Mark Houlahan
Tech Editor, Mustang Monthly
December 21, 2010

If you think back, probably one of the first things you did when you got your license was to find the nearest parking lot or group of friends, and lay some rubber to show off. While few of us have "grown up" since then, a nice peel out/burnout/whatever-you-call-it always puts a smile on people's faces-except for Johnny Law, of course. Most likely, though, after that first time you may have noticed only one tire mark on the ground. Call it the one-wheel-peel or one-tire-fire, either way you only had one tire clamoring for traction back there. This is probably due to the standard, or "open" rear that many cars came with originally, and was most likely the cause of some ribbing from friends and fellow gearheads. You needed to fix this issue, and fix it fast. Usually, the fix entailed swapping out the open rear for Ford's venerable Traction-Lok limited-slip unit, or if you were packing a 9-inch, you could upgrade to the Detroit Locker for some real fun, but that was about it back then.

Today is a different story. The aftermarket choices for Ford 8-inch and 9-inch differentials are much broader, bringing to the market such technologies as electric and pneumatic locking differentials, exotic friction materials, and more. Sure, the Traction-Lok is still around, and a good value for the right builds. The Detroit Locker is still hanging tough too, and available in several designs now. But they've been joined by some pretty big names in the aftermarket driveline parts arena. You've most likely heard of names like Auburn, Powertrax, Eaton, and others.

We dug through all of the choices to show you just what is available for the Ford 8-inch and 9-inch removable carrier rearends. We also delved into the late-model 8.8-inch rearend a bit too, as it's becoming a popular budget rearend swap for classic Fords and Mustangs alike. But first, we're going to walk you through the various differential types and their primary functions, so you'll be better prepared to make a purchase decision based upon your classic Ford's needs, be it street car, drag car, or road racer.

Why You Need A Differential

As your Ford travels down the road both rear wheels rotate at the same speed because they are traveling the same distance. When making a turn, the inside wheel, that is the wheel closest to the inside of the turn, is going to spin slower than the outside wheel due to the smaller circle, and thus distance, that the tire is following. Actually, all four of your tires take different paths when navigating a turn if you want to know the truth, but we're only concerned about the rear axle here, as the fronts are independent of each other in this regard.

Anyway, as the rear tires have different wheel speeds during the turn, there must be a way to separate the left and right wheels from each other, otherwise driveline strain and tire wear/noise would increase to unacceptable levels (though we know of plenty of people that love the sound of a tire chirping in a tight turn with a locker or spool, but we digress). To do this, the rear axle is designed with a differential; a device that can differentiate the incoming power from the driveshaft between the two different wheel speeds when needed.