964 AWD system vs 993 AWD

[Maych87]

I understand the two models have a very different approaches to their AWD system. The 993 system is purely mechanical. Where the 964 system automatically adjusts the diffs from data from wheel speed sensor inputs and yaw sensors to send power to slipping wheels.

Functionally what is it that cause the 964 to have such bad understeer when pdas is activated? Why does this happen with the 964 system and not the 993 system?


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[Paperboy]

On a 964 if various sensors sense a potential oversteer situation the PDAS locks the rear lateral diff up to 100% creating "safe" understeer. This is then reduced in steps when the system feels it is safe to do so.
I believe the 993 C4 had less rear lateral diff lock under similar circumstances with less understeer as a result..It also sends less power to the front compared to the 964c4 system with more rear wheel drive feel.
(The 964 system was basically from the off road Paris Dakar winning cars,and maybe not ideal for normal road use.)

[Bill Verburg]

964 has a static sustained torque split of 31/69 imparted by the epicyclic differential at the front of the gearbox, this was further modified by a multiplate wet clutch in that same central differential controlled electronically and run off signals from the lateral and longitudinal accelerometers and wheel speed sensors the clutch pack could then provide excess torque to the end w/ better grip. there was a similar electronically controlled rear differential that could do the same laterally at the rear wheels.

993 has a simpler lighter awd controlled by a viscous coupler in the central differential, the only electronics involved are the low speed ABD that brakes a wheel when it spins and this can only happen at low speeds. At least 5% of the available torque is always applied to the front axle, this increases to ~35% in normal driving, at most the front receives ~45% ot the available torque, the actual torque split is always changing based on the slip in the central diff. The rear axle again has no electronics, just an asymmetric mechanical 25/40 lsd

[Maych87]

So when the car detects the car about to oversteer it locks the rear diff and causes understeer. That makes sense because the inside wheel would not be able to slip around a corner so it would try to force the car to go straight.

A couple more questions.

1.When the car is sensing an oversteer situation that is achieved through the latitudinal yaw sensor. Is that correct?

2. Can the rear diff vary from 0%-100% (open to locked)? Does this mean it’s earthier fully open or locked or is there an in between amount of slip.

3. The difference between an asymmetric mechanical lsd and the version of lsd in the 964

Asymmetric lsd- applies torque to the wheel with less traction while allowing the opposite wheel to slip the appropriate amount to turn the radius of a corner?

964 rear diff- locks both rear wheels together when detecting slip. which does not allow the inside wheel to slip which causes the vehicle to want to go in a straight line?




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[Bill Verburg]

Originally Posted by Maych87

So when the car detects the car about to oversteer it locks the rear diff and causes understeer. That makes sense because the inside wheel would not be able to slip around a corner so it would try to force the car to go straight.

A couple more questions.

1.When the car is sensing an oversteer situation that is achieved through the latitudinal yaw sensor. Is that correct?

2. Can the rear diff vary from 0%-100% (open to locked)? Does this mean it’s earthier fully open or locked or is there an in between amount of slip.

3. The difference between an asymmetric mechanical lsd and the version of lsd in the 964

Asymmetric lsd- applies torque to the wheel with less traction while allowing the opposite wheel to slip the appropriate amount to turn the radius of a corner?

964 rear diff- locks both rear wheels together when detecting slip. which does not allow the inside wheel to slip which causes the vehicle to want to go in a straight line?




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The 964 clutch packs are able to vary the torque distribution, both longitudinally and laterally.

asymmetric mechanical lsd has different lockup rates on acceleration and deceleration for 993 awd it is a anominal 25% on acceleration and nominal 40% on deceleration. 964 has no such ability. it varies long and laterally only

[Maych87]

So the 993 varies the front diff from 5% to 45%.

The 964 system is static at 31% to the front diff. So it’s range would be from 31% to 100% to the front diff? Is that another factor of why the 964 c4 understeers so much worse than the 993 c4?


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[John McM]

You make it sound like a C4 is an understeering pig. That isn’t my experience e.g. recently I was 3rd at an autocross only beaten by a couple of GT4s

It’s very predictable at the track https://youtu.be/yJwFpm-65Ns

[MikeP.]

I can still hear the echo from John McM dropping the mic...