Limited slip differentials for BMWs are one of the greatest performance upgrades that can be made for your driving machine! They are a relatively inexpensive upgrade that can be quickly installed and provide a serious performance enhancement. Generally speaking, limited slip differences consist of clutch discs that are spring loaded in a carrier mounted within the differential housing. When you push on the gas pedal power is transferred via the driveshaft in your car to the input of the rear differential. The clutches distribute the power to both wheels regardless of traction conditions. In a non-limited slip differential, the driving force is passed to the wheel with least resistance. This means that if one wheel is slipping all the driving force of the car will spin that wheel, while the wheel that is in solid contact with the ground will receive no driving force. With a limited slip diff the clutches within the diff keep the force evenly distributed to both wheels, so that if one is spinning, the other will still receive power. This makes a big difference when you are cornering hard on a freeway onramp, where the inside wheel unloads and you need all the power to be delivered to the wheel with the best traction. Limited slip diffs provide this traction. The word ‘limited’ means that there is slippage allowed within the clutch pack when you are turning corners sharply. This allows for a ‘differential’ movement between the inside and outside wheels in a turning motion, since the outside wheel must travel further to make that turn. If there were no differential motion, your car would bump its way around the turn as one or the other wheels would be forced to skid slightly to enable the turn to be completed.
Checking to see if you have a limited slip differential in your car. If you can put your car on a lift, it is easy to check if you have a limited slip differential. Put the car into neutral and go to one side of the car at the rear and manually turn the wheel on that side. Watch to see what the wheel on the other side of the rear does. Does it turn in the same direction as the wheel you are turning? That is a definite sign of a limited slip differential. It should positively move in the same direction as the wheel you are turning. If the other wheel turns in the opposite direction, you have an 'open' diff or one without clutches. If the motion of the other wheel is moving only slightly either in the same direction or not at all, this also indicates an open diff. The other wheel should turn as if it is directly connected to the wheel you are turning, since it is, by clutches! Also, BMW attaches a tag to the rear of the differential on the cover via one bolt. If you can clean this tag off--it can be pretty dirty or rusty-- you should see the letter 'S' at the start of the tag followed by three numerical digits indicating the ratio of the diff. Finally, generally speaking, if you scrape clean the bottom of the diff, you will probably find an 'S' stamped in white or yellow paint somewhere on the bottom of the case.