September 26, 2005

Power managed foolishly pretty much leaves you sitting there with your pride (or broken parts) in your hand. Managing the power for your project doesn't just mean building a tough-as-nails engine that will crack the 400-horse mark on a dyno. No, you have to consider that the 400 rampaging horses coming off your engine's flywheel have to be channeled through a transmission, driveshaft, and, ultimately, to the rearend where the power is transferred to the pavement.

When we consider getting that power to the pavement, we think about tires, traction bars, shocks, springs, torque arms, and more. But do we ever consider the differential? We don't give that chunk of steel enough of our time, especially during the planning phase of our projects. The transmission gets pulled for a complete overhaul, the driveshaft gets new U-joints and balancing, but what happens to the rear? The rearend usually only gets our full attention when it breaks, or when we get spanked that first Saturday night at the drag strip because we didn't calculate the proper gear for the power on board. We wind up picking up the rearend pieces because we forget to build for 450 hp.

Our point? You can't get there effectively with a smelly, old, worn-out differential. You have to plan your differential just like you plan the engine, transmission, chassis, and brakes. This means setting aside some of your project building budget, thinking about your overall desires and needs, and picking the proper parts to build a sound and reliable rear that can handle what you've got in mind. Remember, building a rear for your project is different than adding a stereo system or a billet grille--you really don't want to go back and upgrade later.

Spend the money now and build the rear your project will need for now, as well as into the future.

Planning For Power
Your differential build plan needs to begin just like the rest of the car's building and driving plan.

  • How do you intend to use the vehicle?
  • How much power do you intend for your engine to make?
  • Will your transmission have overdrive?
  • What's your budget?
  • How do you choose a differential, rear axle housing, and all of the components?

Currie Enterprises offers the most extensive line of differentials and rear axles in the world and has been upgrading Ford rears for decades. We're going to focus on what's most popular with Ford buffs: the removable carrier 8- and 9-inch rear axle assemblies available in Fords since 1957.

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This is Currie's basic 9-inch carrier based on the C7AW-E Ford casting. Currie throws a little more meat into the places where it counts, such as around the pinion pilot. That makes this stronger than the factory C7AW-E casting. For less money, Currie will sell you the stock C7AW-E gray iron casting for under $100.
Used Currie C7AW-E Casting Price: $79.95
New Currie C7AW-E Casting Price: N/A

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Big 9-Inch Options
Ford's super-durable 9-inch differential was the company's first removable carrier rearend when it debuted in 1957. The 9-inch Ford, named for its 9-inch ring gear, has been the industry standard for performance rearends for five decades. In NASCAR, the Ford 9-inch is the only rearend anyone runs, including GM and Chrysler. Currie Enterprises has made the 9-inch Ford something of an institution, offering a variety of 9-inch designs for many uses and budgets. Let's take a look at what will fit your lifestyle and budget.

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Ground Power
Building the right 9-inch Ford for your project begins with a solid foundation (the gearcases just mentioned). There's an array of gearcases (also known as chunks, pumpkins, third members, differentials, and more) to fit a variety of budgets and missions. Your budget, expected mission, and the amount of power your engine makes determine choice.

If you're seeking a limited-slip differential with nice street manors, the Currie Trac Lock is perfect for your Ford street project because it uses clutches to equal the distribution of power to both wheels. Available for both 28- and 31-spline applications, the Currie four-pinion Trac Lock is designed primarily for Ford 9-inch castings. You may also run the Trac Lock in a Currie 9-Plus. Currie Trac Lock: $399.95

When it comes to differentials, there's a whole lot more going on inside the gearcase that mandates good, solid decision making during the planning stages. Noise and erratic operation make locking differentials a bad idea for the street. Every time you turn a corner, locking differentials rear their ugly heads with a lot of noise. This is why limited-slip differentials make more sense for street use.

Most limited-slip differentials use internal clutches, which slip quietly in the turns. Choosing a carrier also depends on how much power you intend to throw at it. Limited-slip carriers have their limits before horsepower and torque rip them to pieces. Each manufacturer will tell you the limits of a particular carrier. This isn't everything that's available from Currie, but here are some of the highlights.

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Full Support
The pinion support is another weak link not enough of us pay attention to. The pinion support fits into the nose of the gearcase and, as its name implies, supports the pinion gear and bearings. The 9-inch Ford was a well-equipped pinion support-wise from the factory. However, the more stable we can keep the pinion, the longer your 9-inch will live. Currie has a couple of nice options for rearend builders.

Currie's 9-Plus pinion support replaces the stock 9-inch pinion support. Not only does it fit your factory 9-inch case, it fits virtually all 9-inch cases. You can abuse this piece with up to 350 hp. A stronger, heavy-duty pinion support is also available from Currie with 50-percent more bearing surface. It is designed for loads greater than 350 hp and fits all of the Currie 9-Plus cases.
Currie Standard Bearing Pinion Support (PN 033): $95.95
Currie Big Bearing Pinion Support (PN 94031): $106.60

New Life For 8-Inch Fords
The 8-inch Ford axle was born for light-duty operation in the intermediate and compact Fords and Mercs. It consumes less power, weighs less, and delivers reliability with great regularity. Currie has long recognized the 8-inch Ford's purpose for a wide variety of applications. For example, did you know the 8-inch Ford isn't just for Mustangs, Falcons, Fairlanes, and Comets anymore? This tough and dependable rearend is used in three-wheel motorcycles, golf carts and more.

Here are the two types of 8-inch gearcase castings. On the right is the '62-'66 casting with horizontal ribs. On the left is the '67-and-up waffle casting, which is stronger...

The 8-inch Ford's weakness is the thin pinion pilot support in the gearcase. This is the part that breaks when we throw large amounts of power at the 8-inch Ford. When the pinion pilot gives out, the ring-and-pinion collide in an unpleasant way. So what to do?

There are two 8-inch castings. Prior to 1967, the 8-inch gearcase had horizontal ribs and a thin pinion pilot support area. And like the 9-inch Ford, the oil filler plug was on the axlehousing. Beginning in 1967, the 8-inch gearcase had waffled ribs for added strength. The oil filler plug was moved to the gearcase, as well. This is the gearcase to go with if strength is important to you.

...Remember, the '62-'66 casting is dependent on having an oil filler plug in the axlehousing. Don't make the mistake of installing this casting in a '67-and-up axlehousing.

Because Currie Enterprises understands the need for a heavy-duty 8-inch differential, it developed the new Alumin8 gearcase for 8-inch applications. The Alumin8 is precision-made with 206T6 aluminum for extraordinary strength. This makes it ideal for high-power street applications, but it is not designed for racing. The Alumin8 eliminates the 8-inch Ford's basic pinion pilot support weakness. Currie puts more meat around the pinion pilot for added strength. This makes it better and lighter than the stock iron casting.

One important issue to remember about the 8-inch Ford is a lot of what we know about the 9-inch Ford applies to the 8-inch, as well. The 28-spline, small-bearing axle shafts interchange between the 8-inch and 9-inch axles, as long as axlehousing width is the same. the two sizes differ in carrier and ring-gear sizing, but the basic principles are the same.

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As these examples show, the small bearing (left) and large bearing (right) show us the differences in surface area for the bearings that would be installed to support the axles.

Large Bearing/Small Bearing
The 9-inch Ford was available with small or large axle bearings and flanges. From the factory, this meant two different wheel bolt patterns: large and small. Big Fords in the '70s, as well as some trucks, had the large bearing and bolt pattern. this is important to performance in durability. The large bearing housing/flange is simply stronger than the small bearing.

Differential SetUp Quick Points

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9-Inch Housing Applications & Types



Hub Size

Bolt Circle

Housing End

'57-'63 Early Sm. Bearing 2.43" 41/2" Small Bearing
'64-'73 Sm. Bearing 2.43" 41/2" Small Bearing
'57-'63 Early Lg. Bearing 2.43" 41/2" Large Bearing
'58-'60 Edsel Large Bearing 2.870" 5" Large Bearing
'57-'63 Early Pickup 2.870" 51/2" Large Bearing
'57-'69 Lg. Bearing 2.430"/2.780" 41/2" Large Bearing
'64-'79 Pickup Lg. Bearing 2.870" 51/2" Large Bearing
'79 1/2 Pickup Lg. Bearing 2.870" 51/2" Large Bearing
'72-'79 Torino Lg. Bearing 2.780" 41/2" Torino Lg. Bearing
'79-'86 Late Pickup Lg. Bearing 2.870" 51/2" Torino Lg. Bearing