Installing An 8-1/4 Rearend To A Muscle Mopar

When it comes to making your car perform better, there's always a set order for performance modifications; front-to-back. We all start under the hood (usually with carb/ intake/headers, often followed by a cam swap). For those who like to row their own, a performance shifter is installed, maybe a clutch; automatic guys modify the valve body and later install a high-stall converter. Last on the list has always been the rearend. A lot of post-muscle Mopars rolled off the line with a standard, "small" not- built-for-punishment 8 1/4-inch rear end. The 8-3/4 is far more popular by virtue of it's easy to set-up drop-out carrier, ease of gear changes (need a different ratio? Just swap the carrier!), no C-clips, stout construction and, of course, the strength of a larger ring gear. If you really want strength on an unparalleled level, you can step up to the legendary Dana 60. Able to handle torque output on the scale of diesel locomotives, the Dana is the last word in rearend strength. During the halcyon days, all it took to get one as original equipment was checking off the right boxes.

If your current street stomper came equipped with an 8-1/4 rear, standard thinking goes something like this -- If it ain't broke, don't fix it. When it breaks, upgrade to an 8-3/4 or Dana. We'll build the engine and trans, but forget about the rearend. When it breaks, we call the car all kinds of nasty names, tow it home, yank the rear and replace it. Most project cars are high-mileage cars, but the engine and trans have at least been gone through, if not outright rebuilt. When's the last time you even changed the fluid in the rearend? Then you get mad when the poor thing breaks after increased horsepower from the front of the car on top of years of neglect.

Truth be known, unless you're building a big-block or a full-race small-block, that lowly 8-1/4 is good enough. There are some guys who prefer a totally built 8-1/4 in drag racing situations because it's lighter and there's less reciprocating mass, so more power hits the tires. And because they're the "cheaper" rear ends, the parts to rebuild and upgrade them are less expensive. Finally -- and this is the real selling point -- it's already under the car! You don't have to pay for a new housing, it doesn't have to be narrowed to fit, you already have brakes on it, even the spring perches are in the right place! The situation we were in with one of our projects was, we had the whole rearend and brake set-up already in the vehicle, but it was an open diff with a highway gear. We thought about replacing/upgrading it, but decided to work with what we had. The engine is going to be warmed over, and the 904 will be upgraded, but it's a daily driver that probably won't see much strip duty. We felt the "little" 8-1/4 would hold up nicely if rebuilt right the first time. We ordered a complete differential, ring and pinion, and rebuild kit from Ring & Pinion Service and a rearend girdle cover from LPW Racing Products (PN 301-10C, fits 8.2 and 8.3 Chrysler 8-1/4). If we were going to put the vehicle on the strip, and especially if we were going to run slicks, we'd have stayed with this rear but purchased aftermarket axles and a C-clip eliminator kit. We had Mike Mott at Pro Automotive Performance Center put together a stout little rear that will survive nicely under an average street car. Certainly less than a full swap and perhaps just right for our needs.