Richard Holdener
September 14, 2007
While the 5.4L Ford GT motor comes factory equipped with a twin-screw supercharger, replacing the smaller 2.3L blower with a massive 2.8L unit from Kenne Bell adds some serious muscle.

Let's face it, if we at MM&FF have something worthwhile to test, or even something less than worthwhile-heck, if we have anything at all to test, you'll find us strapping down an engine and pulling the lever. There's nothing more satisfying than running a motor on the dyno, making changes, and discovering the power gains (or losses) associated with the changes. We live and breathe for dyno graphs, so when the guys at Accufab invited us for another wild dyno session, naturally we were in.

When they said they were running nothing less than a 5.4 Ford GT bullet, we were doubly in. While we've run a number of tests on the chassis dyno with Ford's supercar, we'd yet to see one strapped to the engine dyno. There were, after all, a great many hurdles to overcome just to get the motor running, never mind the performance upgrades.

The dry sump, dedicated blower drive (with attending accessories) and lack of a conventional (mod motor) starter made installing the GT mill much more difficult than your garden-variety Cobra motor. There was plenty of fabrication involved just to get it running, but with a name like Accufab, making pristine pieces from ordinary chunks of steel and aluminum is second nature.

With success a certainty (though a time-consuming one), the boys at Accufab decided to do more than just run the stock GT motor. Sure, it was interesting just to compare the rated power output to the actual power in our configuration, but MM&FF readers deserve so much more. With power gains just a pulley swap away, why not take the opportunity to run a few tests to produce those magnificent power curves?

Let's face it, the Mustang has been and will always be the most popular and successful ponycar. But what Ford needed (and now needs again thanks to the recent demise of the GT) is a car capable of taking it to the gold-chain Corvette crowd. Even GM's mighty 505hp Z06 is no match for the GT. While not quite matching the 7.0L displacement of the original GT40s, the 5.4L modular motor offered an efficient, positive-displacement supercharger pushing 11 psi of boost. The 74 percent hike in pressure increased the "effective displacement" of our 330-inch motor to something more like 575 inches.

The highlight of the 5.4L Four-Valve modular motor used on the now-defunct Ford GT was the 2.3L twin-screw supercharger. Perhaps the best part of owning a supercharged motor is the ability to crank up the boost.

Rated at 550 hp, every test run on a Ford GT has shown that, much like the '03-'04 Cobras, the power numbers seem somewhat conservative. Running the stock motor on the engine dyno would give us a chance to verify the numbers offered by Ford. It would also give us a chance to step things up, since the best thing about owning a motor with factory forced induction is the ability to crank it up. While there's a limit to the power gains available from any blower, the gains offered with additional boost are far greater than the usual array of bolt-ons applied to a normally aspirated combination.

Using a new HDC drive system offered by Kenne Bell, we were able to crank up the boost on the factory supercharger before eventually replacing the stock 2.3L blower with a much larger (and efficient) 2.8L blower (also from Kenne Bell). At the risk of spoiling the results (as if you guys haven't already memorized the power curves), the larger blower allowed us to easily exceed 800 hp. To put this into proper perspective, a good Holman & Moody 427 side-oiler race motor probably put out something near 550 hp (having never officially dyno'd one, we can't say if that was a net or gross rating). In any case, this new GT motor has the ability to far surpass its predecessor in terms of power out.

After getting the dry sump tank positioned, all of the factory accessories (and blower drive) installed, and the Motec wiring harness made for the GT motor, the 5.4L was installed on the Superflow engine dyno at Westech. This particular engine dyno was configured with a dedicated starting system. This is important as the 5.4L GT motor was not equipped with a starter in the conventional location, and there was no room to run it on the dyno in its new orientation. The motor was purposely kept as stock as possible-the only change being a slight increase in exhaust tubing length to clear the dyno-but the factory exhaust manifolds, cats, and muffler canister were retained. Other stock components that usually don't make the transition to the engine dyno included the full accessory drive system and factory air filter and inlet system. The only nonstock component used on the engine dyno was the Motec stand-alone EFI management system used for tuning. This certainly improved the overall power curve of the combination, but not likely the peak power output as the factory runs over 20 degrees of total timing in the stock programming. Big gains (from additional timing) can be found at lower engine speeds, while gains in the upper rpm range come from slightly leaner (in the low- to mid-11s rather than high 10:1) mixtures.

The billet boys at Accufab whipped up a number of custom components to allow this motor to be run successfully on the engine dyno, including a mount for the dry-sump tank.

In bone-stock trim (but with optimized tuning of 21 degrees of timing and an 11.3:1 air/fuel ratio), the 5.4 motor produced 643 hp at 7,400 rpm and 545 lb-ft of torque at 4,900 rpm. While most GT owners don't run their motors (in the car) to 7,500 rpm, we allowed the motor to go there in an effort to find out where it stopped making power. It looks like extending the rpm range might be a good software modification for fellow GT owners since this supercharged 5.4L pulled strong all the way to 7,500 rpm. Were it our motor, we'd probably limit it to 7,000 rpm and call it a day, since the mean piston speed of the GT motor running at 7,500 rpm exceeds that of a (small-stroke) Formula 1 motor at 19,000 rpm. Now imagine how heavy that 5.4L piston is compared to the tiny Formula 1 slug and you can start to appreciate just what the rods are going through during each revolution.

In addition to the impressive peak power output, torque production from the supercharged 5.4L exceeded 500 lb-ft from 3,000 rpm (the lowest test speed) to 6,500 rpm. That is what we call an impressive torque curve. At the maximum revolutions listed by Ford (of 6,500 rpm), the 5.4L motor produced near 620 hp. Even consid-ering the differences in temperature and tune between our test and those run to officially rate the motor, the 5.4L GT motor seems somewhat underrated by Ford.

Though run at the stock power level, we decided to check the effectiveness of a few minor mods before cranking up the boost. First up was an Accufab throttle body followed by its x pipe system upgrade. Replacing the factory GT throttle body with the Accufab dual 75mm unit resulted in a jump in boost pressure (from minimizing the inlet restriction) of roughly 0.3 psi and with it an increase in power (at the top of the rev range) of 10-11 hp. The Accufab exhaust upped the power ante by 12-13 hp, though, as expected, future testing showed much greater power gains at higher boost/power levels. The restrictions imposed by the factory throttle body and muffler increase with the power level of the motor and as such so do the gains offered by their replacement. In an effort to demonstrate the true worth of each modification, the air/fuel and timing curves were kept consistent for each combination.

After our minor mods, it was time to get serious. Changing boost provided by the supercharger is a simple matter of increasing the blower speed, which is itself a function of the drive ratio between the crank and blower pulleys. An increase in blower speed can be achieved by either increasing the size of the crank pulley or decreasing the size of the blower pulley. Increasing the size of the crank pulley on the GT is made difficult by the proximity of the adjacent components and castings in the front cover, so the only option is to decrease the size of the blower pulley. Since the factory pulley is press fit (to discourage such boost-enhancing actions), Kenne Bell decided the best alternative was to replace the entire drive assembly. Its High Capacity Drive (HCD) assembly replaces the factory blower snout and allows the easy installation of a number of different pulley sizes ranging from stock down to 2.50 inches. This allows GT owners to increase boost pressure up to 18 psi. Kenne Bell also offers this upgrade for GT500 owners. Replacing the factory snout with the HCD equipped with a 2.75-inch blower pulley resulted in a jump in power up to 715 hp. The torque production was up as well, from 545 lb-ft to 638 lb-ft. The combination of the Accufab throttle body, x pipe system exhaust, and HCD (with 2.75-inch pulley) netted an increase of 72 hp and 93 lb-ft of torque. Down below 4,000 rpm, torque production improved by over 120 lb-ft.

The best route for improved performance on the GT motor is to replace the factory 2.3L blower with the larger 2.8L from Kenne Bell. The Kenne Bell blower offered increased displacement (which means more power potential), billet construction, and a polished housing.

Having nearly reached the limit of the stock 2.3L twin-screw supercharger, we decided to take Kenne Bell up on its offer to run the new 2.8L blower for the Ford GT. Though the two blowers share a twin-screw design, the Kenne Bell 2.8L H-series blower offers an increase in displacement combined with improved efficiency thanks to a dedicated high-pressure discharge design. According to Kenne Bell, the new H-series blowers were designed to maximize efficiency at boost levels beyond 15 psi. Since we had already taken this motor to 16 psi (with the HCD drive on the stock blower), the new H-series 2.8L seemed like the perfect candidate for our high-boost 5.4L. To illustrate the merits of the 2.8L Kenne Bell blower, it was equipped with the same 2.75-inch blower pulley run on the stock blower. Where the stock 2.3L blower produced 16 psi with the 2.75-inch blower pulley, the 2.8L Kenne Bell blower produced a massive 22 psi. Naturally the increase in boost pressure had a positive effect on power, with the peak numbers now up to 825 hp and 695 lb-ft.

Later testing with the Kenne Bell upped this power output to 875 hp with custom Accufab cams designed specifically for the GT, but don't tell them you heard about it from us because we were sworn to secrecy.