As automotive enthusiasts, we often talk about the different methods of making engine power, as much of our interest lies under the hood of our Mopars. The pros and cons of automatic versus manual transmissions are also a subject of our conversations, as are the various ways to get our cars to run quicker elapsed times, or make more power on the dyno. When it comes to the rear differential in our Mopars, however, most of us are aware that gear ratio selection is an important factor determining how a car will perform, but aside from that we have very little understanding of how the differential actually works.

Since most of our Mopars came with the Chrysler 8 3/4 rear end, we seldom have any major problems with this area of the car. In fact, even a factory 8 3/4 can handle substantial engine power without fatigue, and we've seen high-mileage rear ends handle modified big-blocks without much trouble. So, barring a catastrophic failure of the rear end, we generally just tend to forget about how many parts are in the rear differential and the various ways they can wear out, negatively affecting the performance of our vehicles. Worn ring and pinion gears will whine, and more serious problems will cause vibrations, excess drag, and even rear end lockup.

Swapping the rear end for another used one is an option, and most of us have swapped the centersection of our 8 3/4 to gain more optimal gearing as well. But as even the most recently produced factory 8 3/4 rear ends are 30 or more years old, the centersection you find to install in your car may be just as worn as the one you removed. Thankfully, most 8 3/4 differentials can be rebuilt, and many new parts are available. Since we needed a solid rear for our '71 Road Runner, we contacted Randy's Ring and Pinion to check out our options for the 8 3/4 in our car.

Like most Mopar enthusiasts, we like to perform much of the work on our cars ourselves. One area that we tend to leave to the experts, however, is the rear differential, and specifically the Sure Grip unit and ring and pinion gears of the car. It's not that the differential is beyond our technical expertise so much as this type of work requires specialty tools and must be done correctly so that the rear end has minimal drag and the appropriate amount of gear noise for the application. For these reasons, we decided to let the pros at Randy's Ring and Pinion build our 8 3/4 centersection, and we followed along to show you how it's done the right way.

When choosing a rear differential, the first determination that must be made is the intention of the car. For all-out drag cars with slicks that don't see street duty, a spool (solid) rear axle makes the most sense. The spool is lighter and locks the rear axles together, ensuring both tires spin at equal rpm at all times. At the other end of the spectrum, if you're just replacing the rear in your '74 Dart four-door with a Slant-Six, an inexpensive open style unit will work just fine. For the majority of us, however, a limited-slip type differential is the best choice for our street cars, or even cars that see a blend of street and strip duty. Chrysler called their limited-slip differentials Sure Grip and they were used extensively in muscle cars, trucks, and even station wagons.