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The last-generation cars–2003 to ’04–are the best in terms of how they came from the factory. The OE bushings are a bit stiffer, and this helped reduce wheel hop but still did not eradicate it. The interior on these Cobras is also light-years ahead in terms of trim and comfort.
The 2003-‘04 Cobras also came with a supercharger, which makes the car a lot quicker in a straight line than the 1999-’01 models but adds almost 200 pounds to the front end of the vehicle, which is a detriment to handling. Supercharged Cobras will need an additional, larger heat exchanger and a better radiator to keep heat under control.
Even with the best heat exchangers out there, the car will still heat soak fairly quickly under competition road course use and will pull down timing and power. The factory supercharger just gives off too much heat to control in the long term when constantly under boost on the road course.
The largest rolling stock we’ve seen without any modifications to the rear is an 18×10-inch wheel with a 315mm tire. With steering rack limiters, the front can fit 285mm-wide tires on 18×9.5-inch wheels.
The independent rear suspension is the weakest link in the driveline, particularly if you want to go drag racing. If you see a lot of excess rubber in the fender liners on a car you’re looking to buy and there is no IRS brace or aftermarket bushings, steer clear. Chances are that the rear end in that car is already compromised. If the car spent its life strictly on the public roads, you will not have anything to worry about–even under spirited driving.
The rear subframe attaches to the car via a cantilevered bracket. The outer half of this bracket is completely unsupported and flexes significantly, leading to less-predictable handling as the entire rear suspension system shifts under the car. You’ll start breaking axles or cracking the differential case on a drag strip or under any prolonged road-course use. We generally recommend anyone who wants to compete on track to stick with the solid-axle GT.
If you’re going to use it as a daily driver and see the track or strip once in a while, we recommend replacing all the IRS bushings with our urethane bushings. Then, beef up the IRS with our brace and subframe reinforcement brackets. These parts will completely transform the Cobra into a smoother, gentler-riding car compared to the solid-axle GT.
Supercharged Cobras respond very well to a cold-air intake and a pulley change. Of course, with the additional boost from the pulley and increased airflow from our CAI, you will need to reflash the computer. These changes will add a minimum of 35 horsepower–even more if you add in exhaust.
The 1999-2001 cars will show a 10- to 15-horsepower increase from a CAI, plus 5 to 15 horsepower from exhaust and headers. Underdrive pulleys are worth 10 horsepower on both versions.
The ’03-’04 Cobra came with a forged crank, rods and pistons. It is a stout lower end. We’ve seen as much as 600 horsepower at the rear wheels produced reliably from that car. The naturally aspirated engine is not as robust. It’s capable of making more than 500 at the rear wheels, but if you’re making anything more than 400, you’re on borrowed time.
Kenny Brown Performance
Most of the criticism against the IRS Cobras came from those who didn’t understand the IRS or tried to drag race with it. If you want to go drag racing, you need a stick axle. It‘s that simple. The IRS is great on track, but the geometry, physics and loading for hard launches just doesn’t work.
With some thoughtful upgrades and improvements, the IRS Cobras really do work well. I like driving them over live-axle Mustangs, as they have much better control on bumpy tracks and the IRS affords you a broader range of driving lines around corners. I’m looking forward to what I can do to the new 2015 IRS Mustang.
If I were looking for a good used IRS Cobra to turn into a track car, I would choose a clean 2001 or ’02. In ’99 there was a big flap over Ford overrating the horsepower. The recalls, new parts and re-flashed processors spilled over into 2000.
Cobras lack good spring options for track use. There are plenty of “Sport Lower Springs,” but they’re more for looks and just don’t have the rates you need. In recent times I have converted over to coil-over setups. This gives a far better range of ride height adjustment and an unlimited choice in spring rates to match to each individual driver’s use, experience and skill set.
In upgrading the IRS, I start with the basics: a heavy-duty forward torque brace and aluminum differential bushings to help stabilize the differential under load. When the back of the Mustang rolls, at a certain point the rear wheels will “steer” in the wrong direction, creating sudden snap-oversteer.
I developed a simple rear steer kit that not only helps reduce rear roll steer, but also unintended undergarment soiling. Additional IRS upgrades can include tubular upper and lower control arms, which shave off quite a bit of weight from an already heavy IRS unit. The tubular lower arms are set up for coil-overs. Currently, we are the only company on the market that offers rear upper and lower control arms for the IRS Cobra.
If you plan on doing more than one or two track events with your Cobra, a differential cooler is very important. The small housing does not have enough mass to absorb and dissipate the heat that builds under hard track use. If not cooled, the whole unit will burn up, making for a bad day. At some point you will need a new differential, and a Torsen works best in an IRS.
Using the same-size wheels and tires for front and rear gives you the option to rotate tires to even out tire wear. Make sure you have enough clearance for big front brakes. The brake companies typically have templates of their caliper/rotor to check for clearance to the wheel. When choosing a set of tires, check with the manufacturer’s specs for recommended wheel widths for that size.
The supercharged 2003s and ’04s have awesome power and an upgraded IRS with heavy-duty 31-spline axles, but the added weight and heat of the supercharger doesn’t make for a good track car. The Mustang is already too nose-heavy without the supercharger, and added complexity just means more things to break. Also, these cars will go into limp mode halfway into a session on a hot or even a warm day as the engine heat-soaks.
Kermie, my 2001 Kenny Brown Cobra CSR-OT (Open Track), was the first full-on IRS track car. It was very quick and constantly embarrassed much more powerful cars. Twelve years later, that car continues to consistently win in the Autobahn Country Club’s intermural race series. But most importantly, the IRS Cobra is a dream to drive.
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