Richard Holdener
January 18, 2011

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If some boost is good, then more must be better. Unfortunately for us, we were already pushing the limits of the factory Eaton, so more boost would require more blower. In our case, more came from a Kenne Bell twin-screw upgrade. Designed as a direct bolt-on replacement for the factory Roots blower, the Kenne Bell offered not only increased displacement (blowers available up to the new 3.6L), but improved efficiency which translates directly into lower charge temperatures. In truth, the factory air-to-water intercooler is effective at removing vast amounts of unwanted heat from the charge temperature. Less inlet air temp equates directly to more oxygen molecules, the essential building blocks of horsepower. The Kenne Bell was installed after swapping over the factory intercooler core. The blower was equipped with a 3.25-inch blower pulley to nearly match the 3.2-inch blower pulley run on the stock supercharger. The KB was run with the same Accufab throttle body, though the intake between the blower and throttle body differed between the two superchargers.

We were obviously hoping for more boost and power from the twin screw, and that's exactly what we got. Equipped with the 2.4L Kenne Bell supercharger, the peak boost pressure jumped to 16.5 psi at 6,500 rpm, and the power output followed suit. Though still climbing rapidly, we shut the party down at 6,500 rpm where the 4.6L produced 682 hp. Peak torque checked in at 594 lb-ft at 4,700 rpm compared to 506 lb-ft at 4,400 rpm for the stock blower. With a bigger blower and more boost, the Kenne Bell certainly offered more power than the stock combination, but a comparison is inevitable between the blower/nitrous combo and the boost-only set up. The Kenne Bell offered more peak power (682 hp versus 636 hp), but lost out in torque production (637 lb-ft to 594 lb-ft).

As indicated previously, these are simply two different methods of improving power and the test motor will certainly withstand much more of either (or both). The only thing better than having more power, is having more ways to achieve it. Whether you choose more boost or Zex appeal, either one makes a killer combo.

'04 Cobra-Eaton SC vs. Zex Nitrous (125 hp)

Adding nitrous to any motor instantly transforms it into a serious contender, and this supercharged '04 Cobra motor was no exception. Despite being blessed with a supercharger, the '04 Cobra motor was the ideal candidate for nitrous. Equipped with forged internals and low compression, the 4.6L could easily handle the extra power offered by the Zex nitrous kit. Having run these Cobra motors near 1,000 hp in turbocharged form, they are more than capable of withstanding serious power levels. Literally at the push of a button, the Zex wet fogger kit improved the power output of the modified '04 Cobra motor from 533 to 636 hp, though the gains were as high as 127 hp elsewhere along the curve. For those looking for even more power, the Four-Valve Cobra motor would easily take another 100hp worth of nitrous.

After cranking up the factory Eaton supercharger with the pulley upgrade from South Florida Pulley Headquarters, we replaced the factory blower with a twin screw from Kenne Bell. Retaining nearly the same pulley combination (3.25 blower/7.5 crank), the peak boost pressure jumped from 12.1 psi with the Eaton to 16.5 psi with the Kenne Bell (2.4L). The improved efficiency and extra boost had a positive effect on the power output, as the supercharged Cobra motor now thumped out 682 hp and 594 lb-ft. As indicated by the graph, the power curve was climbing steadily and was well on its way to the 700hp mark had we elected to run the motor past 6,500 rpm. Pulley changes would certainly push this motor past 800 hp, while the new 3.6L blower from Kenne Bell can easily top 1,000 hp.