Myles Maycher

Spokes

newsboy

I agree with the above, that this is a long topic. The activation of the PDAS in the 964 is to induce under steer, a safety measure. In the 10 years that i have had a C-4, I have not been able to get the PDAS to come on.
This includes some pretty extreme maneuvers, including rear out drifts, around a circle of cones. If it does come on, tapping the brakes, deactivates, so really not something to be concerned about.

kos11-12

Myles Maycher

Spokes

golfnutintib

my understanding is .... unlike the more modern systems used by nissan in their GTR and mitsu in their now defunct evo cars, i do not believe the 964 diffs in any way intelligently alters the torque sent to each individual wheel depending on the situation the cars senses... instead, if it senses inputs showing that the car is losing traction at any wheel, it simply locks the rear diff to induce understeer and reapportions wheel torque differently between the front and rear axles... there is no side-to-side differentiation...

is this correct?

Spokes

G,

Hope this helps.

I quote from Adrian's book: "If one axle of the C4 is on ice and the other is on a dry surface, the wheels on the ice could spin during acceleration. To counter this the longitudinal lock will be activated and it will send more drive or torque to the slower turning axle... front and rear toque ratio can be changed infinitely from 0% to 100%..." pg 212.

It is the lateral lock that locks the rear diff axle..

Also from the History of the 964 "The four wheel drive mechanism of the Carrera 4 also utilised a variant of the G50 gearbox (G64), although this obviously had to be significantly adapted to drive the front wheels of the Carrera 4. The front differential housing was linked to the gearbox via a large diameter tube that (through the gearbox's hollow secondary shaft) connected to an epicyclic differential at the front of the gearbox. From there, the drive moved to the rear differential via a shaft passing through the hollow secondary shaft. The front/rear torque split was maintained at 31/69% through the central differential. This central differential wais in turn controlled by a "multiplace clutch" that could automatically actuate whenever wheel spin was identified by the ABS system. The clutch would then provide excess torque to the axle with the "better grip". A similar electronically controlled rear differential would act as a normal differential (split drive between left and right rear wheel) as well as intervene whenever the accelerator was released while cornering, in order to minimise the accelerator pedal off oversteer tendency of the 911."

Normandy 964

If PDAS senses a speed difference between front and rear wheels it locks the center diff.
If PDAS senses a speed difference between rear wheels it locks the rear diff
If you lift off whilst cornering hard PDAS locks the rear diff.

Adrian's book states 'front and rear torque ratio can be changed infinitely from 0% to 100%' which is impossible.
The diff locks are activated by solenoids. Solenoids are either on or off and cannot be controlled at some middle position.
This is why when PDAS kicks in it completely throws the car off line instead of just feathering your line like a modern system.

John McM

dlpalumbo

From the 'Carrera 4 Porsche Allrad 1900-1990' book I gleaned the following: The longitudinal and lateral PDAS power distribution are both through multi-plate clutches. These clutches are controlled through the on/off solenoids at high rates (~ 100 Hz, cant find exact number). So, the control is a lot like an ABS system. In this manner, on average, a continuous range of torque distribution can be achieved. The 964 will have a 31/69 split in torque distribution without PDAS intervention. The 993 is 5/95. I think this is the biggest difference in handling as the 964 will exhibit more FWD understeer under power. I cut my teeth on FWD cars and learned how to drive them during days before traction control and ABS. My technique was to induce oversteer with throttle and/or brakes aiming for a late apex. Applying throttle dials in understeer to compensate. I drove the 964 in a similar fashion with some success never noticing the PDAS upsetting my line much unless things got out of control. At that point, I was glad it was there.

I highly recommend the Allrad book. The history of Porsche awd development is interesting and the technical description of the 964 awd made me appreciate how impressive the system was for its time.

golfnutintib

that the system alters front to rear power distribution is clear

side to side power distribution on demand, esp. at the rear axle? -- i am not seeing from the literature that the car does that

adrian's comments are less than clear on this point

Myles Maycher

Thanks guys for all the responses. We are getting to the issues I was trying to figure out. The explanations in most of the literature are fairly ambiguous or maybe i dont understand the technical jargon well enough.
What I have gathered is
-The rear diff is only able to control the rear wheels simultaneously
-The rear diff acts as an open diff until the vehicle detects slip
-When the vehicle detects slip the pdas activates

Where I am still unsure and sorry if it has been explained in the above replys. I dont know if the theory of being completely locked or unlocked is accurate. It seems to me that because the diff without the clutch pack being activated is an open diff, depending on the amount of activation on those clutch packs, it controls the amount the open diff goes to a full locked rear diff. I think there is a range from 0-100 like most of the literature states. So say you are taking a corner and your inside wheel looses traction. On an open diff the inside wheel would start spin and the outside wheel that had the traction would basically do nothing. In the pdas system with the same scenario the clutch pack in the diff is activated when the difference in wheel speed is detected. The clutch pack holding power is increased untill the difference in wheel speed is gone. There still is a certain amount of wheel slip until the diff is completely locked up. This is basically the same theory of how a mechanical lsd works. The difference is a mechanical lsd never goes to 100%. I think the big issue is that the diff is able to go to 100% lock which creates massive understeer.

I could be completely wrong. Let me know if this sounds accurate?

Goughary

So if that's the case, one could make a small spacer and preclude the system from ever getting to 100% by not allowing the diff slave from fully locking the clutch packs.

Or by shortening the push rod on the slave...

So if someone has an open set of clutch packs...and a diff slave...could be measured.

Further, wouldn't this also mean that cars with worn clutch packs would understeer less?

Myles Maycher

Goughary dont jump the gun here. But you are on track to why I created this thread. I am not 100 percent sure if what i said in my explanation is accurate. I would like someone with more knowledge than me to confirm unless you know that to be true. Lets be 100 percent sure on operation then we can brain storm on ideas to see if we can fix the issue or find a better solution than altering our driving to accommodate. I want to create a new thread on "finding a solution to the c4 pdas system" once we have the operation of the system 100% ironed out.