Richard Holdener
June 13, 2007

Step By Step

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Mmfp_0706_01z 2007_shelby_GT500_mustang Kenne_bell_twin_screw_supercharger
The new Kenne Bell twin-screw blower upgrade in the flesh.
Mmfp_0706_02z 2007_shelby_GT500_mustang Factory_eaton_roots_style_supercharger
The factory Eaton Roots-style blower is replaced by a massive 2.8L and more efficient twin-screw supercharger from Kenne Bell.
Mmfp_0706_03z 2007_shelby_GT500_mustang Supercharger_pulleys
Unlike the factory Eaton supercharger, there are a variety of supercharger pulleys to adjust the boost (and power output) to your liking.
Mmfp_0706_04z 2007_shelby_GT500_mustang Polished_kenne_bell_twin_screw_supercharger
The highlight of the Kenne Bell kit is the polished 2.8L twin-screw supercharger. Big motors deserve big blowers, and the 2.8L certainly constitutes the label BIG. Capable of supporting over 1,000 hp, it should be more than enough for all but the craziest GT500 owners.

In last issue's Part 1 of our "Mods for GT500 Mods" series, we looked at the effects of tuning and higher boost pressure on a mostly stock 5.4L motor. As you probably know, the GT500 comes factory equipped with a supercharged Four-Valve motor displacing a massive (at least for a modern Mustang) 5.4 liters.

Unlike the previous Y2K Cobra R, the Shelby/SVT gang didn't stop at a displacement hike. Taking a cue from the '03-'04 Cobra, Ford saw fit to equip the GT500 with forced induction. Not just any blower, the larger 5.4L required a brand-new Eaton supercharger. Though the blower remained a Roots style, the displacement (and therefore power potential) of the blower has grown compared to the previous 4.6L Cobra motors. As impressed as we were with the Terminator motors, these new Cyberdyne T1000 models look to be even more potent.

While the aftermarket has not yet caught up with the new Shelby, we were able to test both tuning and boost upgrades on the GT500. Our test car, supplied by Earl's Automotive, was treated to a Metco 2.59-inch blower pulley and custom tuning (23 degrees of timing and an 11.5:1 air/fuel ratio). The combination eventually allowed us to exceed 500 rwhp.

While we hope to provide more testing on a menagerie of minor mods (air intakes, exhaust, and possibly underdrive pulleys), we promised something more substantial for this issue. Rather than chip away at the performance of this GT500, we decided to go the major demolition route and installed the new Kenne Bell supercharger.

The Kenne Bell twin-screw supercharger upgrade was so impressive, it can be likened to adding a supercharger to an otherwise normally aspirated motor. Why such impressive power gains from a blower upgrade? After all, isn't the motor already equipped with a supercharger? The answer is obviously yes, but the engineers at Ford were not looking for maximum power production; they were instead looking for the most effective and cost-efficient method of reaching 500 hp. Remember, power output is only one of the design goals for any motor, and we bet the ability to dramatically increase the power output through aftermarket tuning doesn't even make the list.

Obviously, this provides the opportunities that we have come to enjoy on the previous Cobras. While a factory supercharged or turbocharged motor is a joy to behold, the lack of concern on Ford's part for future improvements means the supercharger chosen for the job was sized with only the immediate power production in mind.

Why would Ford equip the 500hp motor with a blower capable of supporting 1,000 hp? Even more importantly, why would it equip the 500hp motor with a blower that is not only twice as powerful, but twice as expensive? Enthusiasts are already complaining about the cost of the Shelby GT500 (never mind the profit-enhancing market adjustments by the individual dealerships). Installing expensive components on the motor would only aggravate the price structure. In addition to increasing the sale price of the Shelby, installing a 1,000hp blower on the 500hp motor would also increase the warranty work, since (as we all know) enthusiasts are likely to modify their motors in search of that 1,000hp mark.

Step By Step

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Mmfp_0706_05z 2007_shelby_GT500_mustang Kenne_bell_designed_inlet_manifold
In addition to replacing the supercharger, Kenne bell also designed a new inlet manifold between the blower and throttle body. Why go to all the trouble of installing a more efficient blower just to restrict the inlet side?
Mmfp_0706_06z 2007_shelby_GT500_mustang Facotry_throttle_body
With such a massive inlet manifold for the blower, it is likely the factory dual-blade throttle body will become a restriction. OK, so we already know it is, but you'll have to wait until next month to find out just how much.
Mmfp_0706_07z 2007_shelby_GT500_mustang Air_to_water_intercooler_temperature_sensor
As with all of its efforts, Kenne Bell takes the time to monitor every variable while testing. Check out the temperature sensor in the cooling system for the factory air-to-water intercooler.
Mmfp_0706_08z 2007_shelby_GT500_mustang Data_logging_computer
All of those precious sensors were connected to the datalogging equipment located inside the car. Every run contains not just power and air/fuel numbers, but boost, timing, and fuel pressure along with every other factory available parameter.
Mmfp_0706_11z 2007_shelby_GT500_mustang Dyno_computer_monitor
Think the guys at Kenne Bell aren't serious about dyno testing? Check out the big-screen monitor. We were tempted to pull up a chair and click on Monday night football.

Blessed with additional displacement (compared to the usual 4.6L mod motor), the 5.4L also received the free-flowing Four-Valve heads from the Ford GT engine program. Also present was an efficient air-to-water intercooler and injectors capable of supporting as much as 650 rwhp. Obviously, the highlight of the motor is the Eaton Roots-style supercharger. Stick your foot in the throttle of a supercharged mod motor and you will be immediately rewarded with a surge of torque not found in any of the normally aspirated combinations.

Huge midrange power is but one of the side benefits of a supercharged motor. While the blower is undoubtedly the lead character of the supercharged 5.4L, it's also the primary restriction. When you go looking for big power gains, the stock supercharger can take you only so far. This is especially true given the current pulley dilemma, as increasing the size of the crank pulley may prove to be difficult, though we suspect some enterprising manufacturer may try a transfer pulley arrangement that will allow use of larger crank pulleys. The external dimension of the blower snout determines the ultimate size of the blower pulley, so (for the time being) there will be a limit to just how much you will be able to crank up the boost on the stock blower.

Even if the aftermarket finds a way, ultimately the style and-more importantly-sizing of the Eaton supercharger will limit the maximum power output. At some point, the stock blower will run out of flow and/or rpm capability, It is with these limitations in mind that we decided to replace the factory supercharger with the Kenne Bell twin screw.

Not just any twin-screw, mind you, and certainly not the usual Autorotor blower, but a brand-new unit designed by Kenne Bell. The new Kenne Bell twin-screw supercharger upgrade for the GT500 displaces a whopping 2.8 liters. This means Shelby owners can replace the 500 hp (in all fairness, a tad more) with a 1,000hp blower. Thus, the KB combines an increase in efficiency from the twin-screw design (see sidebar on new H-series blowers) with a substantial hike in displacement. As we've seen in the past on the 4.6L Cobras and Lightnings, the results are quite impressive.

To illustrate just how much power was available from the upgrade, we ventured once more to Kenne Bell to subject the Tangy Tangerine Shelby GT500 to the rigors of the chassis dyno. For our test, the 5.4L motor was equipped with the stock Eaton supercharger sporting a 3.0-inch blower pulley. As always, the air/fuel and timing were locked in place to minimize variables. So equipped, the Eaton supercharger produced 8.7 psi, allowing the motor to produce 487 hp and 462 lb-ft of torque. After the baseline testing, the gang at Kenne Bell swapped on the 2.8L blower and revised intake system (between the blower and throttle body). Running the same timing and air/fuel and equipped with the same 3.0-inch blower pulley, the Kenne Bell blower upgrade improved the power output to an amazing 610 hp. The twin-screw supercharger also increased torque production from 462 lb-ft to 567. Despite the use of the same blower pulley, the larger (and more efficient) Kenne Bell 2.8L blower increased the boost pressure from 8.7 psi to 12.7. What GT500 owner wouldn't be happy with over 600 rwhp?

Just in case some weren't, we decided to take things a step further. In fact, it looks like the new Kenne Bell blower kit will be offered at this higher-horsepower level. Getting greedy, we replaced the 3.0-inch blower pulley with a smaller 2.75-inch version (pulley swaps are a snap on the Kenne Bell kit). The result of the pulley swap was an increase in boost pressure from 12.7 psi to 14.5. The additional boost pressure had a positive effect on the horsepower and torque curves. Equipped with the 2.75-inch blower pulley, the supercharged 5.4L motor thumped out 656 hp and 641 lb-ft. of torque. Torque production from the twin screw exceeded 600 lb-ft from 2,700 rpm all the way to 5,500, making for one sweet powerband. Even at 2,000 rpm, the motor produced 520 lb-ft. We suspect traction will be in short supply with 520 lb-ft of torque at just 2,000 rpm.

As we indicated, the Kenne Bell 2.8L blower is capable of supporting over 1,000 hp on the proper combination. After all, we saw the smaller 2.4L produce 960 hp on a smaller 4.6L. There's plenty more power to be had from this combination, especially once Kenne Bell cures the restriction in the throttle body and MAF. Producing 700 rwhp is a simple matter of an inlet upgrade, while there's a ton of power left with more boost.

Naturally, we'll check back with Kenne Bell on this GT500, and we'll also look into other mods once they become available.

GT500-Stock Eaton vs. Kenne Bell Blower Upgrade
Well, fellow Mustangers, it looks like the guys at Kenne Bell have done it again. Check out what happened to the 5.4L Four-Valve GT500 motor after we replaced the stock Eaton supercharger (running a 3-inch drive pulley) with the new Kenne Bell twin-screw blower upgrade. The kit featured the massive 2.8L blower capable of supporting 1,000 hp or more. Run with the same 3-inch blower pulley, the larger and more efficient twin-screw provided roughly 4 more psi of boost, from 8.7 psi to 12.7. The Kenne Bell blower upgrade improved the power output of the 5.4L motor from 481 hp to 606. Naturally, the increase in boost offered serious torque gains throughout the rev range.

Kenne Bell Supercharged GT500-3-inch vs. 2.75-inch blower pulley
As if over 600 rwhp wasn't enough, we decided to take things to the next level. By replacing the 3.0-inch blower pulley with a smaller 2.75-inch version, we increased the boost pressure to 14.5 psi and the power output right along with it. Running the same timing and air/fuel ratio, the additional airflow (and boost pressure) supplied by the pulley change increased the peak power output from 610 hp to 656. As always with a positive displacement supercharger, the power gains were every bit as impressive through the entire rev range. The peak torque jumped from 567 lb-ft to an amazing 641.