996-4S 4 Wheel Drive System

[RatBoy M3CSL]

So is the 4 wheel drive system on these a permanent set up, or rear wheel drive only until it starts sliding, I was just taking advantage of the slippery conditions with a spot of slippery snakey junction exits in 1st, not quite enough power to break traction into 2nd as it straightened up today tho', the car does slide around nicely, but not for long enough for me to establish if it was a perm or part time 4 WD system tho'..?

[Magic919]

That's a very long sentence.

It's a viscous coupling and transfers about 5% minimum to the front and more as it heats up. So mostly a 2WD, albeit a bit lardy, but will probably pull you out of trouble.

[mrgill]

i have searched and searched for this answer, different people have different answers...there is a video on you tube of a 4s on a grassy patch, the rears spin and the fronts did nothing, some info i found states that the viscous fluid doesnt last forever, thus if the viscosity doesnt thicken up sufficiently it gives no drive to the fronts.. also the difference in rolling diameters is what gives the 5% figure, a slight difference in speeds of the viscous plates/fans heat to thicken the liquid/gel up enough and its when the plate on the rear side spins faster to the extent of giving up to 40% drive to the fronts...theoretically of course! by the way this is just what i have summed up from internet trawilng!

[RatBoy M3CSL]

..well I've been doing my own scientific research by lighting up the tyres in the wet and trying to figure it out too, I can't see exactly what's going on but so far it feels pretty RWD to me.. , when we get some snow I will try another method, and open the window on some lock and see if it throws snow at me.. !

[mrgill]

lots of people have tried that but none of the threads i have read has anyone seen the fronts spin! maybe the coupling weras out and so doesnt deliver any drive anymore? who knows! maybe both wheels need to slip to spin up the rear coupling plate? but that would need a locking rear diff!? i sure hope my c4s is better in the snow than my last m3!!!

[Callughan]

My 4S very good in snow.

In rain when I floored it you could feel the power shift to the front.

5% driven to front wheels at normal times and upto 40% at other.

[NBTBRV8]

Callughan said:

5% driven to front wheels at normal times and upto 40% at other.

Yep, constant drive to the front variable as above.

[monthefish]

From another thread..

Sunnysidebb said:

monthefish said:

Thanks Frank - useful info.

Sunnysidebb said:

The 996 4wd system is permanently engaged. There is a slight wheel speed difference between the front and rear which ensures that the centre VC is always loaded with a minimum of 5% hence giving a minimum of 5% torque to the front wheels raising to a max of 40% the faster you go.

Can you tell me more about this - i.e. is the drive to the front wheels dictated by the speed of the car, or what is happening at the wheels, and how does that 'fit in' with what the TC system is trying to do?

No problem.
Keeping to basics....
The 996 4x4 system is basically permanent hard drive rear wheel drive. By which I mean there are cogs turning the rear diff. The front diff is not directly connected to the gear box. The torque transmits through a Viscous coupling VC( http://en.wikipedia.org/wiki/Viscous_coupling_unit ). The rear wheels are allowed to turn slightly faster than the front hence the input speed of the VC is faster than the output hence there is a torque movement turning the front diff. the ration never changes and is dictated by both diff ratios. As you travel faster then the amount of speed difference between the two increases which increases the torque to the front. See how a VC works.
With TC switched on it monitors the amount of wheel spin on any of the 4 wheels , if it sees one spinning it applies the brakes to that wheel alone slowing it down. if the wheel or any more wheels spin and the brakes can not slow the spinning down then the DME will intervene with the power delivery of the engine to ensure wheel spin stops. This is why when driving in snow its best to turn Off the TC ( PSM )
With TC off the wheels are allowed to spin without psm intervening with power delivery.
Note: PSM does a lot more than just TC it also monitors other parameters but that’s a different topic.lol

The 997 however has a variable centre VC and can be controlled by the DMS. So I suppose it could be used as part of the TC as well. However I haven’t looked into how the 997 works anymore than that.

It should be noted the 911 996 is not like the Cosworths , Evos, Or Subaru's which utilise a centre differential as well as a centre VC hence there is a mechanical connection to both the front and the rear diffs.


Hope that helps.
Frank

Edited by Sunnysidebb on Monday 19th December 20:45

[mrgill]

so i wasnt far off in my understanding.... apart from this bit, the speed difference between the front and rear wheels will never change during full traction acceleration if there is no wheel spinning, the ratio will be constant and linear surely (not exponential)? i.e 1 full turn from the front wheel will equal 1.1 turns from rear, and will always be the same until the rear wheels slip... does this make sense ?