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October 1, 1998

At Mustang & Fords, we're always talking about power upgrades. Whether it's a conversion to late-model 5.0L power and fuel injection or dropping in that nifty Ford Motorsport 351 H.O. SVO, we approve. If you're bent on converting a six-cylinder car to V-8 power, then you know that a differential change is definitely in the cards.

In many street applications, the standard 8-inch differential is up to the task, especially with an automatic transmission. The automatic trans cushions the whole drivetrain from the shock that comes with manual transmission use. However, when we get into higher horsepower applications and a manual transmission, the picture becomes more and more gray. Remember the 351 H.O. SVO engine we mentioned? Couple that mill with a T5 trans and you're going to be beyond the limits of that 8-inch diff.

Even with an automatic trans, breakage of the 8-inch rearend is likely to occur with high horsepower use.

Original equipment on K-code Mustangs, Shelby GT 350s and 500s, Cobra Jets, and many other high-performance Fords, the Ford 9-inch rearend is so famous even our moms have heard of it. Noted for outstanding durability, it's the choice of many drag racers, whose horsepower figures usually exceed those found on street cars. Using a 9-inch diff in your street rig can give you an extra margin of durability and allow you to make horsepower upgrades without having to worry about the outback. Let's look at what's involved in giving your Ford a bulletproof butt.

Housings for the 9-inch differential vary in width according to the year of your Mustang. Housings for the '67-'70 cars are 2 inches wider than those for the '65-'66 cars. Diffs for '71-'73 Mustangs are 2 inches wider still. Once you've located a housing that's the correct width for your rig, installing it on the car is a fairly straightforward operation. While it is possible to narrow a housing that's too wide for your application, the job demands perfect accuracy, so put away your hacksaw. Have a reputable shop such as Currie Enterprises perform the modifications for you. Currie can help you with all aspects of your rearend conversion.

Many 9-inch differentials came with a 28-spline axle, which offers good durability for most small-block applications. You'll be in good shape for anything up to about 300 hp. When we up the ante power-wise, a move to a 31-spline axle is recommended. This setup is good for up to 400 hp. When we go beyond that figure, there are plenty of aftermarket parts available to further beef up the rearend of your rig. The 9-inch is the most popular choice among racers, and this explains why there are so many components available for it. The basic design features an offset pinion that allows for greater contact between the ring and pinion gears.

Aftermarket housings and third members abound for the Ford 9-inch, so you may wish to go this route if very high horsepower figures are in your equation. These pieces can offer greater strength and less weight than stock components. The housings can be ordered in a variety of widths to suit your application. It's no mystery to racers everywhere that the 9-inch Ford differential is one of the most durable designs out there. When this fact is combined with great availability of parts and technology, it's easy to understand why this diff is considered the way to go.

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