kos11-12

Hi Thilo, Welcome back ! It would be great if the master cylinder could work the LSD as it is, with the actual hydraulic system , I must admit I am bit lost there, so if we machine the front shafts to convert to C2 the ABS sensors will work as normal, so what is in the electronics that makes the C4 system to pass from rear to front wheel drive ?
Regarding the shifter the CAE seams to me to be the best , it's a very neat design, plus it's very close to the steering and offering a real short shift , I agree it's a bit over price tho .... The down side is you may need to get rid of the centre console rather than cut it to fit the shifter, and reposition the 3/4 switches, either by a block off radio or make one yourself . Personally I am getting rid of the centre console as part of a project of lightening the cabin, deleting sunroof, sound proof with RS carpets and lower dashboard delete.
Or you could make a shifter yourself if you know a good engineering place, like Filipo check his recent race car thread, there is few pictures it will probably be much cheaper .

As you said about the box , once we get rid of the planetary gear it will be left about 5kg, not a big deal ,

All the best Konstantin

jack.pe

The four wheel drive system is fixed, the electronics do nothing in terms of shifting drive between front and back. All the PDAS does is lock the centre and rear diff close to the limit to induce under steer. The centre diff is the main culprit for the under steer when it locks. But it will only do this to speeds of 65mph, after that the PDAS does not intervene. This is all according to Adrian Streather who I interviewed some months ago.

alexjc4

I think after all this time we need to recognise that Adrian's information on the pdas wasn't complete or accurate - not his fault.

The pdas system allows of variable front/rear torque split between 70/30 and 50/50 and anything in between.

The rear diff also allows variable lockup between open diff and fully locked.

At least that is my reading of the factory authored "carrera 4 porsche allrad" book http://www.amazon.co.uk/CARRERA-Porsche-Allrad-Wheel-1900-1990/dp/351701124X

The exact strategies the pdas uses for distributing torque haven't been published, but it is seems unlikely porsche would opt for a simple all or nothing for either diff.

koenig_roland

But, with that in mind and with the target of reducing the understeer (which isn`t a big problem on my car): Could it be possible, that a simple disconnect of the center diff (block the hydraulic pipe) could cure the tendency of understeer!?

Only the fact, that all four wheels are driven does not explain the trend of understeering G64.00 equiped cars have. Until the moment, when the pdas system locks something, everthing should be fine. First with an action, related to the center diff, understeer begins?!

nevertheless: 2WD is the aim i`m looking for... And a dynamic, hydraulically activated, lsd would be a fine topic...

alexjc4

The base torque split front/rear is enough to generate a little understeer in dry conditions (ie acceleration is not traction limited) tbh. If there is any significant weight transfer rearwards due to acceleration then the front wheels can quickly be sent a proportion of the available torque that is greater than the proportion of the weight supported by them meaning they will have higher slip rates than the rears (the definition of understeer). In the wet when the car is traction limited the split is more proportional to the dynamic weight distribution and the base torque split offers the maximum overall traction available through all four wheels.

Edit: have done the maths on this, it was some time ago, and I had to guess the height of the centre of gravity based on other 911s as I couldn't find any real world data for a 964.

jack.pe

Hey Doc, so how is that torque difference managed by the PDAS? Though the centre diff?

alexjc4

Yeah, the pdas applies a pwm signal to the solenoid in the frunk which modulates to hydraulic pressure applied to the slave cylinder which acts on the centre diff clutch pack. This varies the degree of lockup between the annulus/carrier and sun gear in the planetary gearset in the centre diff which changes the torque split. (Unlike the cross axle differential action that we are used to visualizing, an epicyclic torque splitting diff is very hard to visual and the mechanism isnt intuitive at all.)

kos11-12

HI guys,
so how is the front diff LSD working , can that be the cause of under steer ? !

alexjc4

The front diff is an open diff.

jack.pe

Alex, what happens when the PDAS is turned off? what is the split between front and back then?

alexjc4

The front diff doesn't effect the torque split.

The torque split due to the epicyclic gearset in the centre diff (aka permanent 4wd) is 30/70ish, the centre diff is essentially an open diff until the pdas comes into play.

John McM

The ratio is stamped on the gear, 25:15

alexjc4

Cheers, that would be a 37/63 split - which is what I remember now I think about it, and this was the figure I used when I did my calcs some time back.

If you take a ford sierra cosworth 4x4 (front engined of course) it had an epicyclic centre diff with a 34/66 split.

jack.pe

I'm not getting this.. so essentially there is a clutch between the front and rear.. and this can be used to vary the amount of torque going to the front. As standard the centre diff is open so all the power sent it's way goes to the front. the clutch could then be activated by the PDAS but only to reduce power to the front.. unless the clutch is permanently open and the PDAS closes it but then I cant see how any power at all would go to the front under normal circumstances..

alexjc4

Sorry guys I'm having some senior moments aqbout the exact statis torque split - I think Porsche publish it as 31%:69% - not sure how that aligns with the ratio stamped on John's gearset though. The 37/63 that jogged my memory is the static weight distribution I think.