After getting the dry sump tank positioned, all of the factory accessories (and blower drive) installed, and the Motec wiring harness made for the GT motor, the 5.4L was installed on the Superflow engine dyno at Westech. This particular engine dyno was configured with a dedicated starting system. This is important as the 5.4L GT motor was not equipped with a starter in the conventional location, and there was no room to run it on the dyno in its new orientation. The motor was purposely kept as stock as possible-the only change being a slight increase in exhaust tubing length to clear the dyno-but the factory exhaust manifolds, cats, and muffler canister were retained. Other stock components that usually don't make the transition to the engine dyno included the full accessory drive system and factory air filter and inlet system. The only nonstock component used on the engine dyno was the Motec stand-alone EFI management system used for tuning. This certainly improved the overall power curve of the combination, but not likely the peak power output as the factory runs over 20 degrees of total timing in the stock programming. Big gains (from additional timing) can be found at lower engine speeds, while gains in the upper rpm range come from slightly leaner (in the low- to mid-11s rather than high 10:1) mixtures.
The billet boys at Accufab whipped up a number of custom components to allow this motor to be run successfully on the engine dyno, including a mount for the dry-sump tank.
In bone-stock trim (but with optimized tuning of 21 degrees of timing and an 11.3:1 air/fuel ratio), the 5.4 motor produced 643 hp at 7,400 rpm and 545 lb-ft of torque at 4,900 rpm. While most GT owners don't run their motors (in the car) to 7,500 rpm, we allowed the motor to go there in an effort to find out where it stopped making power. It looks like extending the rpm range might be a good software modification for fellow GT owners since this supercharged 5.4L pulled strong all the way to 7,500 rpm. Were it our motor, we'd probably limit it to 7,000 rpm and call it a day, since the mean piston speed of the GT motor running at 7,500 rpm exceeds that of a (small-stroke) Formula 1 motor at 19,000 rpm. Now imagine how heavy that 5.4L piston is compared to the tiny Formula 1 slug and you can start to appreciate just what the rods are going through during each revolution.
In addition to the impressive peak power output, torque production from the supercharged 5.4L exceeded 500 lb-ft from 3,000 rpm (the lowest test speed) to 6,500 rpm. That is what we call an impressive torque curve. At the maximum revolutions listed by Ford (of 6,500 rpm), the 5.4L motor produced near 620 hp. Even consid-ering the differences in temperature and tune between our test and those run to officially rate the motor, the 5.4L GT motor seems somewhat underrated by Ford.
Though run at the stock power level, we decided to check the effectiveness of a few minor mods before cranking up the boost. First up was an Accufab throttle body followed by its x pipe system upgrade. Replacing the factory GT throttle body with the Accufab dual 75mm unit resulted in a jump in boost pressure (from minimizing the inlet restriction) of roughly 0.3 psi and with it an increase in power (at the top of the rev range) of 10-11 hp. The Accufab exhaust upped the power ante by 12-13 hp, though, as expected, future testing showed much greater power gains at higher boost/power levels. The restrictions imposed by the factory throttle body and muffler increase with the power level of the motor and as such so do the gains offered by their replacement. In an effort to demonstrate the true worth of each modification, the air/fuel and timing curves were kept consistent for each combination.
Run in bone-stock trim (with optimized tuning), the supercharged 5.4L produced 643 hp and 545 lb-ft of torque at a peak boost pressure of 11.5 psi. | 
This polished stainless steel x pipe system offered by Accufab (the dark photo doesn't do it justice) greatly improved the sound quality and offered a decent power gain. Like the Accufab throttle body, the gains offered by the exhaust increase with the boost/power level of the test motor. | 
Positive-displacement superchargers are sensitive to inlet restrictions. Replacing the stock throttle body (bottom) on the 2.3L blower can be worth 10, 15, or even 20 hp depending on the power/boost level. The dual 75mm Accufab throttle body not only offered significantly more flow, but it looked cooler than the factory piece. |
While the throttle body and exhaust upgrades offer decent power gains, the real power comes from the additional boost provided by the Kenne Bell HCD drive assembly. Replacing the factory snout (with press-fit pulley) with the Kenne Bell HCD allows for easy blower pulley swaps to increase the boost. | 
5.4L Ford GT-Stock vs. Boost and Bolt-ons
The great thing about a factory supercharged car is the ability to dial in more power. The installation of the Kenne Bell HCD drive assembly allowed us to run a smaller blower pulley (2.75 inches) that increased the peak boost pressure from 11.5 psi to 16 psi. The Accufab throttle body and x pipe system were also on hand to improve power. The combination netted an increase from 643 hp at 7,400 rpm and 545 lb-ft of torque at 4,900 rpm to 715 hp at 7,200 rpm and 638 lb-ft at 4,900 rpm. Increasing the blower speed offered sizable power gains throughout the rev range, but the gains tailored off at the very top due to the limited flow rate of the factory 2.3L blower. | 
Run with the 2.75-inch blower pulley, throttle body, and x pipe system, the supercharged motor produced 715 hp and 638 lb-ft of torque. It's possible to install an even smaller (2.50-inch) blower pulley, but the stock blower has reached a level of diminishing returns. |