if its variable then how is this controlled? Can the driver turn a **** or something to what he wants or does the ecu and the state of grip the car has adjust itself?


In other words can i make the car 50/50 split for launching at the drag track?

Can i make it 100% rear wheel drive for some tail happy fun or maybe a burnout to warm tyres at the 1/4 mile track?

 

Nope either awd or disconnect the front drive shaft and make it only rear. That's it no adjusting anything.

 

so you mean there is only one split ratio? the car does'nt change its ratio split at all? What is the ratio split? i read 95/5 but that would make the car pretty much rwd then with a pointless 5% going to the fronts?

 

Hi Mattysupra,
Not sure which cars you've had.
But the 996 is not like a Cossie, Scub, Evo or for that matter a Land Rover, which are all true 4 wheel drive cars with transfer boxes containing a diff and VC. The 996tt is permanently rear wheel drive there is no transfer box on a 996. Some Torque 5% to 40% is sent through the front prop into a VC and into the front dif. The front prop turns at the same speed as the output shaft to the rear diff. That's why you can remove the font prop completely and the car will still be RWD. Do this on the above named cars and unless the centre VC has been uprated they wont go anywhere and if the did the centre Diff and VC would soon burn out.
Basically the 996tt has almost all the characteristics of a RWD car with some characteristics of a 4WD car.
Hope this helps a bit.
Frank

ps on tarmac the grip is outstanding anyway. In the wet/mud/snow my Cossie would leave it well and truly. A Scube and Evo with traction control and variable diffs is another league over a cosworth again....

 

Yes i have all the above cars over the years apart form the land rover!

im trying to compare it to how my current car works (nissan GTR) and yes you can remove the front shaft on that to make 100% rwd . However i have a box of tricks i have managed to get working to make it what ever split ratio i want, if the 996tt works simular i think i can adapt the same box of tricks to work on the 996. Im sure alot of you would like that?

So you say some power as in 5%-40% can be sent to the front wheels that i take the car controls.

What is VC?

How does the car change its split ratio? Does it pump hydrolic fluid maybe it a clutch system a bit like a auto gearbox?

 

Yup. VC= viscous coupling. The 996turbo is the lamest excuse for AWD I have ever seen.

 

And how exactly does it work? the faster it spins the more power to the front wheels or is controlled by a hydrolic pump that i may be able to over ride?


WHen does the car send power to the front wheels? Only on acceleration? on wheel spin? does it have a G-meter and activate around bends maybe? Does the car have any control at all when its 4wd or is simply on acceleration? what about at speed on the motorway?

 

Don't know any of those answers for sure, but I think it works more the faster you go, as I have gotten stuck in the snow when I first got it, thinking I had a Quattro. Boy was I mistaken. I have since removed everything and LOVE it!!! Maybe someone knows for sure and can chime in...

 

If it does work on speed then it is defo a shat system!

 

The faster you drive the more power is transferred to the front via the viscous coupling. I read somewhere that at 150mph, there is 30% power to the front wheels.

 

^ That sounds backwards from what would be desired.

 

The earlier pre 35 GTR's used a combination of sensors/ECU to send power to the front. ANd thus one could add a control systems to control it, (there were several available) http://www.nengun.com/kansai-service...t-s-controller

Here's an explaination of how the older gtr awd systems worked. http://en.wikipedia.org/wiki/ATTESA

I believe the 996tt awd system is entirely mechanical and can't be controlled via electronics. that said it drives a lot like the older GTR's, i owned and modified all 3 generations.

 

Ok, This is how it works.
The rear drive/diff is connected directly to the output of the gearbox, no differentials. Proper RWD and no mistake. the Output shaft for the front prop is in all intense and purposes connected to the same shaft. So any speed increase from the rear wheels spinning will transfer at the same rate to the output shaft for the front prop and of course the front prop its self. If the front prop was connected directly to the front diff( remember its would also be connected to the rear wheels as well) without a VC then Yes the 996tt would be a true 4X4 but would be like a landrover with the diff lock engaged. Totally unusable unless grass tracking or in the snow.
However Porsche have not connected the front prop directly to the front diff, they have fitted a VC. A VC has only 1 input and 1 output. There is no direct mechanical connection. The turning force is transferred through plates and silicone fluid. A bit like an auto box Torque Converter.
So, if the output drive to rear diff and the front prop turn at the same speed and rear diff is driving the tyres then it can be said that the front tyres being on the ground and the same size must be turning at the same rate as the rears and front prop.
So rear box output speed= Front prop output speed.
So Rear tyre speed=front tyre speed
But if the rear wheels spin then they will be moving faster than the front wheels.
So the rear box output will still= to front prop output
But front prop speed will not = front wheel speed.
There will be a difference in rotational speeds.
The one that is spinning the fastest must have the more energy which will be the front Prop and not the front diff so that energy is sent into the VC , the silicone liquid and plates inside the VC transfer a % of the speed difference to the front diff.
If the rears spin a little then the front prop speed differs a little to the front diff so only 5% will be transferred.
If you do a Max Burnout and the rear tyres are spinning at max velocity then the front prop will also be spinning at the same speed. The front tyres will be barely moving so you might think the VC would transfer 100% from the prop to front diff. no. It will transfer a max of 40%.
This can be changed with an uprated VC. But unless you get rear spin its worthless. In other words unless your rear tyre speed differs from your front and thats not very often then there is little drive to the front.
for eg a Cossie has a permanent 66% rear:33 front split no matter what happens to the rears . The ratio is governed by the gear ratio in the Center diff and VC combination in the Transfer Box.
Bottom line is the 996tt uses a form of 4 wheel drive for driving dynamics more than outright traction.
As mention, in the snow they are pants. I live in Scotland and get to drive mine quite a lot in the snow. Its not good.
Hope you have understood my explanation. I'm not that good at putting things down in print.
Frank

 

Thanks for the explination Frank, I didnt know most of that, good read. Quick question though, pants = bad?

Justin

 

Hi Justin

Yes, its sort of a derogatory comment meaning Bad.
Frank