So I was reading something last night that implied that Porsche, when they changed the AWD system from the 964 to the 993, accomplished the 15% constant front drive by means of slightly smaller rear tires vs. front. Does that mean if you go even smaller rears ( say 30 series rear 40's front), that we can induce even more front drive at all times?

 

Theoretically, yes. But I would be afraid of inducing too much heat into the VC and confusing the ABS system.

 

Isn't the VC always in motion anyway? Sorry, I don't quite have a clear understanding of our awd system. How would you confuse the ABS? Isn't that based on sensor speed to brake pressure?

 

The ABS system compares all four individual wheel speeds while on the brakes. This is why I mention ABS trouble.

Yes, the VC is rotating while the car is moving and 'adjusts' based upon the difference between the front and rear wheel speeds. As an extreme example, if one end of the car is spinning its wheels, and the other is held stationary, it will overheat the fluid inside the VC and burn it up. I am not certain what is too much difference that will cause damage..

 

Yeah, your extreme is my understanding.....which is why one needs to make sure to use a 4 wheel dyno! Do you really think a slightly smaller than stock tire would overheat the VC? The theory of different wheel speeds is the basics to how the system seems to work. And under decel load is when the VC is working the most.....correct? I wonder if you could induce a little more front end, would the car push less, handle better....so on?

 

Actually the opposite. While there might be some coupling on decel, it is designed to couple on accel to enhance traction. For the same reason the front diff is not of a limited slip variety, front tires need to freewheel upon decel for best steering balance.

 

I thought the different wheel sizes was to insure that a certain minimum percentage (5%?) of power was transferred to the front wheels under normal driving. Is this correct?

 

Eric I have the same question, and would like to know the answer too. The thread ended with your question, nearly two years ago, so I'm ressurecting it because had an interesting discussion with another well known RL'er who believes that on the AWD cars.\, the front wheels are not engaged during normal driving, but only when a tire (front or rear) begins to spin/slip and thus begins to spin faster than another.

 

The spec tire sizes are 25" diameter rear and slightly larger by 0.1" - 0.4" in front ( depending on brand, model, etc): this indicates that the input shaft to the VC turns anywhere from 0.4% - 1.6% faster than the output shaft ( drive shaft to front diff.) in steady state driving with no tire slippage. This modest difference likely has the AWD versions performing much like the C2 versions in regular spirited street driving .... but with adequate preload on the VC for instant torque transfer if the rear end suddenly gets greasy ...

 

Hi guys, when we go to the technical check (mandatory by law in Belgium) they put our cars on rollers to control the braking action symetry. Of course with a 4 wheel drive, there's a problem as most of the stations are only equiped with a single roller. I don't know where, but I remember having read somewhere that the max allowable speed difference was 6km/h between the front wheels and the rear ones if one or the other are stationary. This could be in the owner manual by the way...but I'm not at home and cannot cross check.
is it the acceptable limit your looking for?

Cheers

 

I am just a newbie here as I took possession of my first Porsche yesterday (95 993C4) and haven't had time to post pictures or get past "user" status to "member" status. In reading Adrian Streathers book (Porsche 993 The Essential Companion) he states that" Porsche does not employ any deliberate methods to ensure some torque is permanently sent to the front wheels. Under normal driving conditions all the drive is at the rear wheels."

He further states "There will be some natural torque transfer to the front wheels coused by:
1. Excessive transmission temperature.
2. Changes in wheel rolling diameter relationships.
3. Weight distribution changes.
4. The rear engine configuration which tends to push the front wheels causing them to rotate slightly slower than the rear wheels (nose down attitude).
All these issues are not only extremely variable and even seasonal, they are subject to changes carried out by the owner (suspension changes).

"In pure technical terms, the Dynamic 4-wheel drive system is only active when traction is lost at both rear wheels".

I couldn't find specifics on tire diameter changes and their affect on the viscous coupling system other than the fact that with 18 inch wheels and rear tires there is a slight difference of 4.5 mm, 0.177 inches) in rolling diameter. The rear wheels being slightly smaller will rotate faster but this is not enough of a difference to provide permanent torque transfer.

Hope I haven't muddied the waters.

 

Wow Dan - this is an old thread. I forgot all about it.

I still don't know the true answer, but interestingly I just read a reprint of a Jan 1995 R&T article on the '95 993 C4. It states that power diversion to the fronts is up to 39% on ice or in snow, 5% to 25% in dry or damp conditions. This seems to imply that there's 5% always going to the fronts in normal (dry) conditions. But, that also seems to contradict Adrian Streather's statement posted by Terbil.

The mystery continues...

 

If I may further muddy the already muddy waters:

A viscous coupling by design transmits little or no torque until it reaches a certain temperature. Then it locks up quite quickly over a narrow temperature band. So I'm not sure it's accurate to say it transmits some contant, small percentage at all times.

 

I'm in the process of buying tires that are outside of the recommended specs. I posted here and read up on the effects of introducing different diameter tires with different RPM numbers front to back to ensure I'm not asking for trouble. What I've found indicates that so long as the variance in either overall diameter or PRM front to back is within ~4%, there is no significant impact on the VC. It doesn't answer your question, but its one more data point in the same vain and at least it shows that the VC may not be overly sensitive to changes in the car's configuration.

 

I read up on the AWD system last night and came upon information that very explicitly states it is, in fact, a permanent AWD system, so Eric is correct. However, Bret (terbil) is almost correct in that it is effectively a RWD because 95% of the power is in the rear wheels when accelerating in a straight line. The system employs variable torque slip that can send up to 50% of power to the front when the rears start to loose grip. It also has variable slip rear differential that transfers power from one wheel to the other for optimum side to side grip (though I believe this is an option) and lastly, an automatic brake system that brakes a spinning wheel up to 40mph.

Further to this, I remember seeing a video on the 993 where one of the drivers being interviewed mentions that one of the benefits of the AWD system is that power transfers to the front as the car reaches higher speeds, thereby providing increased stability.