Savant

Does anyone know the secrets of the PSD? I've read the 928 owner's manual and searched the usual 928 internet haunts (including rennlist) looking for detail on the PSD, but have found only patchy information which sometimes contradicts other references.

From what I've read, it seems the PSD is not an LSD. An LSD offers a degree of fixed differential lock which is active at all times. The PSD is a full locking differential, which via computer control offers 0% lock under normal conditions and up to ?full lock when required (this is certainly what it feels like if turning when it is active).

In theory it seems a better traction solution than an LSD, and much better than "traction control" which applies braking force to the slipping wheel and throttles back the engine if both driving wheels slip. In practice my '90 928S4 consistently outperforms my friend's 97 SL500, especially if his traction control is on...

I would be interested to know:
- is my summary accurate?
- exactly how does it work (mechanically)?
- what gradations of lock and maximum lock are provided?
- what other PSD triva is there?
- do any other performance cars have a PSD type diff, and if not, why not because it seems to work well/reliably

Thanks in advance...

2V4V

Andrew,

Well, lotsa questions, I'll give you a few answers that fall readily to hand...

I'm not aware of any other production automobile that has the exact feature. Rally cars (and production AWD cars) do all sorts of really cool stuff with computer controlled diffs, but I don't believe the PSD-esque solution has been used outside the (RWD) realm of the 928. All wheel drive stuff being the major exception to the rule, but they seem to all use a slightly different way to solve the power-shifting.

Mechanically, there is a very high pressure (~180 BAR) hydraulic pump unit in the rear section of the LH rear wheel fender. This is connected by a hose to a slave cylinder plugged into the diff. Movement of the piston out of the slave cylinder forces a clutch-pack together, locking the wheels at a whatever percentage the computer deems appropriate.

How much lock-up? Well, I recall reading "0-100%" in a factory document, but I don't recall which document, so I can't cite it. The test procedures given in the factory manual for the PSD dictate a breakaway torque in excess of 369 FTLB, so I tend to think it's probaly capable of damn-near 100% lockup, until you really beef up your powerplant.

There is a factory service manual supplement for the PSD (#35, WKD 481621.35) that will give you some more technical insight, but not total illumination.

Suffice to say, for a system that only uses the ABS senders, 1 lateral acceleration sensor, and a (relatively) primitive comuting unit, it's not a terrible system all things considered.

Most cars currently do "traction control" via modulation of brakes at the individual wheels as well as modulation of engine output. IIRC, Porsche's newer version of "save your ***" on the Boxster (PSM) is a system of that ilk, but it manages in more dimensions than just keeping the wheels tractive - it figures out that you screwed up, how you screwed up, then determines a braking effect to try (rather successfully, I'm told) to keep you pointed the direction you'd like to go, rather than in the weeds.

There are many LSDs that have a certain "variance" to the lock-up - they use the thermal expansion of a fluid between the plates to modulate the amount of lock provided. They are generally referred to as "viscous locking". The downside is that they are not capable of the attack or decay response times that an automated "active" system (like PSD) is.

There's more detail and I glossed over some things, but that's the quick and dirty of it.

Greg

John Veninger

It's AWD, but the 911 Carrera 4 uses the system.

Rich9928p

- do any other performance cars have a PSD type diff, and if not, why not because it seems to work well/reliably

>> Mercedes Benz offered it as an option on their cars until traction control became available ... the technology was developed by Bosch and offered to their customers.

Vilhuer

Porsche did 5 to 6 minute long marketing video back in 1989 when they introduced PSD for 1990 model year. It demonstrated 3 different modes of behavior.

1. Accelerating on uneven traction surface
Right rear wheel spins (like it normally does on open differential car what PSD is, until pressure is aplied). PSD is locked up to 100% on speeds under 30 kph. Kph limit is there to prevent uncontrolled manoeuvres happening.

2. High speed cornering with high transverse acceleration
Locking is introduced when needed by calculating cornering speed, curve radius and transverse acceleration. When inside rear wheel tends to spin, locking between 0 to 100% is varied and maintained as situation requires.

3. During load change reactions like steping on the gas and braking
Differenting wheel speeds are sensed, also right or left cornering and momentary wheel behaviour like acceleration or deceleration. To prevent oversteer, 0 to 100% locking is applied as needed.

Measurements are made in thousandth of a second.

To my undestanding PSD tried to keep rear wheel speeds the same or about the same debending is car moving straight ahead or cornering. In straight just same and in cornering minor difference is alloved as different speeds are normal and reguired for inner and outer wheel.

In straight line, locking is limited to 30 kph to prevent sudden movements in higher speeds. Under 30 both wheels will spin same amount as PSD don't limit engine power or apply brakes. Above 30 only right wheel spins like with normal open differential.

In corners right locking % is calculated to try to prevent either back wheel moving in inappropriate speed, too fast or slow. PSD tries to keep them spining at right speed. If it can't, both will spin same amount of 'wrong' instead of one being all 'wrong' and other like it 'should'.

Or I could be totally wrong.

Before anyone asks. There is no video, I didn't mention it, nothing to see here, move along...

Regards,

Erkka

1992 928 GTS 5-speed PSD, maybe
Oak Green Metallic 22L
Classic Gray MX

sweanders

I think you are right on the money Errka, I've seen those movies some years ago...

John Veninger

Do you have the video

Savant

Thank you all - it seems the PSD deserves to bear the Porsche name. I would be interested to see that video if anyone is keen enough to make it possible. I look forward to reading more info or trivia (if there is any).

sweanders

You don't need the video.. What you need is to experience going a tad to fast over a crest that is just at the exit of a very sexy S-curve (whose curves you know as well as your high school crushs hip movements) and realize that there is mud on the road so that when the car is settling it loses it's rear while sliding on the front tires slightly sideways... I did this recently and just kept my foot down.. It took a couple of milliseconds for the PSD-light to come on and a magic hand pulled the car straight again while the two of us in the car held our breath....

I'm not a great driver, but not even Schumacher could've saved that situation without PSD... There was a fellow Porsche owner (968) sitting in the passenger seat while this happened and he just shook his head afterwards...

Scott M.

Acura also used a very, very similar system in the NSX. They also added steering input to the mix so the TCS (traction control system) knows whne you're jerkin' around. Combine that w/ a mid engine chasis and you have one potent handling ride.

Scott
Porsches