Richard Holdener
November 1, 2003

Like the proverbial redheaded stepchild, the 4.6 two-valve Mustang GT has always taken a back seat to the more powerful 4V SVT version. Originally, the '96 four-valve Cobra motors offered a 1.5-second, quarter-mile advantage over the weak-kneed, 215-horse GT. Things changed dramatically when SVT unleashed the supercharged monster known affectionately as the '03 Cobra.

While the new GTs can run in the high 13-second zone stock, adding a positive displacement supercharger to an already impressive four-valve-per-cylinder powerplant literally transformed the Cobra into an absolute over-achiever, capable of 12.4-second e.t.'s in bone-stock trim. The press (yours truly included) was quick to lob rave review after rave review toward the well-deserving SVT, most of which was aimed at the thunder produced by the force-fed, four-valve 281.

The two-valve 4.6 Mustang supercharger kit included everything necessary to add over 100 hp at just 6 psi. The 9 psi, intercooled kit (shown) was good for more than 150 extra horsepower.

Unfortunately, the 2V GT owners, even with the now upgraded 260-horse engine, were quickly left in the dust in terms of performance. Well, GT owners, all that is about to change--enter the new Twin Screw supercharger kit from Kenne Bell. In the spirit of "If you can't beat 'em, join 'em," Kenne Bell decided to give the 4.6 GT owners a taste of supercharged Cobra power by offering what the GT motor was missing, namely boost pressure. Oh sure, the DOHC SVT mill sports four valves per cylinder while the GT motor makes due with half that number, but the addition of the more efficient twin-screw Autorotor blower all but makes up the power difference.

If you follow MM&FF religiously, you will no doubt remember the power gains offered by the addition of the twin screw Autorotor to the equally supercharged SVT Lightning. Simply replacing the Eaton M112 roots blower with the more efficient twin-screw design upped the power to the tune of 60 horsepower. Similar gains are available from Kenne Bell for the SVT Cobra, but we now turn our attention to the GT, as our favorite redheaded stepchild is in dire need of an attitude adjustment. That adjustment comes in the form of a new supercharger kit (intercooled to boot) from Kenne Bell.

Readers may remember testing performed on the prototype of the kit some time back in MM&FF. After witnessing the power gains offered by the twin-screw compared to the traditional positive displacement roots blower, the gang at Kenne Bell recognized the potential for a dedicated kit. Taking their prototype pieces, they set out to build dedicated castings and the associated components you have come to expect of a production Kenne Bell supercharger kit.

The highlight of the new Kenne Bell was the 417 twin-screw Autorotor supercharger.

Naturally the highlight of the kit was the supercharger itself. Like all of the kits from Kenne Bell, the 4.6 GT kit is supplied boost via a twin-screw Autorotor supercharger. Testing has shown the twin-screw design to be more efficient and powerful than the more common (and less expensive) roots-style supercharger.

Due to the cost difference, roots superchargers are commonly employed on a handful of OEM applications. The SVT Cobra and Lightning are two such examples, but look for SVT (not to mention any number of other manufacturers and tuners) to jump on the twin-screw bandwagon in the very near future. After witnessing the additional power available from the design, it is no wonder so many SVT Cobra and Lightning owners are opting for the twin-screw upgrade recently made available by Kenne Bell.

Available in a number of different sizes, or more accurately displacements, Kenne Bell employed the 417 Autorotor in the 4.6 GT kit. The 417 moniker designates the displacement of the supercharger as 1.7 liters. This means each revolution of the twin-screw displaces 1.7 liters of air. To put this number into perspective, the Eaton roots supercharger used on the Lightning displaces 112 cubic inches to the 104 cubic inches of the 417. Fear not for the Autorotor, as the twin-screw design more than makes up in efficiency what it lacks in displacement.

The Kenne Bell kit was not some throw-together blower deal. Kenne Bell spent a great deal of time and money on designing dedicated manifold castings for the new kit, including this lower intake manifold.

Need more convincing? Running elevated boost levels (more on this later), the 4.6 produced over 500-wheel horsepower running the 417 Autorotor. Think about that number for a second, as that equates to more than double the factory power output of a stock GT motor. It is also a great deal more than most '03 4V Cobra motors put out. How long the stock motor is likely to last at this power level is another story, but know there is plenty of power potential in the kit for those so inclined to up the boost pressure beyond the specified limits.

As you might expect, there is more to the kit than simply bolting the supercharger onto the stock long-block. The first thing to go was the factory intake manifold. Off came the composite intake and on went a dedicated (and no doubt expensive) casting that houses an air-to-water intercooler. The 6-psi kit is sold sans intercooler, but the 9-psi kit includes the trick air-to-water system to help lower the charge temperature and eliminate any chance of detonation. Lower charge temperatures allow more aggressive tuning for maximum power.

Just like the 4V SVT Cobra and Lightning setups from the factory, the Kenne Bell 2V GT kit employed a lower intake mounted air-to-water intercooler to minimize the inlet charge temperature. Testing has shown the system to be very effective, even at elevated boost levels.

The supercharger is bolted directly to the lower intake, while another dedicated casting is used to supply air from the stock throttle body to the inlet of the supercharger. A great deal of effort went into the design of the kit to allow the system to bolt in with a minimum of fuss and to retain the majority of the factory inlet components.

Retained are the stock air filter assembly, mass air meter and throttle body along with the associated inlet tubing. Of course additional horsepower is available with the installation of a 90mm mass air meter, 75mm throttle body and dedicated Kenne Bell inlet tubing, but we'll get to that testing in a moment.