By Richard Holdener

Why install a supercharger on a motor already equipped with one? The answer is easy! A Kenne Bell blower upgrade is a tried and true method for improving the power output of the 5.4L (and 5.8L) GT500 modular Ford motor. We all know that the Shelby GT500 is an impressive piece, especially with the most recent 5.8L 4-valve motor. Even with the previous (and smaller) 5.4L, the GT500 was a forced to be reckoned with on the street. Despite the presence of a factory supercharger and impressive power offered in stock trim, the supercharger itself was the limiting factory when it came to additional power. Superchargers are designed for specific power levels. There is obviously wiggle room in their ability, but eventually, the physical size and rpm potential of the blower limits that amount of airflow it can provide. The factory-supplied Eaton supercharger supplied on the GT500 is no different, so making big power gains means steeping up to not only bigger, but Better Boost!

The test mule was a bone-stock, 5.4L four-valve pulled from a 2011 GT500.

To illustrate the potential of Better Boost, we performed a blower upgrade on an otherwise stock 2011 GT500 motor. While most GT500 testing is performed on the chassis dyno, our testing was run with the motor out of the car on the engine dyno. This made working on the engine much easier, though a Kenne Bell, twin-screw blower upgrade is not terribly difficult in the car.

Compared to the stock M122 Eaton supercharger, the Kenne Bell offered both improved efficiency and increased displacement. That’s why Shelby chose them for the Super Snake and 1000 horsepower programs. The additional efficiency came first from the twin-screw design (already superior to traditional roots) but Kenne Bell kicked things up another notch by employing an industry-leading 4×6 twin-screw rotor pack. The result of this design was that the Kenne Bell twin-screw blower offered increased power with less parasitic losses and a lower inlet air temp than the Eaton supercharger. Toss in the displacement hike to 2.8 liters and you get a blower capable of supporting over 1,000 horsepower on the right application.

The theories were all well and good on paper, but how would they translate into real-world numbers? To find out, we installed the GT500 motor on the dyno for testing. Rather than run the factory ECU, the timing and fuel were controlled using a FAST XFI/XIM management system. To demonstrate just how much power the factory Eaton supercharger was contributing to the mix, we ran the GT500 motor first in normally aspirated trim using a modified 5.4L Cobra R intake. We combined the Cobra R lower intake with a fabricated plenum and Accufab throttle body. Remember, with the mild factory cam timing and lower compression, this GT500 motor was designed specifically for forced-induction use. Equipped with the Cobra R intake and stock exhaust manifolds, the normally aspirated GT500 motor produced 384 hp at 5,300 rpm and 413 lb-ft of torque at 4,400 rpm. That the motor produced more torque than horsepower is a clear indication of the mild cam timing. Having established our baseline, it was time for boost.

After the installation of the factory Eaton supercharger and air-to-water intercooler (using dyno water), we run the supercharged combination using the factory 3.06-inch blower pulley. Once again run through the factory cast-iron exhaust manifolds, the supercharger pumped out a maximum of 10 psi of boost and elevated the power output from 384 hp and 413 lb-ft of torque to 594 hp and 559 lb-ft of torque. The difference between this output and the factory rated 550 hp can be attributed to the lack of an air intake, complete exhaust and drive accessories.

The optimized tune no doubt added a few extra ponies as well, as the factory tune was very conservative. Two things the positive displacement (roots-style) blowers excel at are immediate boost and torque production. Though peak numbers are usually the hot topic of discussion, the broad torque curve (exceeding 500 lb-ft from below 3,000 rpm past 6,000 rpm) offered by the supercharged 5.4L is what really helps accelerate the GT500. There was obviously more boost and power to be had from the Eaton supercharger, but we decided to step up not just to more boost, but Better Boost with the Kenne Bell blower upgrade.

To illustrate the substantial power gains offered by the increased efficiency and displacement, we configured the Kenne Bell twin-screw supercharger with the same size blower pulley as the factory Eaton. Both combos were run with the factory crank pulley. The closest we could get to the 3.06-inch stock GT500 blower pulley was a 3.0-inch Kenne Bell version. The Kenne Bell blower upgrade utilized a new lower intake that accepted the factory GT500 air-to-water intercooler. Installation of the 2.8L blower upgrade was simple, easy, and well worth the effort given the dramatic results. Run with the 3.0-inch blower pulley, the Kenne Bell supercharger pumped out 20.0 psi of boost.

This represented an increase of 10 psi over the factory Eaton. Naturally the additional boost increased the power output, jumping from 594 hp and 559 lb-ft of torque to 806 hp and 721 lb-ft. The final test involved replacing the stock exhaust manifolds with 1 7/8-inch long-tube headers. The headers further increased the power output of the Kenne Bell 5.4L to 823 hp and 731 lb-ft, while simultaneously dropping the boost by almost 1 full pound. More power with less pressure is a little something we like to call Better Boost!

Before running our tests on the blower upgrade, we decided it was a good idea to run the otherwise stock GT500 motor normally aspirated. This way we could demonstrate just how much of the power came from boost supplied by the stock blower. Run with a modified Cobra R intake system, the GT500 motor produced 384 hp and 413 lb-ft of torque. After adding the Eaton supercharger, these numbers jumped to 594 hp and 559 lb-ft of torque. Stepping up to the Kenne Bell 2.8L, twin-screw supercharger resulted in 806 hp and 721 lb-ft of torque.

The initial testing was run with the factory cast-iron exhaust manifolds, but we eventually upgraded with motor with a pair of 1 7/8-inch, long-tube headers from American Racing. The headers were worth a decent power gain, as the peak numbers jumped from 806 hp and 721 lb-ft to 823 hp and 731 lb-ft. Boost also dropped by nearly 1 full psi, from 20.0 psi to 19.1 psi.

Sources: Kenne Bell; kennebell.net