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996TT Discontinued Tire Sizes & AWD Implications

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Old 12-02-2016, 08:45 AM
  #61
"02996ttx50
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Originally Posted by jumper5836 View Post
^Super Sports are not made in that size, as well 315 require a 12" wide wheel.
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
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Old 12-02-2016, 09:36 AM
  #62
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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.
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Old 12-02-2016, 11:31 AM
  #63
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Originally Posted by T10Chris View Post
I know how VC units work as the link describes, but I have seen other people say that it causes braking, or makes the car fight itself, or other similar things in many posts and I don't see how that is possible...

All I can see it doing is removing the preload/being 100% RWD drive bias until there is tire slip from the rear for AWD to kick in?

Maybe it is just semantics/phrasing that is tripping me up.
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.
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Old 12-02-2016, 11:52 AM
  #64
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Originally Posted by Third-Reef View Post
I have been running with larger rears for a year now, about 13k miles and living to tell about it ( 225 45 18 front and 295 35 18, came on the car) it did good on my track day at Laguna going to change to stock size micelin PSS soon and i will report back with any significant changes.
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...
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Old 12-02-2016, 11:26 PM
  #65
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Originally Posted by Berra View Post
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.
What's the difference in weight distribution between those two cars?
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Old 12-03-2016, 09:15 AM
  #66
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Originally Posted by Dock
What's the difference in weight distribution between those two cars?
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.
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Old 12-03-2016, 10:29 AM
  #67
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Originally Posted by Berra View Post
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.
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.
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Old 12-03-2016, 12:42 PM
  #68
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Originally Posted by 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.
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
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Old 12-03-2016, 03:40 PM
  #69
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I didn't see a front tire spin in one of those videos.
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Old 12-03-2016, 03:52 PM
  #70
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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...
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Old 12-03-2016, 04:18 PM
  #71
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Originally Posted by kmagnuss
I didn't see a front tire spin in one of those videos.
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.
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Old 12-03-2016, 06:57 PM
  #72
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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.
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Old 12-03-2016, 07:37 PM
  #73
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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.
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Old 12-03-2016, 10:36 PM
  #74
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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.
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Old 12-03-2016, 11:48 PM
  #75
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Originally Posted by Berra View Post
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.
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.
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