Yep, you're right.

 

The difference in the rotation speeds of the front portion of the viscous coupling vs rear is a percentage value of the vehicle speed based on tire size (if you ignore any tire slippage,). So if the tire rotations created a 1% difference then at 100 rpm drive shaft speed, the differential rotation (difference between front and rear couplings on the viscous coupling) would be 1 rpm. At 1000 drove shaft speed the differential would be 10 rpm. So the torque transferred in straight line normal driving is directly related to vehicle speed. Now if to spin the rear wheels the rpm differential goes up dramatically. This is what people refer to when they feel the front pulling when the rear steps out.

 

I think you misunderstand how the viscous coupling works. It requires a speed differential between the front and rear wheels to work as intended (which is why the front tires on these cars are slightly taller in the front than in the rear, that was on purpose). As the speed of the car increases (and thus the axle speed differential), the amount of "slippage" in the coupling increases, raising the temperature of the fluid and increasing the amount of torque transfer, thus adding more "pull" to the front tires. The faster you go, the more torque there is transferred to the front wheels. Rear wheel slippage will also cause this to happen.

 

I think you are both correct. If the car is driving in straight line then it might take until 150 mph to for the AWD to fully kick in on the front axle, but once you start adding yaw to the equation (turning) full power can get sent to front at very low speeds. At east that is my experience..

 

Seems to be an old wives tale of wanting taller front tires on this car. I've never, ever seen it written anywhere including factory books and would love to see it if I may be wrong. AWD cars should have front/rear tires matched as closely as possible. There's a slight variance allowed, hence the % of allowable difference, but always they should try and be matched.

Mechanical diffs are simple. I think the AWD system as simplistic as it is, works fine for it's purpose. Heating the fluid ( difference in speed between front/rear wheels) causes the discs to engage and activate the front wheels getting more power and thus grip.

 

The OEM tire specs speak for themselves. Front tire is slightly larger diameter than the rear. The 959, which has a similar AWD system, has the same slightly larger front.

 

Revs per mile is still slightly less on the rear. The sizing speaks more to what's available to match as close as possible than anything else.

 

The revs per mile are designed to be slightly higher on the rear so that there always a little torque to the fronts. If it is there are more revs per mile on the front than the rears, as the rear starts to slip there is a transition from slight drag on the fronts to applying torque to the fronts. I had a set of the Contisport DWS that had the stagger the wrong way (fronts smaller) but still within the 3%? recommendation. It had significantly more PSM interventions and had a squirreley feet at the limit as i transitioned in to the power. I put a stock size set of PS2s on and the car never felt better. i have since replaced only the PS2 rears and not the fronts and it is a little weird as i think the stagger is off.

 

Quick search revealed this overly technical treatise on the subject, but once you swim past all the differential equations, you get this fairly simple explanation of the type of viscous coupler Porsche utilized...

on page 17:


Front wheel speed to rear wheel speed less than 1:1 = front wheels rotate slower than rear, which means front wheels have larger circumference and diameter than rear.

And theoretically, if you had exactly matched circumference front and rear tires and no additional slippage from traction or turning, there would be zero torque transfer to the front.

If I'm interpreting this correctly, slightly taller rears would also cause the speed differential and create torque transfer to the front during straight line, full traction driving, but at small losses of rear traction (up to the percent that the rears are larger), it would actually reduce the torque transfer to the front instead of increasing it...

 

Didn't the 959 use computer-controlled clutches to continuously vary torque distribution? If so then the tire size difference doesn't seem to be a factor in its AWD system.

 

The sizing was designed so the front would larger making the AWD system work properly. Same as the 959.

 

On the 959 the system could variable or fixed via a selector ****.

https://www.roadandtrack.com/car-cul...e-porsche-959/

This is along video but at about the 15:00 mark they talk about the AWD system and why the tires are sized as they are.

 

What you felt is accurate.

With larger rears the front diff is unloaded, getting no torque, so when the rears slip it takes a millisecond for the torque to surge to the front. With larger fronts the front diff is always torque loaded, ready to immediately react to the rear slipping.

And the PSM is the set up for larger diameters fronts too. I also have had PSM issue when just replacing the rears tires.

 

https://www.ukcar.com/features/tech/.../activedif.htm

 

I might think you mean larger rears, as in larger rears than std. sizes so they are equal to the fronts. But if you were to have rears that are even larger than the fronts again you will load up the clutch plates in the VC so you create a drag in the front wheels. As the multiple clutch plates generate heat when there is difference in the input to the output shaft the silicone oil increase their viscosity rapidly, and the degree of solidity will depend on the temperature generated by the axel differences. There is no difference in what axle that is fastest, it will generate heat. It is much like the syncro system from VAG. They changed to Haldex about 2000, but before that they used a similar setup, but then without the difference in wheel speed. A good VC on those can be seen on camera locking up completely within 90-180 degree of wheelspin. Another way to make a difference in the VC axles is with gear ratios, then you can use equal tires F/R.

I wonder if anyone has more info about the different gear ratios on manuals vs. tiptronic S, as it is stated in the 996 bible. Is it true? What effects on 4wd was intended?

Btw I think the lockup on the 997tt front diff is PWM, so not going to work with just ramping the voltage. Someone did do some tests on controlling this some years ago, but I could not get any more answers in that post.