For daily speeds the wear is probably slow. My car frequently saw sustained speeds of 160-170 before it failed. Fortunately it was only going 90 when it blew out.

All legally of course.

 

HI..Interesting....If I may ask,where did you get or learn that??
The reason I ask is because when I leave the line hard and also power shift 1st to 2nd but not spinning the tires,my front chirp looking to dig in...And my rear tires are are a bigger diameter by .4''..
Thanks for your info..

 

Once you grasp the simplicity and operation of our very basic mechanical AWD system you will realize that it is absolutely imperative that the rear wheels turn JUST A TINY BIT faster than the front wheels to have any kind of constant torque transfer from rear to front in a situation of a car being driven in a straight line at a constant speed. Either that or you need to have a different front and final drive ratios but in our case of the 996tt they are both the same at 3.44 according to Porsche. With that said, diameter "delta" between the front and rear will determine the amount of torque transferred from rear to front (the smaller the diameter of the rears relative to the fronts, the more torque will be transferred to the front). Obviously you can't go with too great a difference or you will overheat the silicone based fluid in the viscous coupling due to constant slippage. Most people who have run SLIGHTLY larger tires in the rear are fine with regards to damaging the viscous coupler if they stay with in the variance mandated by Porsche and they probably can't tell or "feel" the small amount of negative torque (driveline drag) which is created by the front wheels. You can think of it as your front brakes dragging just a tiny bit. The larger the rear tires get relative to the fronts the more torque you transferring from the front to rear and vice versa. To put in another way, if both your front and rear wheels were the exact same diameter than you would get zero torque to the front unless the rear wheels began spinning at which point a certain amount of power (up to 30%) would get transferred to the front wheels.

To look at this from the point of the viscous coupler, when the front wheels are smaller then the rears (as in a 235/35/19 and 305/30/19 combo) they will obviously spin faster then the rears. When the fronts spin faster then the rears then they drive their respective vanes within the viscous coupler faster than the vanes connected the the cardan shaft / rear drive. In essence, the front wheels are transferring torque to the rear through the viscous coupler. This happens until the rears start rotating faster due to slippage for example. An easy analogy would be this: You have a pipe with a motor on the front of the pipe turning a propeller midway in the pipe facing another propeller connected to a motor at the back of the pipe. If the front motor spins the propeller faster then it will push whatever "medium" is inside past the slower turning propeller in the rear. The front wheel drive portion of our AWD system is incredibly basic when you really grasp the simplicity of it's operation. Hope this helps....

The only limitation that Porsche puts on the AWD system viscous coupler is the 5mph difference between the rotation of the front and rear wheels which works out to 21 revs per mile difference of the front to rear axle at 180mph. This limitation most likely has to do with the overheating of the temperature sensitive silicone fluid due to rotational differences of the input/output vanes in the viscous coupler.

Here is a good read that explains the AWD system on our cars on page 257-259:

http://books.google.com/books?id=Raa...eather&f=false

 

Lots of great info! Subscribed!

BTW I am running 225/40/19 and 295/30/19 without any problems!

 

so whats the proper setup to run in 18" if someone wanted to be no smaller thean 315 in the rear?

 

Spec Sidewall Radius Diameter Circum Revs/Mile Difference
225/40-18 3.5in 12.5in 25.1in 78.8in 804 0.0%
235/40-18 3.7in 12.7in 25.4in 79.8in 794 1.3%
295/30-18 3.5in 12.5in 25.0in 78.4in 808 0.0%
315/30-18 3.7in 12.7in 25.4in 79.9in 793 1.9%

on either an 8.5 or 9 front wheel and 12 rear.

 

thanx. now if i could just find them in something with treadwear more than 220!!

 

Here is a tid bit of info..If the tire spec for diameter is given on a certain rim size..
It will be a smaller diameter on a wider rim and a bigger diameter on a narrower rim..
I HAVE measured this many times on my tire installs..A .5 " width can make a .25" diameter difference..
Just sharing my findings

 

thats really interesting. considering the gt2 setup is on a 12" rim yet every brand that supplies the 315/30-18 says the rim width maxes out at 11.5" so on a 12" it should stretch the tire

 

Every tire is different. A Nitto NT01 in a 315/30/18 fits perfectly square on a 12" rim while a Michelin will be stretched a bit. NT01s run wide, a 315 is more like a 325..... A 235/40/18 NT01 will also fit perfectly square on a 9" rim while a PS2 will be stretched..

 

i do realize that but my point was oem is a pirelli p zero which calls for a 11.5" rim and yet porsche runs it on a 12" rim against the recommendations of the tire manufacturer.
i have only found about 6 tire sets that fit the 315/235 combo and they are basically split between the 100 treadwear and the 220 treadwear. michelin, pirelli,kumho,falken,nitto,toyo. any others? for me im leaning towards the pirelli

 

personaly i liked the oem pirelli's.. a lot. though i have stayed with the ps2's nearly religiously since nearly crashing while ( ahem ) scrubbing in some h'kook v12's. ( never again ).

me, i would go with any of these: michi's/pirelli's/b'stones/yokos ( i'm leaning toward yoko advans when i need new front AND rears myself ).

up to now, i've stayed with the ps2's simply because i think those are the best made of the sample group. but i have NO data to back that up, AND they ain't cheap, obviously.. just my experience with the 315/30 size. as for the choices you've got, i wouldn't worry about any of those on a 12" wheel. too many of us have run them for too long without issues.

in my uneducated opinion, those are THE sizes ( the 9x12x18 combo ) for this car.

 

So to beat this dead horse again... I'm keeping my 305/30/19 rear tires. what's the best match for the front? I established my 235/35/19s were a NO GO the hard way. Is a 235/40/19 a good match? or was it 225/40/19?

 

^neither.
put a 245 35 19 and you will be ok.
you can also do a 235 35 19 if you want.
it will not toast your front diff.

 

Isn't this what the problem was??? Out of the sizes mentioned, depending on manufacturer and rim size, the 235/40/19 will likely be the best match. Based on pure size, it is slightly taller, which is desirable and has been mentioned before, and well within the safe range. Out of all the sizes mentioned, 235/35/19 is the worst match. My 2 cents having done a lot of the same research myself and currently running 19" rims.