vinyl_tien

I have a 89 S4 manual with LSD, I want to install G28.55 trans.
What is the big different when driving LSD and PSD?
Can 89 S4 install later PSD system? if could what kind parts should I have?

vinyl_tien

Hi Sterling,
Thanks for your advice, you are right in my point.
I really want to know " what would it really gain?"
PS.I have a GTS(serious body damage) will be parting soon so I will remove the PSD system from them.

9x8

First of all, why are you swapping your old transmission for GTS one?
Is it bad?

What I wanted to say is: PSD is not something that you would add for performance.
While very complex and well-engineered, the system IMHO is still inferior to the conventional LSD units.
So, if you are absolutely sure you want to swap to G28.55 - rather swap an LSD from your old trans and be done with it.

Too much work to get PSD up and running, with basicly nothing to be gained.

vinyl_tien

Hi 9x8
Thanks for your advice.
The reason I want to use the PSD is because I can not find any 5 speeds PSD 928 in Taiwan. I want to know how good it is or just almost same like LSD. Sure it will spend a lot money, so I want to know how good or just almost same.

docmirror

The mechanical installation is not complex. You will need to make tabs for mounting the pump, and run the lines to and from the gearbox. There will be changes to the wiring that will be complex. The force transducers are in a set mounted under the drivers seat. Then, it interfaces with the ABS controller, so you would need the later controller, and add some wiring directly from the PSD, to the ABS, to the force transducers. It's way more than it's worth.

If you want to use the later trans, I would just run it as an open diff, and not install the PSD part.

9x8

I see what you mean there, but I can assure you that it's not really worth it.
It does indeed feel a bit different, but IMO the conventional LSD is just way more predictable (and therefore - better ).

puyi

The main problem is : 90 wire harness is very different as previous. Not to say all has been reworked but most part had. So, even if you get the PSD harness from MY '90, you will not be able to install it "plug and play". You will need to work on all the electric scheme. When you open the Central electric box is about to say open the pandorra's box.
For my 89 race car, I have simplified all the wire harness, It was a mess and it take me lot of hours to do it reliable. As I Have a '90 928, I also can say that It is much better done on a 90 and late as earlier model.

Best

Puyi

vinyl_tien

Thanks docmirror, 9x8, and puyi, you guys answer is so helpful. Now I have some ideas.
Today I see a friend who own a S4(90), PSD auto. I can hard to tell the different from a PSD and LSD, WHY??

9x8

If I understood you correctly: both systems perform similar function, so the both feel very similar.
But, PSD is more oriented for street ("normal") driving, and is not really intended for some hard use like trackdays or trying to have some fun with sideways driving.

vinyl_tien

Hi 9x8
Thanks for your advice. I have another question, right now I am using G28 10 trans. Is G28 55 much better in gear parts?

9x8

If I'm not mistaken, both boxes have identical gear ratios, but 55 should be a bit stronger.
Which means, it makes sense to upgrade only if your old tranny is bad or you have an engine which makes way more power than stock (stroker, supercharger, turbo, etc.). And even then IMHO the best option would be to just run the stock transmission until it grenades (which still could take a lot of time ) and only then swap to the 55 one.

So, in short: I'd keep the G28 55 one as a spare and just enjoy what you have now.

EDIT: Wait a second, I somehow keep calling your G28.55 transmission "GTS" while it actually is a correct type for a GT.
I guess, I got confused by you stating that you have a spare GTS car.

Anyways, it means that it's even not that much better than your old one.
Also, it means that it actually might have a conventional LSD, not the PSD (if it's off '89GT).

docmirror

Ok, LSD diff provides up to 40% locking power to the wheel that is free-spooling(spinning and not making traction). In this event, the locking power is purely mechanical in nature, and is a function of the ratio of the left and right rear wheels spinning at different rates. There is a spool in the diff that will wind up and a set of clutches that begin to lock the axle when the difference is too great. Of course, we need to allow for a certain amount of difference in rotation for cornering because the outside and inside describe different radii during normal cornering.

PSD is a completely different animal. I will disagree with some on here that the PSD is more for normal type driving. Most drivers under normal conditions will not even know the car is equipped with PSD, however, get the car on gravel surface, or snow, or get it into serious track work and the PSD will come alive.

The PSD system has 6 inputs into it; lateral acceleration(cornering loads L & R), Fore and aft acceleration from the combined ABS sensors(speeding up, and braking), and the two rear ABS sensors for differential wheel rotation sensing. From this, the PSD controller performs a complex sigma and delta of these parameters and decides which wheel to begin locking to the crown and pinion gears. It can limit the slip of right or left rear wheel up to and including 100% torque.

In the most extreme example, suppose you were standing still on a surface where the right wheel is on a sheet of glare ice, and the left wheel was on hard asphalt. The moment you begin to move, the right wheel will spin. The fore and aft sensors will determine the acceleration of the vehicle, and the amount of wheel spin on the right side and begin to lock that wheel, thus transmitting more and more power to the left wheel, until the right wheel is running at basically the same rotational speed as the left.

Another extreme example is if you were on the track, and braking hard into a left turn, while going slightly downhill. The left rear tire will lose traction and begin to slow down. The RIGHT side is now considered to be 'slipping', even though the left side is the one that is slipping under braking. The four accelerometers sense the turn, and the braking and the speed sensors tell us that the right side is going to fast, so the diff will begin to lock the right wheel until it is spinning the same rate as the left. Since you are slowing, and not accelerating, the locking diff will not affect the traction as much.

In all cases, the PSD will attempt to even out the speed of the two rear wheels by engaging the clutches in the diff by hydraulic pressure in the clutch pack. PSD will only have an affect when the two rear wheels are operating at different speeds, and the difference isn't accounted for by the different radii of the turning car, and the acceleration of the car forward.