[dr bob]

PSD (Porsche spier (sp?) differential) was included on US 928's starting with the 1990MY. Prior to that, 928's were available with optional clutch-type limited-slip (LSD) units. Look for option code 220 on the build sheet or on top of the gas tank. Sometimes there is a sticker still attached to the diff cover announcing the LSD presence.

The PSD system includes a pump and reservoir for hydraulic (brake) fluid in the left rear wheelhouse area. There's a controller linked to the ABS sensors that detects wheel slip under acceleration. When activated, the PSD pump pressurizes a hydraulic cylinder that increase the preload on the clutch pack in the diff until slip is eliminated. Because the allowable slip is determined dynamically, it's great for street use but not necessarily great for track use. Many track users remove the heavy pump and the rest of the hydraulics, and replace the slave cylinder on the diff with a simple steel cover plate with an adjusting bolt. Clutch pack preload is therefore adjustable, just not while driving.