Richard Holdener
July 1, 2009
For the first test, the GT1000 5.4L was equipped with a stock GT500 supercharger. Obviously the motor was never going to produce 1,000 hp with the stock Roots blower, but it did allow us to run our header test at a more realistic power level.

With our understanding of header (and intake) theory now complete, we can take a look at our header test. Having run our three sets of American Racing headers on the normally aspirated GT1000 motor, it was time to apply the same test procedure to a supercharged combination. To facilitate this test, off came the modified Cobra R intake and on went the stock blower assembly from the GT500. The M122 supercharger was installed along with the factory air-to-water intercooler.

All testing was completed with dyno water running through the intercooler core. Actually, the intercooler received water after running it through the engine--not ideal, but pretty real world in terms of water temps. (We kept the water temps to the motor low for this reason). We took the liberty of installing a set of larger Siemens injectors since we planned to install a Kenne Bell blower and crank things up to reach our goal of 1,000 hp.

Since Ford blessed the GT500 with drive-by-wire throttle activation, we had to make our own mechanical version. Apparently the boys at Westech didn't take to my vice grips on the throttle shaft, so they machined this aluminum throttle lever. Not only did it look much better, it also didn't damage the throttle shaft.

The Eaton supercharger was run with the stock blower and crank pulleys, which produced a peak boost pressure of 10 psi (with the stock exhaust manifolds). So equipped, the supercharged 5.4L produced peak numbers of 630 hp at 6,500 rpm and 547 lb-ft at 5,100 rpm. Compared to the normally aspirated numbers with the stock exhaust manifolds, we increased the power output by nearly 73 percent with an increase in boost pressure of just 68 percent.

On the surface, it appeared that the stock exhaust manifolds were less affected by the wild cam timing in supercharged form, but the only way to know for sure was to install the first set of headers. On went the 1-inch stainless headers from American Racing, and once again, up went the power. Equipped with the long-tube headers, the supercharged GT1000 motor produced 655 hp at 6,500 rpm and 578 lb-ft at 4,800 rpm. It should be noted that these gains came despite a drop in boost from 10 psi to 7.2. Whenever you get an increase in power with a drop in boost, you know you are on the right track. The peak-to-peak difference between the stock manifolds and 1-inch headers was 25 hp, not the 100-plus hp witnessed on the normally aspirated combination. True to form, the scavenging effect of the long-tube headers improved the power output from 3,500 rpm all the way to 6,500 rpm, a sure indication that the extra power came from scavenging and not simple additional flow rate. Were the gains based solely on flow rate, we'd have seen the power gains increase with engine speed, meaning less of a gain down at 3,500 rpm than at 6,500 rpm. The second test run with the Kenne Bell at a much higher power level confirmed these gains were a result of the scavenging effect. Without the wild cam timing, these are the types of gains we'd expect to see on a normally aspirated mod motor as well, possibly slightly less, but consistent gains through the entire rev range.

The first order of business was to establish a baseline with the stock exhaust manifolds. After the results of the normally aspirated combination, we were anxious to see their response to this supercharged application.

When it was time to run the larger headers, things got really interesting. Installation of the larger 1 7/8-inch headers (with larger 3.5-inch collectors and extensions) improved the power output slightly to 658 hp at 6,500 rpm and 580 lb-ft at 4,800 rpm. Note that both long-tube headers reduced the peak torque engine speed compared to the stock exhaust manifolds (4,800 rpm versus 5,100 rpm). This trend continued with the larger 2.0-inch headers (same 3.5-inch collectors and extensions), as the supercharged motor produced 663 hp at 6,500 rpm and 583 lb-ft at 4,800 rpm.

Basically, the peak power numbers improved (albeit slightly) with each subsequent increase in header diameter. What was interesting was that unlike the normally aspirated combination, there was no loss in power elsewhere along the curve. The big-boy 2.0-inch header produced as much torque down at 3,500 rpm as the smallest 1-inch headers. The only difference between the headers was at the top of the rev range.