2003 Mustang Cobra Gears Upgrade - Grabbin' Our Gear And Headin' Out
Swapping In A Set Of 3.73s Into This Terminator Gives Us A Newfound Best E.T. At The Dragstrip
From the January, 2009 issue of Muscle Mustangs & Fast Fords
By Vinnie The Hitman
Photography by Michael Galimi
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.
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.
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.