This is probably a really dumb question. But since I spend most of my time messing with engines, and the electrical aspects of cars, I never really paid attention to transmissions or differentials. The question is what the hell is the difference between a regular old, plain diff, a limited-slip, and a "posi-trac". I've got a good generalization about the differences, but my redneck friends all have different opinions. And for once it would be nice to get the right info to shut them up.

First of all, "posi-trac" is slang that has developed from GM calling thier clutch type limited slip diff positraction back in the '60s.

There are really only four types of differential.

An open differential is just what it sounds like. It has no provision to keep the drive wheels spinning at the same speed. It transfers power from the wheels that grip to the wheels that slip, to use some Subaru slang.

A limited slip differential has some sort of means of transferring power to the wheel that has more traction available. There are about a billion different ways to make a limited slip differential. I can think of a clutch pack, a cone type or "auburn", a torsen and a viscous coupling.

A spool is a differential with no differential if that makes any sense. The drive wheels are forced to turn at the same speed under any circumstance. This allows all the power to be transferred to one wheel. It does not allow the different wheel speeds nessicary to negotiate a turn normally. One of the drive wheels has to slip against the road to turn the vehicle.

A locker is a spool that can be disenguaged. Some are electrically operated by the driver, some operate from the application of torque others are operated by compressed air, and there is the infamous "gov-lock" that GM puts in most of thier full size trucks. It combines a weak clutch type limited slip with a govenor driven locker. That one makes for some pretty entertaining explosions.

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First of all, "posi-trac" is slang that has developed from GM calling thier clutch type limited slip diff positraction back in the '60s.

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Positive traction.



An open diff distribute torque equally to both wheels. A limited slip transfers torque unequally to both wheels.

You dont know the power of the darkside