To start the reassembly process,...
To start the reassembly process, we installed a new front pinion gear bearing and then installed a new front seal. In case you were wondering, yes, these parts are exactly the same as the ones for a solid-rear 8.8, so parts are readily available.
Gearing Up
With our small stack of new parts sitting pretty on our cluttered workbench, we dropped the IRS center section from the vehicle, which requires you to undo practically the entire rear suspension. Start with the bolts at the outer end of the rear control arms where they attach to the rear hub carriers. Using an 18mm wrench and ratchet makes short work of them, and once everything else is undone, you can gently pry out the rear halfshafts from the center section. Then remove the front differential crossmember, the driveshaft, and the two rear support bolts. With the center section unbolted and carefully lowered to the ground, we put it on the bench and swapped the gears in and out with our assortment of tools. It should be noted that air tools do speed things up greatly, and that you will eventually need access to a hydraulic press to shim the front bearing for the proper pinion depth. Also, a dial indicator with a magnetic base will be required to give you an accurate backlash reading. If you don't feel confident doing this yourself, enlist the help of a shop that is versed with 8.8s.
With our new gears ready to...
With our new gears ready to install, we started with the pinion gear. Oftentimes, you can install gears without a pinion depth tool by starting off with the shim that the rear originally had. Ford Racing gearsets usually line right up by using the stock shim. Non-OEM gearsets (like the Pro 5.0 gears here) like to go in with no shim at all. Like all 8.8 Fords, a crush collar is used to set pinion bearing preload and with the pinion nut tightened slowly by an impact gun. We looked for a resistance that was snug to turn by hand (about 20 lb/in for new bearings, 12 lb/in for used bearings).
After several hours of dealing with the stench of used gear oil, our center section was fully cleaned, assembled, and ready for reinstallation. Before we slithered back underneath our Cobra though, we wanted to solve two major issues that typically plague IRS cars. For one, we immediately replaced the original rubber support bushings on the front of the carrier with heavy-duty polyurethane units by Steeda (PN 555-4015). These bushings prevent excessive movement during acceleration and deceleration. Secondly, we beefed up the center section's cast aluminum rear cover/mount. Because of the increased stress that will be placed here once we start adding more power and traction, reinforcing the rear cover/mount with Steeda's Cobra IRS Differential Cover Brace (PN 555-8118) will prevent major catastrophes down the road. For the uninitiated, the rear cover's mounting ear can and will break off under heavy abuse and power-both of which we intend to dish out.
With the new Pro 5.0 3.73 gearset slapped in, and the car back on the ground, we made our way back to Raceway Park in Old Bridge Township, New Jersey. We gave the Nittos a hellacious burnout and lined our freshly-geared Cobra to the staging beams. At the drop of the hammer, we quickly realized a much stronger launch as the boards displayed our best-ever 1.841 short time. Once underway, we banged off a few more powershifts, continued down the 1,320, and once we passed the stripe, were rewarded with an 11.925 at 118.92 mph. Based on our previous best of 12.74 at 109 mph, our new performance gave us an incredible drop of 0.82 seconds. Granted, we were about 33 rwhp stronger than before, but with the 86-degree temps and mild headwinds running right into our 32-valve faces, laying down a high 11 at almost 120 mph is certainly impressive for a car that is so mildly modified.
So once again, the age-old gear swap proves to be one of the best bangs for your buck. Aside from more ideal gearing off the line, it typically wakes up almost any car, adding lively throttle response on the street and serious performance on the strip with little sacrifice to the daily grind. Few other mods can make that claim. As for our Cobra, we'll continue to find ways to update this five-year-old mega Mustang to prove that indeed, the '03-'04 Cobras are the best bangs for the proverbial buck to have ever rolled off the River Rouge assembly line. Hang in there and let's see where our Cobra travels take us.

With our 31-spline Traction-Lok...

With our 31-spline Traction-Lok free of its original 3.55 gearset, we fastened the new 3.73 ring gear into place with new bolts. Go around in a slow and steady pattern to draw the ring gear into place evenly.

Setting up backlash in a Cobra...

Setting up backlash in a Cobra rear is just like setting it up in a conventional solid version of the 8.8. Here, we play with the shim pack that came with the Ford Racing installation kit (PN M-4210-B) to get to our desired backlash. As a starting point, make sure that both shim packs equal the combined thickness of both the original shims. Note that the rear caps are also made of aluminum in this rear.

Because the housing is made...

Because the housing is made of non-magnetic cast aluminum, we can't put the magnetic base of our dial indicator on the rear cover's flange. Instead, we stick it to the differential unit, which is steel, and measure backlash off of the rear cover flange at a similar height as the ring gear. Like all 8.8s, you want 0.008- to 0.010-inch backlash for silent and street-friendly performance.

With a little more pattern-marking...

With a little more pattern-marking grease to check our gear mesh, we close up the operation and torque the rear bearing caps to a final 80 lb-ft.

IRS Cobras have halfshafts...

IRS Cobras have halfshafts that slide into the side of the center housing and are supported by a separate bearing that rides on the stub shaft. These do go bad, so we ordered a fresh set from our local Ford Racing dealer under part number M-4413-A. Simply tap them in from the outside until they are flush with the machined recess and then install the seal until it's flush with the outer flange. Don't forget to put a little gear oil here so they won't run dry the first time they rotate.

After a few years of even...

After a few years of even normal use, the upper and lower halves of the front support bushings begin to wear, or even worse, begin to grind their way into the aluminum housing, damaging the mounting ear and creating excessive play in the process, just like on our car. This is a common problem that should be addressed right away on any IRS Cobra.

For our Cobra, we installed...

For our Cobra, we installed a set of Steeda IRS bushings to stiffen things up. Part number 555-4015 rectifies things nicely, and priced at just $39.95, it's an economical move.

The rear cover's mating flange...

The rear cover's mating flange has a groove that runs the entire perimeter. This is designed to accept a nice bead of silicone sealer to prevent any internal fluid from becoming external fluid.

As a preventive measure, we...

As a preventive measure, we installed Steeda's Cobra IRS Differential Cover Brace to reinforce one of the IRS' weakest points, the rear cover and integrated mount. A lot of torque is placed here, and with the added traction provided by sticky tires, even mildly-modified Cobras like ours will see this cover crack and the mount will rip itself away from the rest of the rear. The good thing about Steeda's design is that it uses six of the factory cover's mounting bolts for secure attachment, and access to the fill plug is unhindered. With our rear complete, it was back to the garage floor and then back to the track, where we found a new best e.t. of 11.925.