After our minor mods, it was time to get serious. Changing boost provided by the supercharger is a simple matter of increasing the blower speed, which is itself a function of the drive ratio between the crank and blower pulleys. An increase in blower speed can be achieved by either increasing the size of the crank pulley or decreasing the size of the blower pulley. Increasing the size of the crank pulley on the GT is made difficult by the proximity of the adjacent components and castings in the front cover, so the only option is to decrease the size of the blower pulley. Since the factory pulley is press fit (to discourage such boost-enhancing actions), Kenne Bell decided the best alternative was to replace the entire drive assembly. Its High Capacity Drive (HCD) assembly replaces the factory blower snout and allows the easy installation of a number of different pulley sizes ranging from stock down to 2.50 inches. This allows GT owners to increase boost pressure up to 18 psi. Kenne Bell also offers this upgrade for GT500 owners. Replacing the factory snout with the HCD equipped with a 2.75-inch blower pulley resulted in a jump in power up to 715 hp. The torque production was up as well, from 545 lb-ft to 638 lb-ft. The combination of the Accufab throttle body, x pipe system exhaust, and HCD (with 2.75-inch pulley) netted an increase of 72 hp and 93 lb-ft of torque. Down below 4,000 rpm, torque production improved by over 120 lb-ft.
The best route for improved performance on the GT motor is to replace the factory 2.3L blower with the larger 2.8L from Kenne Bell. The Kenne Bell blower offered increased displacement (which means more power potential), billet construction, and a polished housing.
Having nearly reached the limit of the stock 2.3L twin-screw supercharger, we decided to take Kenne Bell up on its offer to run the new 2.8L blower for the Ford GT. Though the two blowers share a twin-screw design, the Kenne Bell 2.8L H-series blower offers an increase in displacement combined with improved efficiency thanks to a dedicated high-pressure discharge design. According to Kenne Bell, the new H-series blowers were designed to maximize efficiency at boost levels beyond 15 psi. Since we had already taken this motor to 16 psi (with the HCD drive on the stock blower), the new H-series 2.8L seemed like the perfect candidate for our high-boost 5.4L. To illustrate the merits of the 2.8L Kenne Bell blower, it was equipped with the same 2.75-inch blower pulley run on the stock blower. Where the stock 2.3L blower produced 16 psi with the 2.75-inch blower pulley, the 2.8L Kenne Bell blower produced a massive 22 psi. Naturally the increase in boost pressure had a positive effect on power, with the peak numbers now up to 825 hp and 695 lb-ft.
Later testing with the Kenne Bell upped this power output to 875 hp with custom Accufab cams designed specifically for the GT, but don't tell them you heard about it from us because we were sworn to secrecy.
Knowing the sensitivity to inlet restriction, the Kenne Bell blower featured a downright huge oval inlet designed to accept the dual 75mm Accufab (or stock) throttle body. Note the O-ring sealing. | 
5.4L Ford GT-Stock vs. Kenne Bell 2.8L H-Blower Upgrade
The first thing you should notice is that, unlike the stock supercharger, the Kenne Bell 2.8L blower did not sign off at 700 horsepower. Equipped with a 2.75-inch blower pulley, the supercharged Ford GT motor produced 825 hp and 695 lb- ft of torque. Credit the additional displacement and improved (high pressure) efficiency of the Kenne Bell 2.8L blower for the additional power. We know the 2.8L blower is more than capable of supporting 1,000 hp, having produced 960 hp on a 4.6L with the smaller 2.4L blower. Originally, we were skeptical about the number of GT owners wanting to produce 800 hp, but the evidence suggests they are every bit as enthusiastic as the average '03-'04 Cobra owner. | 
In addition to the displacement increase over the factory 2.3L supercharger, the Kenne Bell twin-screw blower also featured a revised discharge design. According to testing run by Kenne Bell, the H-designation (for high-pressure) blowers offer improved efficiency at higher boost levels than their non-H counterparts. With our boost level certainly exceeding 15 psi during testing, naturally we selected the high-pressure blower. |
The new Kenne Bell 2.8L H blower was designed to mount directly to the factory lower intake, which housed the efficient air-to-water intercooler. | 
After removing the stock blower, on went the Kenne Bell 2.8L H. Once installed with the Kenne Bell 2.8L blower and matching 2.75-inch blower pulley, the peak power numbers jumped to 825 hp and 695 lb-ft of torque (at just over 22 psi). After the installation of some prototype cams from Accufab, the power numbers jump to 875 hp at less than 20 psi, but we were sworn to secrecy on that test until the final profiles have been tested. | 
As was the HCD drive used on the factory blower, the Kenne Bell 2.8L H blower was designed to easily accept different blower pulleys. |