Four-wheel drive on 993 TT, 4 and 4S

[orangecurry]

Just to add, after doing some research on the matter, the difference in tyre 'diameter' as the way the 4WD system works on the 993 is a complete red-herring. Data: Don't forget that as the tyres wear, the diameter will decrease by up to 14mm 225/40/18 = 637.2mm 285/30/18 = 628.2mm so with worn tyres, the difference will vary from 27mm to 0mm And the narrow bodied C4 is even less likely to be controlled by tyre-diameters, sitting on 265/35/18 = 642.7 So how will that achieve any drive to the front axle? So... what DOES generate CONSISTANTLY a 5% 'drive' to the front through the VC? Also apparently not the different ratios in the actual diffs on each axle, which are (according to the technical data in Porsche's own 993 repair manual) the drive ratio's front and rear being the same at 3.444:1

 

[Dave Wilkinson]

ORIGINAL: orangecurry So how will that achieve any drive to the front axle?

I think you're reading too much into it. The difference, even a minor one, in the rolling radius of the tyres will cause a difference in the number of rotations over a given distance. This will ensure that the two sides of the viscous coupling will rotate at slightly different speeds. This will invoke the shear within the silicone fluid and raise it's temperature. The rise in temperature will increase the viscosity and provide resistance to the shear - hence providing the torque transfer. Regards Dave

 

[orangecurry]

But in practice there will be times (and for the NB cars MOST of the time) due to tyre-wear when the rolling radii are so close that there is no difference, therefore there would be no AWD effect. Porsche state a minimum of 5%. I can't imagine that anything less is a 1990s-Porsche solution. If they say 5%-35% (extremes to 50%) then there will be an engineering solution which delivers those figures. Are you suggesting that when on a WB car the rear tyres are highly worn, and the front tyres are new, that is when the (say) 35% is transferred to the front in normal driving, and it would never drop below 35% to 5%? Do you see what I'm getting at?

 

[Dave Wilkinson]

ORIGINAL: orangecurry Do you see what I'm getting at?

I do, indeed, see what you're getting at. The problem I have is that I don't have any detail of the internal workings of the VC and your assumption that it's ALL to do with wheel speed differential may not be all there is to it. I presume that ANY rotation within the VC will help to raise it's temperature, even if both sides are rotating at the same speed. Certainly the varying differences between tyre sizes and wear will have some impact on the temperature gradient but I can't believe that even with equal tyre radii the VC would remain cold for long. My technical understanding is fast reaching it's limit on this subject and I don't want to guess my way into an ever deeper hole!! It would be great if anyone with further knowledge could give a definitive answer on both the torque split and MOT roller issue. Regards Dave

 

[MarkG]

ORIGINAL: Dave Wilkinson Matthew, Your inference is exactly right about the tyre sizes. However, there is no conspiracy with the tyre manufacturers as you will see from the attached comparison, a 285/30 IS actually considerably smaller than a 225/40 in terms of overall circumference, doing 841 revs per mile on the rear compared to 829.3 revs per mile on the front. This difference is obviously enough to heat the viscous fluid sufficiently to give the 5%-15% torque feed to the front axle. As far as the 964 is concerned the performance difference was small - but it was there. Because of the different gearing and, I suspect, the added sure-footedness of the C4, the C4 was actually quicker off the mark than the C2 up to 50mph where they appear to match each other. Beyond that, the C2 creeps ahead beating the C4 to 60 by 0.1 sec (5.1 compared to 5.2). By 130mph the margin has grown to 3.8 seconds (23.7 compared to 27.5). Top speed is only 1 or 2mph different around 160mph (+ or -). The torque split on the purely mechanical centre diff on the 964 C4 is 31% to the front,, 69% to the rear. With the hydraulic actuated locking mechanism under the control of the PDAS this could be infinitely varied from 0% to 100%. This answer has considerably stretched my research at 7 in the morning!!! Regards Dave

Hmmm, I thought the narrow bodied cars ran on the 265/35 rear tyres and according to the chart they are slighly bigger than the fronts, so does this viscous-warming-up-malarkey work both ways then ? I'm considering running 11" rears with 295/30 tyres which will only produce a speed difference of 0.48% slower compared to my fronts, so will this be enough to keep the 4WD system happy ?