Very simple. When the front wheels are smaller then the rears, 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.
In the case of our AWD system, it is absolutely imperative that the rear wheels turn just a hair faster than the front wheels to have any kind of 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 have different front and final drive ratios but in our case they are both the same so 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. Most people who have run the 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 (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. 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.....

 

Thanks for the detailed explanation - know for the wrench. If what you are saying were true, then why is it that the porsche recommended winter tire sizes are larger in diameter in the rear than in front. Seems that would really be a bad scenario for winter driving when you really want to have a quicker reaction to the front axle. If the axle is running negative torque back to the center diff, that is exactly the opposite of what you would want in snow and ice.

 

With regards to the winter tires, I have no idea. The Porsche winter tire size according to the maintenance manual is 225/40/18 and 265/35/18 which makes the rear 0.2" larger than front. No clue. I was just explaining how the viscous coupler works. I suspect that the torque transferred back to the center diff is probably very small bordering on insignificant. As soon as the rears would slip than the torque would be transferred back to the front. I have seen several instances where stuff was written in the Porsche maintenance manual that has the numbers wrong or backwards. Who knows....

 

It just seems strange to me. Can't a visous couple be designed to provide a specific ratio of power front to rear with fluctuation? I just find it hard to believe that the static ratio is dependent upon tire diameter. Subarus as an example have open diffs front and rear with a viscous center and they are set 50/50 static (non STI of course). I remember reading somewhere that the TT was 5/95 static with up to 30/70 change. - duh it was in that link you provided! Man I'm getting old

 

One thing is for sure, after reading that section on AWD, I have reaffirmed that my 964 C4 is the bomb in the snow. It is too bad they cheapened out with the 996 AWD system and abondoned the 964 system.

 

The only thing I can think of is that Porsche figured drivers wouldn't be doing high speed runs on snow tires and the benefits having narrower snow rears with more bite outweighed the costs (potential less-than-ideal torque transfer)...

On a side note, Porsche recommended putting a 265/35/18 tire on the OEM 11" rear wheels??

Also, the 245/35/19 295/30/19 setup that BOOSTTT just put on his car is very similar (8 rev per mi faster fronts) to the 225/40/18 and 265/35/18 snow tire rec (~7 rev per mi faster fronts depending on brand) that pwdrhound quoted... Maybe it's perfectly safe... (would-be good news )

 

Whew thank God!!! That is some really good info there my freind. Thanks for posting! I can tell the car really feels much more stable now and still no dash lights flashing as B4. The fronts really feel like they are pulling me out of the turns when the rear gets a bit loose. All I have to say is beware that those fronts are like 1-2mm from rubbing, but mine are not. (Lucked out) I really can't belive they do not rub, as it is so close. I feel like I can enter corners much quicker with the meaty fronts. It just feels right to me. Now I need to do something with my old, barely used 235 35's. CraigsList, I guess. Thanks for the info!

 

It's not the width, but the diameter I was commenting on. The previous discussion was around the fact that the rear wheels needed to be slightly smaller in diameter than the fronts for the AWD system to provide static power to the front axle - and not negative torque. My comment was questioning this logic, when in fact, Porsche recommends a winter rear tire size that is taller in diameter than the fronts. Porsche recommends the 265 be mounted on a 10" rim. I have this exact setup.

 

I don't think there is any way to get a definitive answer to this one unless you set up some type of a dyno that would be able to measure the torque produced and the effect of different diameter wheels. At the end of the day, any way you slice it, a viscous coupler must have a difference of speed across the input and output vanes to produce torque. The input vanes (driven by the cardan shaft) must spin faster than the output vanes (front diff) in order to transfer torque from rear to front. There is no rocket science here. Our AWD system is very basic and purely mechanical without any electronics involved. WITHOUT the rear wheels slipping, this difference of speed across the vanes in the viscous coupler must be as a result of either different final drive ratios at the front and rear wheels (which is not the case for us as the 996tt has a 3.44 ration both front and rear according to the Porsche maintenance manual, assuming what is printed is correct) or a difference in tire sizes (smaller in rear will cause the input shaft to spin faster). This seems to be the route Porsche has chosen and this is the reason why as speed of the car increases, there is a corresponding claimed increase in the amount of torque transferred to the front (the greater the speed difference in the input/output vanes, the greater the torque transfer). I may be completely wrong here but if anyone has a better explanation I'd love to hear it....

 

Geez I just read all this and I run the 305/30/19 on HRE and 245/35/19 Toyo R888's.

Sorry to bump an old thread, but I swapped from 235/35/19 and 315/25/19. The 305 set up doesn't rub, and the car feels way better. My ET's are perfect so the tires will fit and I have coils so no rub. Smash corners and can feel way more grip. I think this is all in people's heads.

 

i just put on 305/30/19;s with eibach lowering springs fit like a glove with rolling the fenders

 

What are your fronts? Any mods? Issues?

 

Do you love the R888?

 

I do! They are pretty awesome.

 

305/30/19 & 245/35/19 Toyo R888 stickest tire. Camber slightly adjusted although stil in green. Car lowered 2 inches in rear and 2 1/4 front. No rubbing. Have coils on stiffer setting and eibach sways. Best combo I have had. The 25 sidewall is just too skinny for the power, especially if you have tune and all that good stuff. 235/35 is also not wide enough.