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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.
Myles, if you can read a copy of Carrera 4 Allrad from start to finish and out-think the engineers who created the system then you are a better person than me. They specifically mention why they dismissed the system later used in the 993 i.e. viscous coupling.
As for finding a solution to the c4 pdas sytstem, I'm still unsure what you mean. What is the problem you have observed driving your car? As I posted on the Pelican thread, I was third out of 36 cars in a Porsche Club New Zealand autocross. I was beaten by two GT4s and left a long line of capable modern Porsches in my wake. It was a third gear track with slaloms and esses, plus a hairpin. A well set up C4 with a driver who knows how to get the best out of them is competitive without needing to butcher the system.
Sorry if this comes across as aggressive, but I love my C4 as is, and while I'm all for improving the breed I worry when the 'problem' isn't defined and we struggle to even come up with a clear understanding of how it works. Not a great platform to go playing with a machine that goes so fast.
John- the issue is that the system isn't ideal for performance driving. It over locks the diffs-
There are ways around this that we have all known about for ages. From wringing out the C4, we learned to left foot brake or trail brake to cause the system to not lock. Then we learned to unplug the PDAS control unit and drive without it- and then a few years ago we learned that holding the switch for ten seconds on the console deactivates the system, which should be similar to what happens when you trail brake, though turns off the abs as well, which trail braking doesn't do.
However - what we also know is that the lightweight used a manual set of ***** to adjust the amount of lock from in the cockpit. Which has always made me feel like there could be some benefit in not just having open diffs, but adjusting the system in such a way where if makes some, but not quite as much and not quite as drastic a change to the locking - and therefore a more controllable response when the system senses slip.
Further - the system was designed for street driving and not at the limit track use. So learning a bit to make some changes that would be better under certain circumstances would be fun...to see if they make a real difference. I've been interested in this since my first day at the track- just haven't done too much yet to explore it.
Don't get me wrong, I'd love to see someone recreate that controllable system in a raw track oriented machine but it seems like the base knowledge isn't there yet to do it.
What's needed is a systems analysis as to how the sensor information is gathered, analysed then output to the solenoids.
Then the work can start on modifying the output for a desired result.
In the back of my mind though is the experience I've had with other mods. If you don't have the factory spend then be prepared to experiment a lot. It might be cheaper just to buy a C2.
Agreed. There is very limited understanding and expertise regarding the system. And I'm not an expert there by any means. Happy to experiment and do some guesswork...
That said...we have a huge community here and some of these threads in the past have elicited a strong response from the experts out there hiding in the wings...
I think one of the issues in understanding stems from those that state that the system was "designed to induce understeer". In reality, the way it was designed induces understeer, and if the focus is on first reducing the aggressive nature of the locks, then it can be tweaked from there...
I love my C4 and wouldn't switch to a C2...and honestly believe that a C2 and a C4 of equal weight, driven correctly, the C4 is a faster track car on most tracks...when both are set up properly...on street tires.
But- i did a winter track day at Monticello a month or so ago and the awd while fun...was interestingly bad in certain situations in cornering. Driving with the system off was better but not by much and losing the locking of the diffs all together made it somewhat more controllable w the throttle but over all slower.
The awd system is amazing at climbing a steep frozen hill. And driving on the streets in the snow at slow speeds is easy and assuring...but on the track that day, the Subaru rally car that was out there was seemingly significantly better and the spec Miata on snow tires was also better (as were the two drivers)...but nonetheless, there were a few corners that were solid ice, and getting the car to point was a pain in the *** - it just wanted to go straight. My feeling was that if the locking had been more gradual or less aggressive, it would have been easier to get the car to point and therefore easier to control the car with the throttle. But open diffs were terrible in the snow in relative terms.
Anyway- you are right 100%...and at this point i think the way forward is some real life experimentation...and some physical understanding of the mechanics of the system and hopefully someone with electronic knowledge can jump into the electronic management and give some idea of whether that can use some programming help.
Short of programming help, i feel like maybe there is an option to reduce the max locking of the diffs by some percentage. As the system seems to often go from 0-100% almost immediately in snow and on a track where you are driving at the limit and start to lose some control...where effectively you still have control, but the system thinks you don't because the wheels are spinning or losing grip.
Hey guys thanks again for the great input. There are 3 ways to modify the system.
1. Electronically
- through the pdas/ abs control module
- or Piggy back system
- latitudinal yaw sensor
I installed a megasquirt engine management system on my car last year and it has a wide variety of options for setting up inputs and outputs and all kinds of other things that I can probably make it piggyback to the pdas system if needed
issues with a piggyback is you would get the Christmas tree on the dash and the warning horn would go crazy. If you were willing to not use the abs you could completely unplug the pdas/abs control module and wire everything to an external management system to control the differential lock slave to the actuators. This would eliminate warning lights but also eliminate abs.
Latitudinal Yaw Sensor - all the information I can find on this sensor is when the sensor reaches the limit where the car is about to oversteer it goes full lock on the rear diff. So why not get rid of this sensor and place a dummy static circuit in place of the yaw sensor that makes the vehicle think it is never oversteering. Unless the pdas/abs sensor uses the latitudinal yaw for something else.
2. Hydraulically
- pressure reducing valves
- or pressure limiting valves depending on the system function
depending on how the system sends pressure to the hydraulic differential lock slave. if there is a max pressure applied to the actuator that causes the diff to lock, a pressure limiting valve could be placed inline that only allowed a certain amount of total pressure then directed the excess fluid to the reservoir (ex: anything over 600 psi would be directed back to the reservoir). These valves are typically adjustable and could be backed off until the diff didnt lock.
although I have my doubts on this system
There is probably a different diff lock up rate depending on the available traction the tires have (the differential lock slave would need more pressure to hold the clutch pack if the tires had more grip). This would mean that the above paragraph wouldnt work because the differential lock slave would continue to get more and more pressure until the diff locks. If this is the case the max pressure at which the rear diff locks would always change. You may be able to put a valve that reduces the the pressure just before the differential lock slave. The issue is that the pump would probably continue to output more pressure until the diff locked and the wheel speeds matched from wheel to wheel. There would be a max that the pump could output, so it might work. It would probably be very hard on the super expensive hydraulic pump.
I assume the c4 lightweight system is something like this. If you did the same thing for the center diff you could limit the amount of split to the front diff.
Modify the pushrod on the differential slave. This may limit the amount the clutch pack engagement. It would be hard to know how much to shorten the rod and would not be adjustable. There maybe other issues as well
Completely change the rear diff to a true lsd. This would probably be very difficult.
There may be other options or a combination of the options. But to figure out anything we still need to fully understand the system. I love tinkering on my porsche and am willing to try just about anything. Worst case scenario I can install pressure sensors into the outputs of the pdas solenoid and hook the wheel speed sensors and yaw sensors into my engine management system and datalog everything to see whats going on. I don’t want to prematurely do that until I have a complete grasp on the system. Also I have a 12 day old baby and its hard enough to find the time to reply to this forum.
Again here are the things we know
-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
-When the yaw sensor detects an oversteer situation it locks the rear diff
- the front rear bias is 31/69
Things we do not know
-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?
-Is there a maximum pressure pdas solenoid will output? How does the hydraulic output to the diff slaves work?
-is the latitudinal yaw sensor only used to detect an oversteer situation? Or is it used in another calculation for the pdas?
I hope we can keep the rest of this thread factual to the pdas system, please don’t post your personal experience with the car unless it is content directly related to the pdas system.
John McM My thought was as well that they were hydraulic valves. I couldn’t find much on the lightweight or ruf system. Maybe Goughary has found that info some where? Or are you just basing that off of the switches you see in the picture Goughary?
From the pics- it looks as though it's a screw system that pushes a master cylinder- thereby adjusting the torque transfer directly. But i don't own a lightweight! So hard to tell.
Someone here may have owned or driven a lightweight at some point....
Great thread guys thanks as a C4 owner learning about these great machines, picked from the internet looks like cable controls to me similar to aircraft prop or fuel/mixture controls.
IIRC from reading the 964 lightweight used the 953 transmission so they may have used the same differential controls? So completely manual with no pads.
953 rally car Jacky Ickyx driver
Very interesting and thought provoking. Here's a couple of comments.
1. It is typical FWD behavior to go straight on spacked snow and ice when throttle is applied. Don't be so quick to blame the pdas.
2. As long as the car is rotating the rear diff will be at 100% lock. I have experienced the lock going on and off when I hung the tail out. This is a relatively slow dynamic unlike the longitudinal lock based on the ABS sensors.
3. The yaw sensor is an accelerometer. A rotation is always applying acceleration forces. A lateral drift may or may not, especially on ice, wet grass, etc.
4. IIRC from my reading, the 959 used the same center clutch pack and tranny with the exception that the planetary gearset was added to yield the 31/69 split and both clutch packs were dynamically controlled instead of preset. So it seems if one looked at the 959 you could figure out how to setup the 964 to reduce the 100% lockup situation. But you'll still have the 31/69 power split with it's inherent understeer.
5. One way to dial out the understeer is to put a thicker roll bar on the rear. The one from the C2 is recommended. I have one if anyone is interested.
I have a c2 rear sway bar, Rennline adjustable rear sway bar links, Bilsteins, eibach springs, Bridgestone re-71r, and a very aggressive alignment. The car is very neutral and I love it. But the pdas system can still make the car temperamental and require a lot of unneeded driver effort.
I haven’t had a chance to look into the 959 or lightweight system very much. I think you could mimic there complete manual diff control by unhooking the the hydraulic lines to the solenoid. Then hook those lines up to two master cylinders one that actuated the rear diff slave and one for the Center diff slave. You would have to fab up a bracket and a screw system for the adjustment to the master cylinders. So you could incrementally change the pressure the master cylinder was sending to the diff slave.
I think this would make the rear diff adjustable to how much you wanted an open diff and fully locked in the rear. You could also adjust the Center diff from 31 to 100 bias to the front. I’m not so sure you would want to tho.
The minimum bias of 69 rear 31 front may be to much front bias even at its minimum. But if that is static and not constantly changing like with the pdas on. Maybe you can set the suspension up to be neutral always at that bias.
My concern would be that if you were driving around with to much lock in the rear you would destroy the clutch pack or transaxle. But maybe if you went to this system if the car is a daily driver you would back off the rear diff engagement to save these components.
Also these are all assumptions based on the little knowledge I have gathered on the system. Please let me know any thoughts if I am accurate or not.
I do know the 964C4 pdas was very advanced and way ahead of its time. It’s very close in theory to electronically controlled diffs that are coming out today 30 years later. Porsche just had a different implementation for the system for safety vs all out performance.
I know with some tweaking there’ll be a very good solution. If you guys want to go to Ruff and ask you can do so. I am still at the point where I am interested in learning more on the system and possibly figure out a solution myself.
04-15-2018, 11:56 PM
