996T power distribution

[JapanRed]

Simple question which I'm sure someone can easily answer.

Does the 996 Turbo split power to the wheels evenly 50:50 or, as with the 997 C4S can it change distribution according to conditions? If it's the latter, what is 'normal' distribution?

Thanks

[Mark A S]

I think your find its 95% rear under normal situations decreasing to 60% when traction is on the wane!
Mine could really do with a rear LSD TBH.

[JapanRed]

Mark A S said:

I think your find its 95% rear under normal situations decreasing to 60% when traction is on the wane!
Mine could really do with a rear LSD TBH.

Thanks Mark. Much appreciated.

[Rockster]

JapanRed said:

Simple question which I'm sure someone can easily answer.

Does the 996 Turbo split power to the wheels evenly 50:50 or, as with the 997 C4S can it change distribution according to conditions? If it's the latter, what is 'normal' distribution?

Thanks

Nominally the 996 Turbo AWD torque is split 5% to 40% going to the front axle with the remainder going the rear axle. Actual torque numbers can be zero at the low end. I do not know what they can be at the high end. 45%?

My info is at around 150mph the front axle receives its max. torque, be it 40% or whatever.

The 996 AWD system isn't intended to dynamically -- on a "real time basis" at any rate -- split torque.

The torque split is handled by a viscous coupling - which relies upon a special silicon based fluid that becomes more viscous as it gets hot, to its working temperature (300F+ degrees). This fluid is heated by the different rotational speeds of very closely spaced discs. Every other disc is connected to the cardan shaft from the transmission, while the other discs are connected to the input shaft of the front diff.

There is a built in axle speed difference arising from the different tire sizes used at the front and rear of the car. This causes the discs in the viscous coupling to rotate a different speeds. This causes friction of the fluid and heats the fluid which as I mentioned above causes it to become more viscous and this transmits some engine torque to the front axle. The amount is based on speed. As I mentioned above at 150mph or thereabouts the difference in speed between the frotn and rear diff results in 40% torque being fed to the front diff.

My info is viscous coupling damage can occur if the car is used in marginal traction conditions that result in the rear wheels spinning. This can overheat the fluid in the viscous coupling and this can damage it. (It is also imperative that if the car has to be towed it is towed with all 4 wheels/tires off the pavement.)

There are videos on the web that show owners of 996 AWD cars hooting around in snow and it is clear that even though the rear wheels are spinning like crazy there is no torque being transmitted to the front axle as the front tires are in some cases are clearly nearly stationary and not spinning as would be expected as they are more lightly loaded than the rear wheels.

OTOH, I have viewed at least one video of a 997 AWD car in similar marginal traction conditions and it is clear in the video the front wheels/tires are in fact being driven by the AWD system as one can see the front tires spinning upon take off along with of course the rear tires.

The AWD system used in the 997 cars is a different design using a clutch pack whose percentage of engagement is controlled by the DME and is probably better able to handle a case where the rear tires are spinning. I would however recommend owners of 997 cars with the new AWD system avoid this situation just in case to avoid possibly subjecting the clutch pack to too high temperatures.

[Jibaro]

http://youtu.be/2-pYen94aPc

[JapanRed]

Thanks for an impressive description Rockster.
Thanks also to Jibaro - a very informative video.