the technical answer.

In my own experiences, when I ran Pirelli PZeros for a limited time after getting the car, the psm did not come on. I made the mistake of not changing all the tires and put Potenza 050's on the back. the psm started coming on a lot. when it comes on in a curve its just **** distracting and feels scary to me. its gotten better since, but for the curvy mountain driving I often do I just turn it off.

When it kicks in I get a jolt as the wheels seem to lock up. It seems to me its the rear tires. I've had recent shoulder surgery and I really feel it in my shoulder when that happens. If I have the psm off and brake in a curve the abs comes on, and thats also pretty distracting, but that cant be turned off as I understand it.

so I think my problem is probably the tires. and my driving is hold-over from my older car and I'm still exploring the awd system capability (within reason that is) and that could also have something to do with it.

I'd have to say autocross with the psm off and find out what the car can do. I'd be interested in other answers to this thread and to your autocross experiences going both ways. Cheers, ....

[judgejon]

wow...this is a great thread... http://www.renntech.org/forums/index.php?s...amp;#entry58986 .. . it's noted other current awd tread.

Edited July 2, 2008 by Loren
Corrected link

[Malakas]

judge,

I think your link is broken... it just sends me back to the main forums. ( i want to read, I want to read)

I will definitely post about the autocross experience - and I know what you mean (first hand) about the PSM being a little distracting when it starts to intervene. To my (albeit simple) brain, PSM and a RWD C2 make sense, but Im just not sure how it is going to help you on an AWD car unless the PSM is mostly stability (when the arse is WAY out) more than traction control.

[JeTexas]

I think he meant this one:

http://www.renntech.org/forums/index.php?s...amp;#entry58986

[Loren]

I think he meant this one:

http://www.renntech.org/forums/index.php?s...amp;#entry58986

Um... I fixed his link above.

[Malakas]

Arg... its still sort of confusing.

But from all that I read I surmise that: the AWD system is probably most pronounced during

1. cornering: the car is rotated for several seconds, allowing the front/rears to rotate at different speeds therefore increasing the "thickness" in the VC and routing some power to the front to "pull" the car toward the exit (instead of the armco)

2. high speed cruising: where the different tire sizes (and their relative differences in friction) cause different rotational speeds... and... oh wait, they cant spin at different rates... oh crap, Im confused again...

[wwest]

Personally I have strong reason to believe the VC in my '01 C4 is there for "show" only, marketing purposes.

I believe I have read somewhere that disabling, turning off, PSM, does not disable PSM's Traction Control, rear "virtual" LSD, etc, aspects. If that is true then our VC can NEVER come into "play" and basically we C4 owners have been sold a "load of poles".

But then that would be my second time.

My '01 AWD RX300 has a VC. But it also has VSC & TC. My RX300 is primarily FWD and my experience has been that the instant front wheel slippage is detected the front wheel brakes are applied, BOTH front wheel brakes, and the engine is simultaneously dethrotted via EFI fuel stavation. So there is NO opportunity for the VC to come into "play", EVER..!!

Which is undoubtedly why the VC was dropped from the Toyota/lexus F/AWD models subsequent to the RX300 series.

[Malakas]

Interesting article regarding awd systems

Wwest,

it must be setup so that the response of the diff is fairly quick, and the PSM must be set to wait until really crazy slip angles are detected before it reigns in the fun. Otherwise you are right - the VC would never come into play if the TC stopped the very thing that activates the diff.

[wwest]

Interesting article regarding awd systems

Wwest,

it must be setup so that the response of the diff is fairly quick, and the PSM must be set to wait until really crazy slip angles are detected before it reigns in the fun. Otherwise you are right - the VC would never come into play if the TC stopped the very thing that activates the diff.

PSM activation...

My personal experience seems to indicate that PSM's stability control aspects detect the "out of whack" condition immediately and then "stand-by", wait for a few hundred milliseconds, for the driver to take, make, corrective action/measures and if not then will use the FULL-FORCE of measures (engine dethrottling selective braking/unbraking, etc.) to reign the matter in.

VC response MUST be delayed otherwise it would/might stiffen too quickly and provide a serious level of driveline windup and/or wheel hop/scrubbing in a tight accelerating turn. Something it MUST NOT do.

A bit in the extreme but think about that circumstance in a 4WD/4X4 with the center diff'l locked.

Broken thumbs, anyone..???

[judgejon]

Interesting article regarding awd systems

Wwest,

it must be setup so that the response of the diff is fairly quick, and the PSM must be set to wait until really crazy slip angles are detected before it reigns in the fun. Otherwise you are right - the VC would never come into play if the TC stopped the very thing that activates the diff.

PSM activation...

My personal experience seems to indicate that PSM's stability control aspects detect the "out of whack" condition immediately and then "stand-by", wait for a few hundred milliseconds, for the driver to take, make, corrective action/measures and if not then will use the FULL-FORCE of measures (engine dethrottling selective braking/unbraking, etc.) to reign the matter in.

VC response MUST be delayed otherwise it would/might stiffen too quickly and provide a serious level of driveline windup and/or wheel hop/scrubbing in a tight accelerating turn. Something it MUST NOT do.

A bit in the extreme but think about that circumstance in a 4WD/4X4 with the center diff'l locked.

Broken thumbs, anyone..???

this explains the jolt I feel in my shoulder when the psm kicks in. M, if the awd didn't generally work as advertised all of us C4 owners would likely be involved in one massive class action lawsuit. I'll give you a full report when I'm back to running the same rubber all around. till then, I guess I'll be turning the psm off on twisty roads.

ps: like we've talked about before, the color of your car is outstanding. best...j

Edited July 7, 2008 by judgejon

[wwest]

Interesting article regarding awd systems

Wwest,

it must be setup so that the response of the diff is fairly quick, and the PSM must be set to wait until really crazy slip angles are detected before it reigns in the fun. Otherwise you are right - the VC would never come into play if the TC stopped the very thing that activates the diff.

PSM activation...

My personal experience seems to indicate that PSM's stability control aspects detect the "out of whack" condition immediately and then "stand-by", wait for a few hundred milliseconds, for the driver to take, make, corrective action/measures and if not then will use the FULL-FORCE of measures (engine dethrottling selective braking/unbraking, etc.) to reign the matter in.

VC response MUST be delayed otherwise it would/might stiffen too quickly and provide a serious level of driveline windup and/or wheel hop/scrubbing in a tight accelerating turn. Something it MUST NOT do.

A bit in the extreme but think about that circumstance in a 4WD/4X4 with the center diff'l locked.

Broken thumbs, anyone..???

this explains the jolt I feel in my shoulder when the psm kicks in. M, if the awd didn't generally work as advertised all of us C4 owners would likely be involved in one massive class action lawsuit. I'll give you a full report when I'm back to running the same rubber all around. till then, I guess I'll be turning the psm off on twisty roads.

ps: like we've talked about before, the color of your car is outstanding. best...j

There would be no basis for a lawsuit, the AWD system works as advertised....

I would imagine the marketing hype goes something like, along the following line.

"if the rear wheels lose roadgrip/traction the front and rear driveline rotational rate will differ and the VC will stiffen up accordingly and thereby increase the engine torque to the front wheels.."

And if TC, Traction Control, always activates "first" and thereby prevents any significant level of traction loss, roadgrip, at the rear, what, then...??

On the other hand I would imagine a closely coupled series of hard/tight accelerating turns (autocross anyone..??) would result in the VC stiffening and putting more torque to the front.

[wwest]

And this "color"..??

[Malakas]

Which "color"??

[judgejon]

And this "color"..??

Malakas has a beautiful, imho, orient red 996...

re the awd, this is one of the reasons I have a C4S. I'd driven a 993 and 997 and could feel, I think anyway, the pull of the front end coming out of several turns. I think I'm getting that same "feeling" now. I like the idea of the 5% minimum to 40% max of the front end. I dont think its just mental... but I've never put the car up on jack stands or roller stands to test the system.

And you're also really right re the advertising. If the awd system did not work as advertised by Porsche, my God, can you imagine the repercussions, all the C4 owners from the very first would be up in arms.

This is a great thread.... good info...thanks. jl

[wwest]

And this "color"..??

Malakas has a beautiful, imho, orient red 996...

re the awd, this is one of the reasons I have a C4S. I'd driven a 993 and 997 and could feel, I think anyway, the pull of the front end coming out of several turns. I think I'm getting that same "feeling" now. I like the idea of the 5% minimum to 40% max of the front end. I dont think its just mental... but I've never put the car up on jack stands or roller stands to test the system.

And you're also really right re the advertising. If the awd system did not work as advertised by Porsche, my God, can you imagine the repercussions, all the C4 owners from the very first would be up in arms.

This is a great thread.... good info...thanks. jl

Who has said, what has been said, indicating the AWD systems does not work as advertised..??

My '01 AWD RX300 has a VC mounted across the center differential but it NEVER comes into play because Trac activates the instant wheelspin/slip (front wheelspin/slip for F/AWD) is detected. Since the adoption of Trac made the VC virtually useless it was discontinued for the later RX models, the RX330 and RX350 and is not used on the Highlander or Sienna which have the same AWD system.

Sort of like some folks putting a mechanical LSD in a 996 that has PSM.....

[Malakas]

And this "color"..??

Malakas has a beautiful, imho, orient red 996...

Thanks! I love the color more every day I have the car (its having the ignition replaced currently).

Well, there has to be more depth here... I wish there was a PCNA engineer we could elicit an opinion from. The basic crux is this: if the AWD system needs slip, and PSM stops slip, then there is an issue.

Is it possible that the sequence is this: (1) rear wheel slip -> VC increased viscosity -> front wheels get power -> car moves forward ... no PSM intervention

-or-

(2) rear wheel slip -> VC increased viscosity -> front wheels get power -> rear wheels continue to slip -> PSM traction control intervenes

[wwest]

The answer is....

You didn't buy your C4 for wintertime adverse road condition driving.

At least I sincerely hope not.

If you expected the VC to be an aid in the above conditions you were sadly mistaken, led astray.

Absent driveline windup, wheel hop, or extreme tire wear VC implementations simply cannot have a QUICK attack rate, they cannot be allowed to come into play quick enough to prevent wheelspin/slip at startup, starting out or accelerating on a somewhat slippery surface. Traction control, on the other hand, will handle those instances very satisfactorily.

Your C4's VC will stiffen up with semi-continuous disparate F/R driveline rotational rates, just exactly why you bought a STELLAR performing SPORTS car.

And yes, absent TC intervention, the VC would eventually "stiffen up" and help you initially start out or accelerate from a lower speed on that slippery roadbed surface, but by the time it does so most driver's would probably be off the road and into a ditch somewhere.

TC in the "short" term, plus VC, a good hand-in-hand working combination.

Edited July 8, 2008 by wwest

[Malakas]

Wintertime... I grew up in Wisconsin, and had enough winter (minus skiing) to last a life time. No, the C4S is with me now more for the widebody, suspension, and brakes. The AWD was just a bonus, and after driving the wife's A4 3.0Q, I started to believe in AWD stability in poor weather.

Well, I am soon taking to the track so we will see how the car fares there. I was hoping to autocross last weekend, but we have a 7wk baby at home - he (and mom) deserve my attention first so...

I was hoping to run with both PSM on and off to see the difference... We'll see next time.:)

[wwest]

Wintertime... I grew up in Wisconsin, and had enough winter (minus skiing) to last a life time. No, the C4S is with me now more for the widebody, suspension, and brakes. The AWD was just a bonus, and after driving the wife's A4 3.0Q, I started to believe in AWD stability in poor weather.

Well, I am soon taking to the track so we will see how the car fares there. I was hoping to autocross last weekend, but we have a 7wk baby at home - he (and mom) deserve my attention first so...

I was hoping to run with both PSM on and off to see the difference... We'll see next time.:)

You can leave PSM on and it will not "interfere" unless your seat-of-the pants "yaw" sensor is out of "calibration", or if your reflexes are simply too slow.

Also, you could/can probably "initialize", "condition", your VC for more optimal autocross performance by driving aggressively, just to the edge of having PSM activate, in a turn circle about 4 cycles just prior to running the course.

Edited July 9, 2008 by wwest