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

We've yet to see someone mod an '03 or '04 Cobra where it does not respond positively. The 4.6 DOHC is amazing, and we've shown it's easy to make big power with nothing more than a Cobra Power Magicpak by Mustang Magic. With the kit's cold air inlet, tune, 3.0 pulley, cat-back exhaust, and catted x-pipe assembly, we've belted out 453 hp to the tires and have cranked out torque that oozes like hot tar on a summer afternoon in Atlanta. On the street, this Dark Shadow Gray Cobra is downright sinister in both looks and acceleration. Just like its namesake, you don't want to mess with this coupe. A simple prod will make it recoil and attack, often leaving anyone looking to instigate no time to react.

So now that we're making good power, we need to get it to the ground more effectively. If there's one part of a Terminator that could use some attention, it's the driveline. Last month, we tackled the clutch and input shaft on the T56 six-speed, and now that pedal effort is halved and torque capacity is practically doubled, we're ready to tend to our next hot topic, which is the rear.

The Low-End Theory
Like most T56 transmissions, the Cobra version uses a rather tall 2.66 First gear to keep the subsequent gear ratios tight. While this makes for great fun by keeping your engine in its powerband after each upshift, it comes with weaker out-of-the-hole performance. To help remedy this, the best thing you can to do is go up in gearing. Don't overdo it as you want to take advantage of more torque multiplication without revving so quickly that you lose out on the blown 4.6's meaty torque band. So, after years of seeing what works and what doesn't, we consistently believe that a switch to 3.73s is worth every dollar spent on these cars. It will give you about 5 percent more gearing, but in this 3,600 pound car, it makes a great difference without sacrificing drivability or fuel economy.

The rear axle setup in the '03-'04 Cobra uses an aluminum center section that is lifted right from the Ford parts catalog under the chapter that covers the '89-'96 Thunderbird Super Coupes. It was originally equipped with 28-spline axles for that car and in the '99 and '01 Cobras. But for the Terminators, it was beefed up to 31-spline halfshafts with a matching Traction-Lok differential that was upgraded with clutch-discs faced with carbon-fiber-reinforced linings.

The good thing about parts sharing here is that buying and setting up gears for an IRS Cobra is as simple as doing it for a solid-rear car, without dealing with that hunk of pig-iron that gives up excellent handling and stability for brute strength. So, to order gears for an '03 Cobra, any Ford Racing or aftermarket set for an 8.8 ring gear will do. We went with a set of proven Pro 5.0 gears in the desired 3.73 ratio. While we were at it, we also picked up a shim kit with and all the bearings inside with an 8.8 ring and pinion installation kit from Ford Racing (PN M-4210-B.) Part number M-4413-A is required in addition as it includes the side support bearings and seals that are specific to the IRS setup.

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.