Richard Holdener
July 31, 2007

In the previous two installments of "Mods for GT500 Mods," we took a close look at the hot, new Shelby GT500. Fresh from the factory, the GT500 offers impressive performance even in stock trim. Blessed with the most powerful motor ever offered in a factory Mustang, the GT500 came equipped with no less than 5.4 liters of displacement combined with impressive Four-Valve cylinder heads (lifted from the Ford GT program) and-miracle of all miracles-artificial aspiration.

Perhaps the best news for enthusiasts is that, like the '03-'04 Cobras before it, the GT500 came factory equipped with a supercharger. Ford even went to the trouble of stepping up in terms of blower size (compared to the previous 4.6L Cobra motors) to provide the necessary airflow to the larger 5.4L motor. What this means for GT500 owners is that extra power will be just a pulley swap away. Not surprisingly, Ford went to great lengths to ensure pulley swaps were kept to a minimum by making life difficult for the various tuners and manufacturers. Blower-pulley sizing for the factory Eaton super-charger is limited by the physical size of the blower snout and the need to have some wall thickness in the pulley itself. So far, the minimum (relatively) safe pulley size seems to be 2.60 inches.

On the crank side, it's possible to increase the size of the crank pulley to increase the blower speed, but you won't see the 8-, 9-, and 9.5-inch crank pulleys common on the Lightning/ Cobra motors. Casting bosses in the front-engine dress limit the maximum crank-pulley diameter, unless someone finds a way to build a dedicated blower pulley arrangement (possibly with a transfer assembly). Until then, GT500 owners will be limited in terms of maximum power (using the factory blower) from their new 5.4L Four-Valve motor.

While the factory certainly went to great lengths to ensure reasonable boost pressure for the 5.4L, extra boost and power from the GT500 is just a blower swap away. In addition to allowing the user to dramatically increase the boost pressure with pulley swaps, the new Kenne Bell twin-screw supercharger (and others too from Whipple, Ford Racing Performance Parts, and Roush) offer a sizable jump in both boost and efficiency by virtue of the increased displacement. The new Kenne Bell blowers retain the twin-screw design for efficiency but step things up significantly. Our GT500 kit came with the big, bad, voodoo daddy of superchargers, the 2.8L (H-series).

Compared to the factory Eaton supercharger, the Kenne Bell offered nearly a liter in extra displacement, a significant improvement in efficiency (power gains per pound of boost), and the ability to run much higher levels of boost. Stepping things up one more notch was the addition of the new H-series blower. Using top-secret intake and discharge modifications (which control port timing and pressure ratio), the new unit offered improved efficiency and power in the 13-plus psi range. The new H-series Kenne Bell blower was right at home during the 16-18.5 psi of boost run for this series of testing.

In Part 1 of "Mods for GT500 Mods," we extracted power from the GT500 motor using a smaller blower pulley and custom tuning. The mods improved the power output from 443 hp and 428 lb-ft of torque (at the wheels) to 517 hp and 521 lb-ft. In Part 2, we replaced the stock Eaton supercharger with the Kenne Bell 2.8L blower upgrade and were rewarded with a jump in power to 656 hp and 641 lb-ft of torque.

Kenne Bell SC GT500-Stock Vs. Modified MAF
Since positive-displacement superchargers are ultrasensitive to inlet restrictions, the guys from Kenne Bell took a hard look at what was in front of the throttle body. Rather than speculate about different components, the Kenne Bell crew data logged the vacuum readings present during the dyno runs. According to their instrumentation, there was more than 4.5 inches of vacuum in the inlet system at this power level. No wonder the gains from the smaller blower pulley leveled off at the top of the rev range.

Starting with the first component in the inlet system, Kenne Bell cut out the factory MAF from the airbox lid. After a quick contour and the installation of a free-flowing Kenne Bell filter, we were in business. Due to the MAF alterations, tuning was naturally necessary to achieve the same air/fuel and timing as the previous runs. Running an identical air/fuel and timing, the new MAF increased the power output of the supercharged combination from 656 hp to over 700, with most of the gains coming past 3,700 rpm.