A few notes on Posi ,trackloc etc.
Positraction and TracLoc(k) are simply the brand names that companies - GM and Ford respectively - have chosen to describe their limited slip differentials. There are lots of different types of limited slip differentials. However, both positraction and tracloc limited slips operate in the same manner. Contrasted with a simple open differential which sends power only to one tire and allows the other to freewheel (when turning), limited slip differentials send power to BOTH rear tires. However, there are spring loaded clutches in the differential which, when they sense a speed differential between the two rear tires (as when turning), allow some 'limited slip' in the clutches. This allows the outside tire to turn faster than the inside tire eliminating any scrub or 'chirping'. There are other ways to achieve this limited slip effect besides with clutch packs. Torsen-type LSD's and the TrueTrac brand use a series of worm gears and sliders to achieve a similar effect - but in the case of the Torsen - more power is sent to the tire with the MOST traction. Whether clutch-type or gear-type -- LSD's generally work completely unobtrusively. You don't know they're there -- you just know the car corners quietly, but when you get on it - you have traction to both tires. However, the TrueTrac and Torsen units have nothing to wear out; unlike clutch-type LSD's which have to have springs/plates replaced when they wear out.
There are also automatic and manual locking differentials, and spools. A spool basically LOCKS the two axles together -- THERE IS NO DIFFERENTIAL. The term differential exists because that unit allows a SPEED differential between inner and outer tires when turning -- a spool doesn't allow this. A spool-piece usually slides into the female ends of the axle splines in the gutted differential between the axle ends. The cheapest and least elegant version of a 'spool' is simply welding the spyder gears together -- done by MANY racers.
Lockers -- automatic and manually locked rear ends -- have a series of dog clutches that are either locked or they're not. The automatic units lock both axles together UNLESS they sense a speed differential between inner and outer tires -- when they sense that they unlock. They are typically quite noisy as they 'ratchet' in and out of locked position, although newer models have gotten better. Manual locking units work the same way (dog clutches) however - you choose whether and when they are locked. This can be done a number of ways - usually you'll see them electronically or pneumaticallly operated.
For street cars limited slip diffs seem to offer the best compromise, although some like lockers and are willing to live with the noise compromise. For drag racers and some oval track cars a spool is all that's needed. For road racers and autocrossers, a good LSD is usually preferred since they have to negotiate turns. However, you'll see some spools there as well.
