Setting Up an 8.8 Axle

The Ford 9 inch is legendary for its strength, but using this rear end can be expensive and is overkill for most builds. Fortunately, your Fox body probably came with an axle that can handle serious power with the right modifications: the 8.8. Here's what you need to know to get the most out of this rear end.

Identifying Your Rear End

The 8.8 gets its name from the ring gear's width in inches. This axle has been used in various forms in the Mustang for over 25 years, and it also saw plenty of use in other rear wheel drive Ford vehicles. Here's a look at what Mustangs came equipped with from the factory.

All Fox bodies up to 1985 as well as all four cylinder and V6 Mustangs from 1986 to 2010 used the 7.5 rear end. It can be identified by its differential, which has a flat top and bottom.

All V8 Mustangs from 1986 onward and all V6s from 2011 until 2014 use the 8.8 axle. CV axles were used with the independent rear suspension fitted to '99-01 and '03-04 Cobras. Internally, the differential on the IRS axle is the same as the solid 8.8. The 8.8's differential has four flat sides with rounded corners.

2005 and later V8's, 2011-2014 V6's and IRS-equipped Cobras use a 31 spline axle. All other axles have 28 splines.

'79-93 Mustangs have a 29 3/16 inch rear axle. This was lengthened to 29 31/32 inches in 1994 and then 30 11/16 inches in '99. '99 and later differentials also have a wider differential housing, even though the cover is the same size.

After being in the hands of the few owners, it's not unlikely that the axle that came in your car has been replaced by something different, whether it was upgraded from a 7.5 to an 8.8 or from a 28 spline to a 31 spline axle. To get the performance you want for your build, you'll probably need a mix of parts from stock axles across the Mustang's production along with some aftermarket pieces. Here are the key features that should be addressed when setting up your axle:

Splines

More splines means more surface area between the axle and differential, and that means more strength. Replacing the axles and differential with 31 spline units will increase the axle strength by about 30 percent, and they'll fit in the 28 spline axle housing. 

Limited Slip Differential

An LSD lets the wheels turn at different speeds to get around corners, but if the wheel speeds spread too far apart, they'll lock together. This helps the car launch in situations where an open differential would just let one wheel spin. 

Ford uses a Traction-Lok limited slip differential in some 8.8 rear ends, including a heavy duty version used in the Cobra's IRS. Ford Racing still makes the Cobra differential, while used differentials can be restored with a new clutch kit. A friction modifier needs to be added to the gear oil for the clutches to operate correctly.

The only way to identify a Traction-Lok equipped rear end from the outside is to read and decode the original axle sticker, which is probably long gone. However, if you lift the wheels off the ground and spin one of them, an LSD differential will make the other wheel spin in the other direction.

If you're building a race-only drag build, Strange Engineering makes drag spools that keep the axles locked for maximum traction. This causes tires scrubbing on turns, making these parts unsuitable for street driving.

Ring and Pinion

These two gears determine the final drive ratio. Manual transmission equipped cars came with a 2.76:1 or 3.08:1 ratio, while automatics used a 3.27:1 ratio. Fitting a set of lower (higher ratio) ring and pinion gears increases mechanical advantages for faster acceleration at the cost of top speed and a higher engine speed on the highway.

Girdle Cover

The standard differential cover is a piece of stamped steel, offering little in the way of support. A girdle cover supports the back half of the housing, reducing flex that can wear out the differential and axles prematurely.

Disc Brakes

The brakes from an SN95 ('94-04) Mustang can be adapted to earlier axles by using the SN95's axle flange and brake caliper mount. The SN95's axle can also be bolted into a Fox body with a little effort, although the added ¾ inch width on each side can cause problems with tire clearance.

Where Can I Get the Parts to Upgrade my 8.8?

Anderson Ford Motorsport has been helping their customers build high performance Mustangs for over 30 years. We carry all the parts to turn your Mustang's axle into something that can handle anything from street cruisers to race cars.