Our test motor was augmented with a quartet of Comp Xtreme Energy (XE2672AH) cams, an Accufab throttle body and inlet and Flowtech 1 5/8-inch, long-tube headers. The DUB pulley system from South Florida Pulley Headquarters allowed easy pulley changes to adjust the boost pressure on both the Eaton and Kenne Bell, though Kenne Bell also supplied a variety of different blower pulleys for the twin-screw. The engine was tuned with the F.A.S.T. management system and equipped with a set of 65-psi injectors to provide sufficient fuel flow from the Aeromotive A1000 fuel pump. A Kenne Bell Boost-a-Pump and Boost-a-Spark were also employed on the test motor to ensure adequate fuel flow and spark energy.
Pulley swaps are equally easy...
Pulley swaps are equally easy on the Kenne Bell blower, requiring removal of only a single retaining bolt. Combining the 3.75-inch blower pulley with an 8.5-inch crank pulley resulted in a peak boost pressure of 14.5 psi.
The factory Eaton M112 was used as the baseline against which the three other forms would be judged. Equipped with a 2.93 DUB blower pulley and an 8.5-inch DUB crank pulley, the Eaton supercharger pumped out a maximum boost pressure of 14.2 psi. The peak power checked in at 583 hp while the torque stood at 574 lb-ft. To really appreciate the torque production offered by the Roots blower, check out the supplied power graph. It is important to point out that the peak power only increased by 11 hp when we upped the peak boost pressure from 11.7 to 14.2 psi, a sure indication that the M112 had very little flow left in reserve.
After our baseline, we replaced the Eaton with the Kenne Bell 2.2L twin-screw supercharger. As before, it was necessary to dramatically reduce the speed of the twin-screw compared to the Eaton to limit the peak boost production to near (as possible) 14 psi. In this case, the twin-screw was run over 4,000 rpm slower than the Eaton, yet managed to up the power peak from 583 to 704 hp (a gain of 121 hp). The peak torque was up slightly as well, from 574 lb-ft (with the Eaton) to 597.
Where the horsepower curve fell off with the Eaton, it continued to climb with the Kenne Bell (to a peak of 14.5 psi). In fact, we know that the power would continue to climb had we elected to run the motor higher than 6,600 rpm. The Kenne Bell blower was run using the factory air-to-water intercooler and lower intake manifold. Like the Eaton, we installed a free-flowing Accufab throttle body to minimize any airflow restrictions at this elevated power level. Of all the systems, the Kenne Bell was probably the easiest to install, requiring nothing more elaborate than an intake manifold (and intercooler core) swap.
Equipped with the twin-screw...
Equipped with the twin-screw from Kenne Bell, the 4.6 pumped out 704 hp and 597 lb-ft of torque.
Next up was the Vortech centrifugal supercharger. As before in the previous test at 11 psi the Vortech easily out-paced the Eaton in terms of peak power. Upping the boost pressure to a maximum of 14.0 psi came courtesy of a 3.12-inch blower pulley (up from the 3.48-inch blower pulley used at the 11 psi level). Like the Kenne Bell, the Vortech utilized the factory air-to-water intercooler thanks to a custom upper intake manifold. The fabricated intake also positioned the throttle body in the stock location. Breathing through the custom intake and factory air-to-water intercooler, the Vortech upped the power peak to 725 hp. The peak torque checked in at 575 lb-ft.
Looking just at the peak numbers, the Vortech handily out horsepowered the Eaton, by a solid 142 hp and even managed to produce one additional lb-ft of torque. While the peak numbers looked impressive, the graph clearly shows that the centrifugal supercharger lagged behind the positive displacement supercharger up to 5,250 rpm. From 5,250 to 6,600 rpm (and beyond) it was all Vortech, but below that point, the immediate boost response of the Eaton offered as much as 200 additional lb-ft (at 2,500 rpm). Were this a 5,000 to 7,000 rpm motor, the Vortech would be the clear winner (in terms of acceleration), but it takes one heck of a top-end charge for the centrifugal to overcome all that low-speed response on the street.
 Since centrifugal superchargers...  Since centrifugal superchargers were basically responsible for starting the blower wars (back in the days of the 5.0), it was only natural that Vortech would offer a supercharger upgrade for the '03-04 Cobra. |  Like the Eaton and Kenne Bell,...  Like the Eaton and Kenne Bell, the Vortech kit retained the factory air-to-water intercooler located under the stock intake manifold. |  To retain the factory lower...  To retain the factory lower intake and intercooler core, Vortech whipped up a fabricated upper intake designed to accept the stock (or in this case, Accufab) throttle body. |