2007 Shelby GT500 - Bigger, Better Boost - Part 2

Blessed with additional displacement (compared to the usual 4.6L mod motor), the 5.4L also received the free-flowing Four-Valve heads from the Ford GT engine program. Also present was an efficient air-to-water intercooler and injectors capable of supporting as much as 650 rwhp. Obviously, the highlight of the motor is the Eaton Roots-style supercharger. Stick your foot in the throttle of a supercharged mod motor and you will be immediately rewarded with a surge of torque not found in any of the normally aspirated combinations.

Huge midrange power is but one of the side benefits of a supercharged motor. While the blower is undoubtedly the lead character of the supercharged 5.4L, it's also the primary restriction. When you go looking for big power gains, the stock supercharger can take you only so far. This is especially true given the current pulley dilemma, as increasing the size of the crank pulley may prove to be difficult, though we suspect some enterprising manufacturer may try a transfer pulley arrangement that will allow use of larger crank pulleys. The external dimension of the blower snout determines the ultimate size of the blower pulley, so (for the time being) there will be a limit to just how much you will be able to crank up the boost on the stock blower.

Even if the aftermarket finds a way, ultimately the style and-more importantly-sizing of the Eaton supercharger will limit the maximum power output. At some point, the stock blower will run out of flow and/or rpm capability, It is with these limitations in mind that we decided to replace the factory supercharger with the Kenne Bell twin screw.

Not just any twin-screw, mind you, and certainly not the usual Autorotor blower, but a brand-new unit designed by Kenne Bell. The new Kenne Bell twin-screw supercharger upgrade for the GT500 displaces a whopping 2.8 liters. This means Shelby owners can replace the 500 hp (in all fairness, a tad more) with a 1,000hp blower. Thus, the KB combines an increase in efficiency from the twin-screw design (see sidebar on new H-series blowers) with a substantial hike in displacement. As we've seen in the past on the 4.6L Cobras and Lightnings, the results are quite impressive.

To illustrate just how much power was available from the upgrade, we ventured once more to Kenne Bell to subject the Tangy Tangerine Shelby GT500 to the rigors of the chassis dyno. For our test, the 5.4L motor was equipped with the stock Eaton supercharger sporting a 3.0-inch blower pulley. As always, the air/fuel and timing were locked in place to minimize variables. So equipped, the Eaton supercharger produced 8.7 psi, allowing the motor to produce 487 hp and 462 lb-ft of torque. After the baseline testing, the gang at Kenne Bell swapped on the 2.8L blower and revised intake system (between the blower and throttle body). Running the same timing and air/fuel and equipped with the same 3.0-inch blower pulley, the Kenne Bell blower upgrade improved the power output to an amazing 610 hp. The twin-screw supercharger also increased torque production from 462 lb-ft to 567. Despite the use of the same blower pulley, the larger (and more efficient) Kenne Bell 2.8L blower increased the boost pressure from 8.7 psi to 12.7. What GT500 owner wouldn't be happy with over 600 rwhp?

Just in case some weren't, we decided to take things a step further. In fact, it looks like the new Kenne Bell blower kit will be offered at this higher-horsepower level. Getting greedy, we replaced the 3.0-inch blower pulley with a smaller 2.75-inch version (pulley swaps are a snap on the Kenne Bell kit). The result of the pulley swap was an increase in boost pressure from 12.7 psi to 14.5. The additional boost pressure had a positive effect on the horsepower and torque curves. Equipped with the 2.75-inch blower pulley, the supercharged 5.4L motor thumped out 656 hp and 641 lb-ft. of torque. Torque production from the twin screw exceeded 600 lb-ft from 2,700 rpm all the way to 5,500, making for one sweet powerband. Even at 2,000 rpm, the motor produced 520 lb-ft. We suspect traction will be in short supply with 520 lb-ft of torque at just 2,000 rpm.