"02996ttx50

i ran, as have others, a 315/30 on a stock twist without issue for quite some time.
re the pss; i've also run the PSS in a 235/40 before.

http://www.discounttire.com/dtcs/tir...LCINT&pc=35523

Berra

AWD works just fine. Had my car out from the garage the other day and had no trouble driving back in. I couldn't do that with my M5. PS2's on both cars, on snow...but one of them is AWD and that system got the car back in.

For those of you that seem to have problems with it might wanna check your front diff.

Carlo_Carrera

I think it is just semantics. To me the car does fight itself. The coupling is fighting itself. Trying to send power to engine and then having to reverse that flow when the rear wheels begin to slip.

996tnz describes it well in post #58.

Running larger rears than fronts has that preload reversed. When the rears start to spin up, the front diff first has to pass through a neutral point where the front and rear axles are at the same speed before starting to engage again and transfer torque forward. Which would normally delay the response. If the rears are much bigger, the front diff is already preheated beyond normal levels so the momentary loss and then a - delayed but sudden - ramp up of torque transfer to the fronts jerks the car around much more than the factory intended. Not ideal in snow perhaps, but much more of a problem if tracking out on the limit on track when the rears start to spin up.

strathconaman

Wait...what 225/45r18 tire are you running up front? And how did you get it to fit? That's a 26inch diameter tire. When I look at my front wheel well, 26" would rub the liner in a number of spots...

Dock

What's the difference in weight distribution between those two cars?

Berra

49/51% for the M5 according to BMW and I think the 996 Turbo is at 62/38%. M5 is about 800 lbs heavier though and has LSD, the Turbo doesn't.

Dock

My guess is that the +11% difference in rear weight bias in favor of the 996 Turbo is the real answer as to why it as better traction on your driveway than the M5.

Berra

Yes might be, but I would think that the difference of 800 lbs in the M5 would somehow give it better traction, plus having a LSD.

Anyhow, this "issue" won't affect many Turbo owners as most of them don't drive during winter and if they do, make sure to have the front diff checked.

I have however a hard time seeing how this system will send 40% of power to the front, but it definitely does work:


Some videos show that the fronts are basically dead which tells me that the system isn't working properly, maybe due to leaking seals.



Here is a 993 Turbo:



My take on this is that the fluids aren't changed at all/or heated up to the point where it breaks down and thus can't make the proper transfer, or seals OR the diff itself.

Anyone care to read, please do, good info on how this system works, and reasons when it stops working:

http://www.syncro.org/VCTest.html

kmagnuss

I didn't see a front tire spin in one of those videos.

greencalex

Yeah, people really have to keep in mind that VC systems don't last forever; all the more so in cars that are "driven hard" frequently (because of more heating cycles of the fluid).

Buying most older "AWD" cars tends to be doggy for this reason; folks with "4WD" systems (the ones that you have to explicitly lock) like to make fun of the SUV crowd, because their systems stand a good chance of not really working, and they just don't know it...

Berra

English is my third language but I'm pretty sure I was very clear with what I wrote. First video shows front tires spinning, the rest don't.

I also wrote as to why that might happen IF the front wheels won't move.

Carlo_Carrera

There is a really simple way to check if your VC is still funtioning.

Put the car on a lift or jack stands so all four wheels are freely hanging and spin the tires in the forward direction one by one. If you do this and the other three tires begin to rotate your VC is working fine.

I check mine four or five time per year when my car is being tech inspected for the track. So far my VC is working as the Porsche gods intended.

Third-Reef

Another method to check the VC, Jack up the rearo with the wheels off the ground and a floor jac under the cross member on flat concrete. start the car, psm off let out the clutch carefully. you will see that the VC transfers some torque to the front wheel as the front wheels pull the car forward on the floor jack. the key to the 996 AWD system is "some torque". it does not ever light up the fronts.

Freddie Two Bs

An interesting experiment would be: same brand of tires, two pairs with slightly different size mismatches, one with front 2â„… taller and one with rear 2% taller. Drive with a full tank of gas for 10 mins at the same constant speed. The 2% taller rear setup should burn more gas if the front wheels are fighting the rears.

kmagnuss

English is my first language... don't be a d*ck for no reason. At what time in the first video do you see a tire spinning... because as I said before in my native tongue, I don't see it.