Guide To Limited-Slip Differentials

The limited-slip differential (LSD) is one of the greatest automotive achievements since the invention of the automobile itself. Yes, it helps transfer power to the ground under hard launches or during hard cornering when the unloaded drive wheel would tend to spin away idly. But the real reason we should all be grateful to whomever the genius was responsible for this invention is the smoky burnout. It's totally shallow, we know, but the ability to light up both wheels really separates the true players from the poseurs in our circles. A performance car must light up both rear wheels.

If your car already has a limited slip, you're golden. But what's the guy with the open differential to do? Save your pennies, buy a posi, and install it as soon as possible. That really should be the first modification you do to your car.

A couple of us felt the scourge of limited-slip envy. Our art director's '85 Monte Carlo and your lowly author's Crown Victoria Police Interceptor were severely lacking in the differential department-a situation we remedied here in the process of writing this article. Tech Editor Smith's longtime friend and contributor Tim Moore fixed what was ailing us by installing an Auburn limited slip in the Monte and a Ford Traction-Lok unit in the 8.8-inch rear of our ex-cop car. Now we can hang with the best of them.

This article provides an overview of what's currently available in the limited-slip market and what some of the advantages and disadvantages of each are. Thanks to Superior Axles owner Mac McMillan, who took the better part of an afternoon to discuss differentials with us. Here's the lowdown.

Nomenclature
There are several variations in designs of LSDs, and each manufacturer referred to its LSD with different terminology. Here's a list of what they call them.

Types
As mentioned before, there are several different ways of locking both axles together, including viscous couplings and Torsen differentials, but our market generally uses clutch, cone types, and sometimes gear types as seen in the Detroit Truetrac. We will break down the list by manufacturer.

Note: The horsepower ratings are guidelines. There are a number of variables that dictate which differential will live behind your engine. Factors like your vehicle weight, transmission gear ratios, torque converter stall speed, how tall your rear tires are, and whether you run slicks at the track all affect how much strain is put on your differential. Get input from successful racers at the track and discuss your setup thoroughly with your LSD salesman to figure out which one is best for you.