Different-ial Strokes For Different Folks
You have the axles, gears, studs, and everything else to put together your 8.8-inch rear. Now you need something to turn the axles, and that task is handled by the differential. There are numerous types from which to choose. You can go with a locker, a spool, a mini-spool, a limited-slip, or an open differential, however, for street-driven applications with higher-than-average horsepower numbers, there are a few that can be eliminated right away.
Let's look at the open differential first. Because it drives only one tire rather than both, if that tire spins, you'll be left sitting in your tracks. While the open differential is great for driving around town, for performance applications, it is, in a word, useless. It is commonly referred to as a peg-leg, or one-wheel wonder, for obvious reasons. Since the differential transfers power to just one wheel, trying to put all of the power your supercharged 4.6 Cobra motor makes to the ground is impossible.
The opposite of the open differential is a full locking differential, better known as a spool. The spool is great for getting all of the power to the pavement, but it drives both axles with no slip, so when it comes to street use, it is not recommended. For example, the spool and mini-spool are both great for drag racing because they allow both axles to get 100 percent of the engine's and drivetrain's potential. When it comes to going around corners or cruising on the street, however, instead of taking the corner nicely, the rearend of the car will hop and chirp. That's because it is necessary for the inside and outside tires to spin at different speeds when negotiating a corner. In a turn, the inside tires cover less ground than the outside tires, so they will turn slower. Without the ability of the rear to slip, the tires would go the same speed, and the inside rear tire will skip and break traction.
Therein lies the true advantage of the limited-slip or locker differential. More commonly referred to as the "posi unit," the limited-slip differential, or its upgraded, more sophisticated sibling, the locker, transfers all power to both rear wheels when going straight. In a corner, the unit senses an increase or decrease in wheel speed or torque on one side as opposed to the other, and it transfers or decreases the amount of power from side to side, thus allowing slip. It's the best of both worlds.
In addition to the gearsets, axles, and other parts offered by each respective manufacturer listed herein, we have also included the different types of differentials available. Keep in mind that choosing the correct differential and associated parts depends on the application and/or type of racing/driving you plan on doing.