2003 Mustang Cobra Gears Upgrade - Grabbin' Our Gear And Headin' Out

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.

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.