If you own a '99 or '01-'04 Mustang Cobra, you're undoubtedly aware of the independent rear suspension that resides beneath. Ford's attempt to equip its top-of-the-line ponycar with world-class hardware is truly admirable, but-as is often the case-it's tough to please everyone. In particular, drag racers have lambasted the IRS for its poor launchpad characteristics, yet this reality hardly negates the fact that IRS has redeeming qualities for a large cadre of enthusiasts.
An IRS is clearly a more sophisticated design than a traditional live axle. It's ideally suited for imperfect roads where the independent suspension should be decidedly more settled and composed, with each wheel being able to absorb a bump without sending the message directly down a solid-axle tube. Additionally, an IRS can be adjusted for camber similar to a typical front end, giving a better contact patch during hard cornering. Of course, one problem with the Mustang IRS is that Ford engineers really didn't have a clean sheet of paper to start with because, for budgetary reasons, the design had to use the same mounting points as the '99-'04 Mustangs with a traditional axle. Such limitations meant inherent design compromises were part of the program from the start. But if there's an upside here, it's that the IRS can be swapped into earlier cars with relative ease-even '04s share the same rear subframe dimensions as all Mustangs since 1979. One enthusiast who's decided to take advantage of this flexibility is Brian Holsten, whose '92 LX was the subject of a complete Kenny Brown tubular front suspension in our June issue ("Totally Tubular," p. 184).
The genesis of Brian's Fox swap occurred when coworker Mike Collins was lamenting the stock IRS in his '04 Cobra he drag races on a regular basis. With a beefed-up stick 8.8 from a previous project gathering dust, Brian offered to exchange it for the nearly new Cobra assembly, sweetening the deal by offering to turn the wrenches on the entire swap. That end of the affair went off without a hitch, and another Terminator Mustang owner hit the track with his rearend cares behind him (see our Sept. '04 issue for a similar story). Brian quickly alerted us to his effort going back the other way, and we decided to follow along to see what's involved. All work was accomplished where Brian turns professional wrenches at Brad's Custom Auto in Seattle, with generous help from fellow mechanics Scott Hicks and Mario Brown.
This is the whole basis for an independent rear in a Mustang-the IRS subframe. Even if you're familiar with such assemblies, this picture may not look remarkable, but the subframe was sent to Kenny Brown's Gasoline Alley digs for significant modifications that include relocating the upper and lower control-arm mounts, as well as the pickup points for the tie-rod ends.
Perhaps the obvious question regarding this project revolves around the why. One big reason would be for improved ride quality in a car that sees considerable real-world use. As much as you've heard about negative issues with the Mustang IRS, there's no doubt this design is hands-down superior in regard to ride quality on anything less than a glass-smooth surface-Ford engineers clearly accomplishing their prescribed task. In the case of Brian's project, however, swapping a stock IRS into his dual-purpose '92 was barely a fleeting thought, being aware that Kenny Brown spent considerable time developing the IRS into an admirable track performer. Given the opportunity, Brian wanted to see how an IRS would perform when treated to a slew of pertinent bolt-ons, and also to see how such a setup would perform in a lightweight Fox chassis. For us, it was a chance to kill two birds with one stone, simultaneously covering Kenny Brown's IRS upgrades along with a Fox install. Is it worth the effort? Well, that's part of what we're here to find out.
Brian went the full-throttle route on this swap, sending the IRS subframe to Kenny Brown for optional geometry and mounting modifications, followed by installation of the company's tubular upper and lower control arms, rear steer kit, Koni/H&R coilovers, and more. Since none of these products had any bearing on installing IRS in a Fox chassis, they're clearly intended for IRS Mustangs of any year. Kenny Brown touts its IRS wares as a big help on stock subframes as well, so '99-and-up Cobra enthusiasts have a variety of options to choose from when they decide it's time for their own extreme makeover.
In addition to the list of modifications Kenny Brown normally makes when charged with modifying an IRS subframe, Brian opted for what's known as the hardtail upgrade. In stock form, the forward and rear mount points have a nearly identical-appearing rubber isolated mounting system, the rear of which fits into brackets that are in turn bolted to the framerails. The hardtail changes these rearmost mounting points, eliminating the rubber bushed ends and frame brackets in favor of an arrangement that bolts directly to the frame. | 
The first piece to go onto the subframe is the 8.8-inch carrier. Straight from an '04 Cobra with just 12,000 miles, the rearend still sports standard 3.55 gears and a 31-spline Traction-Lok dif. Kenny Brown likes to use the stock mount on the back side of the rearend, but you'll see revisions to the front mount in a few frames. | 
Before the 8.8 can be totally secured, Brian first installs the new tubular lower control arms. It may be necessary to slightly spread the cradle's mounting ears to get the new arms into place. |