Richard Holdener
July 1, 2009
Since time was in short supply, we stepped right up to the 2.0-inch headers (we wanted them for our shot at 1,000 hp). Equipped with the 2.0-inch headers, the boost dropped to 11.4 psi, while the peak numbers jumped to 788 hp and 645 lb-ft of torque.

Unlike the test run on the normally aspirated combination, the 2.0-inch header performed best on the supercharged combination. In fact, the largest of the three headers not only offered the highest peak power, but there was no trade in power elsewhere along the curve. Had we elected to rev the motor to 7,000 rpm, the differences between the sizes would have been even greater. It's obvious from the graph comparing the stock exhaust manifolds to the long-tube header that the motor responded to the scavenging effect of the tuned header length. Less evident is the fact that it may have been the short-runner intake even more so than the boost supplied by the supercharger that made the motor less responsive to the different header configurations. Since the header scavenging effect must work in conjunction with the reflected waves produced by the intake manifold, the short-runners used on the blower motor provide no such ram tuning (at least, not in the rpm range of this motor). The only way to be sure is to run the motor in N/A trim with the short-runner blower intake (without boost pressure) to see if the trend continues. Another option is to run the motor equipped with a centrifugal supercharger and long-runner intake to see if the motor was more or less responsive to the changes in the headers.

Header Test: Stock GT500 Motor
After running our header test on a normally aspirated and supercharged 5.4L combination, the last thing we should want is more testing, but we decided to do just that. Since the cam timing had such a dramatic effect on the header comparison of the normally aspirated GT1000 combination, we decided to run the same test on a standard GT500 motor.

After running the header tests, we performed a pulley swap. The smallest pulley offered for the Kenne Bell measures 2.5 inches. Though this pulley has been run successfully, a better option is to increase the size of the crank pulley using this new unit from Innovators West. This will allow you to run a larger (2.75-inch) blower pulley to minimize belt slippage.

We pulled a stock GT500 motor out of a car, installed it on the engine dyno and ran the exact same series of tests. The GT500 motor was run in normally aspirated rim (using the same Cobra R set up), with the stock Eaton supercharger and then again with the Kenne Bell. In each configuration, we ran the motor with the stock exhaust manifolds; then the three different header configurations. Run in normally aspirated trim with the stock exhaust manifolds, the stock GT500 motor produced 384 hp and 413 lb-ft of torque. After installing the 1-inch headers from American Racing, the peak power numbers jumped to 410 hp and 432 lb-ft of torque. As with the normally aspirated GT1000 combination, the larger headers improved power (albeit slightly) at the top of the rev range, but lost power up to 5,000 rpm. Unlike the wild cam timing in the GT1000 combination, the stock exhaust manifolds did not kill the power on the GT500 motor.

The header test was run once again with the GT500 equipped with the stock Eaton supercharger. Run with the stock pulleys and stock exhaust manifolds, the supercharged 5.4L produced 594 hp and 559 lb-ft. Installation of the American Racing headers improved the peak power numbers to 602 hp and 566 lb-ft. As with the test run on the GT1000 motor, the headers improved the power output through the entire rev range. The larger 2-inch headers further improved the peak power output (compared to the smaller 1 headers), and the headers were worth as much as 10-11 hp and 12-13 ft-lb of torque over the stock exhaust manifolds. After running the stock supercharger, we installed the Kenne Bell 2.8L H-series Twin Screw supercharger. Equipped with the stock exhaust manifolds, the Kenne Bell-equipped 5.4L produced 719 hp and 615 ft-lb of torque.