Richard Holdener
July 31, 2007

In the previous two installments of "Mods for GT500 Mods," we took a close look at the hot, new Shelby GT500. Fresh from the factory, the GT500 offers impressive performance even in stock trim. Blessed with the most powerful motor ever offered in a factory Mustang, the GT500 came equipped with no less than 5.4 liters of displacement combined with impressive Four-Valve cylinder heads (lifted from the Ford GT program) and-miracle of all miracles-artificial aspiration.

Perhaps the best news for enthusiasts is that, like the '03-'04 Cobras before it, the GT500 came factory equipped with a supercharger. Ford even went to the trouble of stepping up in terms of blower size (compared to the previous 4.6L Cobra motors) to provide the necessary airflow to the larger 5.4L motor. What this means for GT500 owners is that extra power will be just a pulley swap away. Not surprisingly, Ford went to great lengths to ensure pulley swaps were kept to a minimum by making life difficult for the various tuners and manufacturers. Blower-pulley sizing for the factory Eaton super-charger is limited by the physical size of the blower snout and the need to have some wall thickness in the pulley itself. So far, the minimum (relatively) safe pulley size seems to be 2.60 inches.

On the crank side, it's possible to increase the size of the crank pulley to increase the blower speed, but you won't see the 8-, 9-, and 9.5-inch crank pulleys common on the Lightning/ Cobra motors. Casting bosses in the front-engine dress limit the maximum crank-pulley diameter, unless someone finds a way to build a dedicated blower pulley arrangement (possibly with a transfer assembly). Until then, GT500 owners will be limited in terms of maximum power (using the factory blower) from their new 5.4L Four-Valve motor.

While the factory certainly went to great lengths to ensure reasonable boost pressure for the 5.4L, extra boost and power from the GT500 is just a blower swap away. In addition to allowing the user to dramatically increase the boost pressure with pulley swaps, the new Kenne Bell twin-screw supercharger (and others too from Whipple, Ford Racing Performance Parts, and Roush) offer a sizable jump in both boost and efficiency by virtue of the increased displacement. The new Kenne Bell blowers retain the twin-screw design for efficiency but step things up significantly. Our GT500 kit came with the big, bad, voodoo daddy of superchargers, the 2.8L (H-series).

Compared to the factory Eaton supercharger, the Kenne Bell offered nearly a liter in extra displacement, a significant improvement in efficiency (power gains per pound of boost), and the ability to run much higher levels of boost. Stepping things up one more notch was the addition of the new H-series blower. Using top-secret intake and discharge modifications (which control port timing and pressure ratio), the new unit offered improved efficiency and power in the 13-plus psi range. The new H-series Kenne Bell blower was right at home during the 16-18.5 psi of boost run for this series of testing.

In Part 1 of "Mods for GT500 Mods," we extracted power from the GT500 motor using a smaller blower pulley and custom tuning. The mods improved the power output from 443 hp and 428 lb-ft of torque (at the wheels) to 517 hp and 521 lb-ft. In Part 2, we replaced the stock Eaton supercharger with the Kenne Bell 2.8L blower upgrade and were rewarded with a jump in power to 656 hp and 641 lb-ft of torque.

Kenne Bell SC GT500-Stock Vs. Modified MAF
Since positive-displacement superchargers are ultrasensitive to inlet restrictions, the guys from Kenne Bell took a hard look at what was in front of the throttle body. Rather than speculate about different components, the Kenne Bell crew data logged the vacuum readings present during the dyno runs. According to their instrumentation, there was more than 4.5 inches of vacuum in the inlet system at this power level. No wonder the gains from the smaller blower pulley leveled off at the top of the rev range.

Starting with the first component in the inlet system, Kenne Bell cut out the factory MAF from the airbox lid. After a quick contour and the installation of a free-flowing Kenne Bell filter, we were in business. Due to the MAF alterations, tuning was naturally necessary to achieve the same air/fuel and timing as the previous runs. Running an identical air/fuel and timing, the new MAF increased the power output of the supercharged combination from 656 hp to over 700, with most of the gains coming past 3,700 rpm.

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Next in line in terms of airflow restrictions was the stock throttle body.

If you've never experienced a 600-rwhp street car, believe us, it's something you'll always remember. Talk about an ear-to-ear smile. The 656 hp and 641 lb-ft of torque numbers were produced with the new 2.8L blower sporting a 2.75-inch blower pulley running in conjunction with the factory crank pulley. Running 23 degrees of timing (race fuel for safety) and an air/fuel ratio of 11.5:1, the 5.4L GT500 motor produced a peak boost pressure of 14.5 psi. This compares to just 11 psi produced by the stock blower using an even smaller 2.60-inch blower pulley. There's something to be said for the improved size and efficiency of the new Kenne Bell 2.8L H-series blowers.

While the need for a 650hp street car can be argued, naturally we went looking for more. We couldn't call ourselves Muscle Mustangs & Fast Fords if we didn't. When you think of improving the power output of any supercharged motor, your first inclination should be more boost. At least that's how it was with us. If the 2.75-inch blower pulley was good, then shouldn't a smaller one be better? As it turned out, the answer was both yes and no, as the installation of the smaller pulley netted some impressive power gains. The boost gauge showed a slight increase in peak boost pressure from 14.5 psi to 16.1 psi, taking the peak torque numbers from 641 lb-ft of torque to 677 lb-ft. As expected, the increase in boost pressure resulted in a solid gain throughout most of the rev range, the exception being at the very top. Where the pulley swap netted an additional 36 lb-ft of torque, the peak power gain was only 8 hp. Either we were running out of supercharger (not likely given the 1,000-plus-horsepower potential) or there was another restriction.

Knowing the flow potential of its 2.8L blower and intake manifold, Kenne Bell saw fit to machine the manifold with an adapter to fit the massive throttle body from the Ford GT motor.

Our first thought was the factory exhaust system, since the motor was still breathing through the factory cats and a Bassani after-cat exhaust. While the factory cats would certainly be limiting exhaust flow at this power level, the induction side is much more critical on positive-displacement superchargers. Both Roots-style and twin-screw superchargers are ultrasensitive to inlet restrictions. Choke the inlet of a supercharger and you're rewarded with less flow and less boost, the combination of which equates to a power loss.

Not wanting to speculate on the inlet losses, Kenne Bell relied on its extensive data-logging equipment to determine the culprit. The surefire way to detect inlet restrictions is to monitor the vacuum readings before the supercharger. Kenne Bell set up not one but a number of different fittings to measure the losses inherent in all of the factory intake components, including the filter box/MAF assembly, the inlet tube (between the MAF and throttle body), and the factory dual-blade throttle body itself. Recognizing the need for additional flow, Kenne Bell designed a huge inlet manifold (capable of flowing more than 1,600 cfm) for the new 2.8L supercharger. With an efficient supercharger and free-flowing inlet manifold, it decided to take a look at the factory MAF and throttle body.

Kenne Bell SC GT500-Stock Vs. Dual 75mm Throttle Body
WITH VACUUM still showing in the inlet system, Kenne Bell turned its attention to the throttle body. While the factory throttle body is perfectly adequate for the factory power level, it obviously becomes a restriction at some point. This graph illustrates that the stock throttle body represented a major restriction at this power and boost level. With no larger throttle body yet available for the GT500, Kenne Bell got together with AccuFab to modify a dual-75mm Ford GT throttle body. Since the Ford GT throttle body from Accufab featured a manual throttle cable, it was necessary to adapt factory drive-by-wire electronics to convert the manual throttle body. Some additional tuning was necessary in order to make the ECU happy with the new throttle-body combination (no small feat), but the results were well worth the effort. With a not-so-simple throttle-body change, the power output of the Kenne Bell supercharged GT500 motor jumped from 706 hp and 684 lb-ft to 793 hp and 737 lb-ft. A slightly cooler run netted over 800 rwhp with this combination. Imagine, 800 rwhp from bolting on a supercharger, a throttle body, and a mass air meter. These GT500s kick butt.

Accufab offered this dual-75mm throttle-body upgrade for the Ford GT, so why not install it on this GT500 motor?

What better place to start your search for a restriction than at the beginning of the inlet tract? The first component in the factory induction system was the air-filter box and mass air assembly. Ford saw fit to incorporate the mass air meter assembly into the top of the filter housing (obviously a cost cutting procedure). Until now, all of the testing had been run with the factory induction system, including the filter assembly, the rubber inlet tube, and the throttle body. During testing, the data logging indicated significant restriction present in the inlet tract, meaning there was most certainly power to be had by removing the restrictions. The first step was to attack the mass air meter. With nothing available from the aftermarket (while this was being tested), the guys at Kenne Bell simply took the cutters to the factory airbox. The cure was to remove the MAF assembly from the airbox and equip it with a free-flowing cone filter. Minor tuning was necessary due to the major change in the flow through the meter (to say nothing of the torque limiting features), but Ken Christley had the GT500 ready for dyno runs in no time flat. The MAF mods improved the power output significantly, upping the power peak from 664 hp and 677 lb-ft to 706 hp and 684 lb-ft. The MAF revision allowed more airflow to the motor, and as such, the peak boost pressure increased from 16.1 psi to 16.9 psi. The modification also dropped the vacuum present in the inlet system by nearly 2 inches.

While the 700hp mark came and went, we were still concerned about the presence of nearly 3 inches of vacuum in the inlet tract. While additional gains will likely be realized with a larger mass air meter (are you listening aftermarket?), Kenne Bell turned its attention to the next system in line, namely the rubber inlet tube and stock throttle body. Replacing the rubber inlet tube was a simple matter of swapping it out for the larger (free-flowing) unit on the Ford GT motor. The swap also required the use of the larger dual 75mm Ford GT throttle body available from Accufab.

The one hitch to the throttle body upgrade was that the Ford GT relied on a conventional (cable-operated) throttle, where the GT500 uses drive-by-wire. No sweat, the guys at Accufab and Kenne Bell simply combined the Ford GT throttle body with the electronics from the GT500 and-voil-instant dual-75mm GT500 throttle body. The company was clever enough to machine the inlet manifold (between the throttle body and supercharger) to accept both the smaller factory GT500 throttle body and the larger dual-75mm Ford GT unit. This was accomplished with a simple adapter plate.

This is perhaps the largest gain we have ever measured from a not-so-simple throttle-body swap. After making sure the ECU was happy with the new hybrid throttle body, the motor was run once again. With 23 degrees of timing and an air/fuel mixture of 11.5:1, the GT500 motor responded with an amazing 793 hp and 737 lb-ft of torque. The vacuum in the inlet tract was down by nearly 2 inches (leaving just over 1 inch still present), and the boost jumped from 16.9 psi to 18.5 psi. Inlet restrictions restrict airflow and boost production, which is why we saw such a huge gain in power from this throttle-body swap.

It should be noted that you will not see a gain of nearly 90 hp from a throttle-body upgrade on a stock motor, and may in fact see nothing at all, since the throttle body represented little or no restriction at the stock power level. The gains offered by the throttle-body upgrade will increase with the potential power level. On a 500hp motor, you will see less of a gain than on a 600hp motor. The same goes for a 700hp motor and (like ours) an 800hp motor. This motor did manage to exceed 800 rwhp on a slightly colder run. Oh, the power of simple bolt-ons.

Kenne Bell SC GT500-2.75 vs. 2.50 Blower Pulley
After installing the Kenne Bell supercharger in "Mods for GT500 Mods, Part 2," we were naturally curious about cranking up the boost. Unlike the factory Eaton supercharger, it was possible to easily replace the blower pulleys on the Kenne Bell blower. Stepping down from a 2.75-inch blower pulley to a 2.50-inch pulley resulted in a significant power gain. Torque production was up by 36 lb-ft (from 641 lb-ft to 677 lb-ft), but the peak power gain was less significant (from 656 hp to 664 hp). Since the power gains diminished with engine speed, we suspected there was a restriction elsewhere in the motor.