We are now offering a PSD replacement hose made by Greg Brown. The system works at a very high pressures and nearly 20 year old hoses may be doing some swelling by now.
http://www.mailordercentral.com/928i...944 355 081 01

 

Cool!!

But your linky needs fixing, it comes up with a 968 piston which I don't think is going to be able to handle the pressure.

Cheers,

 

http://www.mailordercentral.com/928i...355%20081%2001

This one should work... try searching for part number 944 355 081 01
if it doesn't..

 

Mark is technology challenged.

 

Probably just a ploy to get folks to buy the 968 pistons for the 6.5 liter upgrade as a while you are in there fixing the PSD hose trick

 

Dumb question? ... is this just a high-quality 944 rear brake hose or does it have some special/different qualities for PSD use?

 

This hose is made from non-expanding PTFE material, covered in stainless and then coated for further protection. Yes, it is different and more expensive than the stock 944 rear brake hose. Note that in the 944 application, the rear hoses are limited in the amount of pressure that they "see" by the pressure limit valves, installed in the master cylinders. In this PSD application, there is not this pressure limit and these hoses "balloon" at a very high rate.

The PSD system works from pressure sent from a pump/reservour to a slave cylinder, which pushes on the "fork" which increases the "limited slip" action of the differential. After initial activation, the system continues to "look" at the percentage of slip and continues to "adjust" the pressure until the "speed differential" of the rear tires is within design specs.

The faster the system responds, the better the system works. Having a "spongy" rubber hose in the system is actually a very bad idea...especially as the hose ages. These hoses get so soft and "balloon like" that many times the required pressure to control the speed differential of the two rear tires is not achieved until the vehicle is actually all the way past the corner that caused the system to activate. The time from "zero lock" to "required lock" gets so long (in terms of time) that the differentials slip way too much and the oil turns black very quickly, due to slippage.

The PSD system is a very, very clever engineering marvel. It is a very high quality system, which when working properly, is one of the better ideas that came from Porsche. However, it is extremely overlooked and generally works very poorly. Many systems haven't had the fluid changed in years. Many are so filled with "old dirty fluid" that the filter in the reservour is completely clogged. We found that changing the fluid, installing a new filter, and bleeding the system isn't satisfactory, in most cases. Getting rid of the "expansion" of this hose can reduce "lock" time by over 50%...and that makes the vehicles faster, safer, and more fun to drive, through corners.

 

Indeed.

 

How do you know if the PSD is functioning properly?

 

If its not it will raise error to dash.

 

When the PSD light is flashing all the way through first gear, and part of second.

 

Only if it malfunctions electrically...there is nothing in the system to tell it if the slave ever moved or the "limited slip" did anything at all.

 

Good explanation Greg, thanks. Now, out of curiousity, where does the hose go exactly ... I'm thinking the hard line goes all the way to the slave on top of the diff, so hoping the hose isn't buried so far up underneath something that you have to take the whole car apart to change it !

 

Tom-

Here's a shot of the flex hose , it connects to a check valve that sits inside that half box-y bracket on the left rear of the diff. ('Scuse the filth, this is the GTS when I got it. Note the rust milkshake dripping from the line...) It bascially goes between the hardline from the pump and the hardline around the top rear of the diff:



You can sorta see the hardline from the pump, dangling in front of the battery box heatshield- it is normally bolted to the floor of the rear deck in a its little 'U' bracket:

 

Question:
If you change the transmission fluid, does the PSD also gets changed? Is it same for an auto or 5speed?
Thanks
Bilal