Bred for the burnout box,...
Bred for the burnout box, our '03 Cobra test mule puts some heat into the Nitto Drag Radials for better traction. With our new 3.73 gears, we'll need all the bite we can get off the line.
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.
First order of business is...
First order of business is to remove the entire center section from the vehicle. This requires you to basically disassemble the entire rear suspension, wheel bearing carriers and all, in order to remove the halfshafts. Then, with it properly sitting on a workbench, remove the front driveshaft flange by undoing the retaining nut.
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.
 From the backside (our favorite...  From the backside (our favorite place to be) we then remove the rear cover, the differential caps, the carrier and the pinion gear. This carrier housing is a rather light piece and an amazing feat of engineering as it has proven to hold up in high-horsepower applications. After a thorough cleaning, we gave it a thorough check as we saw signs of dealer personnel having been in here before (there were new OEM bearings installed at one point and based on the age of the repair, it was probably done under warranty). No issues were found, so we pressed on. |  Speaking of pressing on, we...  Speaking of pressing on, we extricated the factory pinion shim from the original gearset by using a bearing separator and a hydraulic press. Once out, we discovered a 0.018-inch shim underneath, and used it as a starting point. |  Yes, Pro 5.0 is best known...  Yes, Pro 5.0 is best known for its shifters, but gears are now a staple of its business as well. For our Cobra, we relied on Pro 5.0 for a set of 3.73s. Its ring-and-pinion sets are just as high in quality and nicely machined as the big-name kits on the market, so we knew we'd be in for a smooth-running and quiet ride. |