 Since the vacuum readings...  Since the vacuum readings indicated a substantial vacuum present in the inlet system, Kenne Bell started with the factory airbox. The first modification was to remove the MAF assembly from the factory airbox. |  Removing the MAF assembly...  Removing the MAF assembly from the factory airbox allowed Kenne Bell to combine the MAF assembly with a free-flowing cone filter. |  The modified MAF and filter...  The modified MAF and filter netted some impressive power gains. The boost pressure jumped from 16.1 psi to 16.9 psi and the power peak now stood at 706 hp. The peak torque was up slightly from 677 lb-ft to 684 (most of the power gains occurred past the torque peak). |
Next in line in terms of airflow...
Next in line in terms of airflow restrictions was the stock throttle body.
If you've never experienced a 600-rwhp street car, believe us, it's something you'll always remember. Talk about an ear-to-ear smile. The 656 hp and 641 lb-ft of torque numbers were produced with the new 2.8L blower sporting a 2.75-inch blower pulley running in conjunction with the factory crank pulley. Running 23 degrees of timing (race fuel for safety) and an air/fuel ratio of 11.5:1, the 5.4L GT500 motor produced a peak boost pressure of 14.5 psi. This compares to just 11 psi produced by the stock blower using an even smaller 2.60-inch blower pulley. There's something to be said for the improved size and efficiency of the new Kenne Bell 2.8L H-series blowers.
While the need for a 650hp street car can be argued, naturally we went looking for more. We couldn't call ourselves Muscle Mustangs & Fast Fords if we didn't. When you think of improving the power output of any supercharged motor, your first inclination should be more boost. At least that's how it was with us. If the 2.75-inch blower pulley was good, then shouldn't a smaller one be better? As it turned out, the answer was both yes and no, as the installation of the smaller pulley netted some impressive power gains. The boost gauge showed a slight increase in peak boost pressure from 14.5 psi to 16.1 psi, taking the peak torque numbers from 641 lb-ft of torque to 677 lb-ft. As expected, the increase in boost pressure resulted in a solid gain throughout most of the rev range, the exception being at the very top. Where the pulley swap netted an additional 36 lb-ft of torque, the peak power gain was only 8 hp. Either we were running out of supercharger (not likely given the 1,000-plus-horsepower potential) or there was another restriction.
Knowing the flow potential...
Knowing the flow potential of its 2.8L blower and intake manifold, Kenne Bell saw fit to machine the manifold with an adapter to fit the massive throttle body from the Ford GT motor.
Our first thought was the factory exhaust system, since the motor was still breathing through the factory cats and a Bassani after-cat exhaust. While the factory cats would certainly be limiting exhaust flow at this power level, the induction side is much more critical on positive-displacement superchargers. Both Roots-style and twin-screw superchargers are ultrasensitive to inlet restrictions. Choke the inlet of a supercharger and you're rewarded with less flow and less boost, the combination of which equates to a power loss.
Not wanting to speculate on the inlet losses, Kenne Bell relied on its extensive data-logging equipment to determine the culprit. The surefire way to detect inlet restrictions is to monitor the vacuum readings before the supercharger. Kenne Bell set up not one but a number of different fittings to measure the losses inherent in all of the factory intake components, including the filter box/MAF assembly, the inlet tube (between the MAF and throttle body), and the factory dual-blade throttle body itself. Recognizing the need for additional flow, Kenne Bell designed a huge inlet manifold (capable of flowing more than 1,600 cfm) for the new 2.8L supercharger. With an efficient supercharger and free-flowing inlet manifold, it decided to take a look at the factory MAF and throttle body.
Kenne Bell SC GT500-Stock Vs. Dual 75mm Throttle Body
WITH VACUUM still showing in the inlet system, Kenne Bell turned its attention to the throttle body. While the factory throttle body is perfectly adequate for the factory power level, it obviously becomes a restriction at some point. This graph illustrates that the stock throttle body represented a major restriction at this power and boost level. With no larger throttle body yet available for the GT500, Kenne Bell got together with AccuFab to modify a dual-75mm Ford GT throttle body. Since the Ford GT throttle body from Accufab featured a manual throttle cable, it was necessary to adapt factory drive-by-wire electronics to convert the manual throttle body. Some additional tuning was necessary in order to make the ECU happy with the new throttle-body combination (no small feat), but the results were well worth the effort. With a not-so-simple throttle-body change, the power output of the Kenne Bell supercharged GT500 motor jumped from 706 hp and 684 lb-ft to 793 hp and 737 lb-ft. A slightly cooler run netted over 800 rwhp with this combination. Imagine, 800 rwhp from bolting on a supercharger, a throttle body, and a mass air meter. These GT500s kick butt.