Richard Holdener
May 1, 2007

Much has been written about the Shelby GT500, and for good reason. The new super Stang is quite possibly the quickest and fastest production Mustang ever offered by Ford. We understand that the word production leaves a little room for interpretation, but in this context, it means a Mustang readily available from your local dealership.

While some of the specialty Stangs (Saleen, Roush, and Steeda) are also available via the dealership, this Shelby puts them all to rest thanks to an honest-to-goodness 500hp super-charged 5.4L Four-Valve cammer motor. The retro fastback styling of the '05 Mustang had us smiling, but we were less than amused about the loss of the supercharged Four-Valve Cobra motor at the close of 2004.

Let's face it-the supercharged 4.6L Cobra motor was the most impressive performance piece to be offered under the hood of a Mustang since the demise of the Boss 429. What made the '03-'04 Cobras so special was not so much the impressive (and obviously underrated) power output of the supercharged 4.6L, but the way the Four-Valve motor took to modifications. Adding a solid 100 hp to the '03-'04 Cobra motor was as easy as swapping pulleys and adding the proper tune. If our luck held, the supercharged 5.4L mod motor in the GT500 used for testing would follow suit.

This is a supercharged 5.4L, so adding serious power to the new GT500 modular motor requires only a few minor bolt-ons.

Given the impressive power output of the 5.4L in stock trim, many enthusiasts may be satisfied to simply leave well enough alone. For those looking to purchase a GT500 and park it in their hermetically sealed storage cocoon next to that never-enjoyed '00 Cobra R, you may want to skip the next few pages. This story-in fact, this entire series-is for true performance enthusiasts looking to take one badass "Shelby-ized" factory Mustang and make it even meaner.

Despite the ability to pump out an honest 500 hp, we think the 5.4L motor feels a bit underpowered. Ah, who are we kidding? Every motor (regardless of the power output) can use more power. After all, we have to make up for the extra heft that everyone is complaining about. For those old enough to remember them or interested enough to do the research, the original GT500s were a tad on the portly side as well. Though they lacked the response and ultimate handling (and overall track performance) of the original '65 (and '66) GT350, the 7.0-liter GT500 offered equal or better straight-line acceleration, to say nothing of the mystique of the big-block. Just check out the price difference between a fully restored GT350 and GT500 and you will begin to appreciate the appeal of having the big motor.

In our opinion, every motor should sport some sort of factory forced induction. The Shelby GT500 came equipped with an Eaton Roots-style blower, pumping out 8.5 psi to the 5.4L Four-Valve motor through an air-to-water intercooler. We upped power with these pulleys from Metco.

Speaking of big motors, the GT500 is blessed with a serious chunk of performance metal. By combining the displacement of the 5.4L Lightning with the four-valve heads of the Cobra, Ford has produced an affordable but still powerful version of the all-aluminum Ford GT motor. True, the GT500 makes do with an iron block (the GT is lightweight aluminum), wet-sump oiling (dry-sump for the GT), and a smaller and less efficient Roots-style blower (twin-screw for the GT). On the plus side, the GT500 short-block is topped off with a set of free-flowing four-valve heads pirated from the GT motor program. The combination of the longer stroke, the GT heads, and the Eaton supercharger allow the GT500 motor to thump out an honest 500 hp.

Unlike the big-block Shelbys of yesteryear, that 500hp rating is a real number-not some jacked-up reading achieved on the engine dyno with the motor ice cold, devoid of the accessories, and tuned to precision. By comparison, the 500hp rating applied to the GT500 motor comes at operating temperature with full induction and exhaust systems while running the factory ECU tune. If this GT500 motor is anything like the previous supercharged Cobra motors, look for the 500hp rating to be a tad conservative. Even if the rating is accurate, 500 hp is some serious power from a stock Mustang.

Our first modification was the removal of the factory air filter. At the stock power level, this resulted in a dramatic change to the air/fuel curve. Power will definitely be gained from the induction system after cranking up the boost.

While definitely excited about getting our hands on a stock Shelby for dyno testing, we were every bit as anxious to perform a few modifications. Given the recent introduction and the fact that few of the cars were actually in the hands of customers, the list of available performance parts for the GT500 was rather short. As a teaser, know that we have a complete Kenne Bell twin-screw supercharger upgrade kit slated for this GT500. For now, we will start the "Mods for GT500 Mods" series with more boost from the factory blower.

Not content to just crank it up and be done with it, we also took a look at the air-filter assembly, the exhaust system, and even some programming. Since we did not have access to an aftermarket exhaust or intake system for the GT500, we simply pulled off the stock after-cat to determine if the system represented any restriction to exhaust flow. On the induction side, we removed the factory air filter to see if there was any power to be had on the inlet side. Of course, this doesn't rule out power gains from after-cat exhaust or cold-air (or MAF) systems. We just wanted to get as much testing accomplished before replacing the Eaton supercharger. Look for more testing on the bolt-on components as soon as they become available.

For our next trick, we removed the factory after-cat exhaust system. Equipped with an x pipe system from the factory (how cool is that?), there appears to be 10-12 hp in the exhaust system (there'll be even more once enthusiasts start removing the cats).

The first order of business was to establish a baseline for the stock GT500 motor. While you might think this would be as easy as running the motor on the Dynojet chassis dyno and calling it a day, the modern EFI motor is somewhat temperamental. This is actually a good thing as Ford has designed a great many safeguards into the factory management system to prevent the average knucklehead from grenading a perfectly good supercharged motor. What this means for serious enthusiasts is that the performance of the motor will change according to factors such as inlet air temperature, coolant temperature, and even air/fuel (which affects the timing curve).

Shelby owners should try this experiment the next time they are on the dyno. Run the car on the dyno after a good cool-down period, then immediately run it again. More than likely you will see a dramatic difference in the power production. The hotter the motor, the lower the power output, due in part to a reduction in ignition timing and minor adjustments in the air/fuel ratio. This difference in power is important, especially for those contemplating any type of performance upgrade. If the power difference between a hot and cold run is 20 hp, how do you test for an exhaust system that may only be good for 10-12 hp?

The first major change in power came from reprogramming the factory ECU. With a custom tune from Kenne Bell (23 degrees of total timing), the peak power output jumped from 443 hp to 478 hp, while the torque improved from 428 lb-ft to 456 lb-ft.

The unscrupulous dyno operator (the one who talked you into purchasing the trick exhaust in the first place) can provide a dyno testing that shows a solid 30hp gain from the magic exhaust. In reality, 20 hp was from the temperature differential and only 10 was from the additional exhaust flow. This is but one of the difficulties in performing proper dyno testing and why the Internet is full of unrealistic test results.

Our need for precision once again took us to the guys at Kenne Bell. Actually, the precision thing was only one reason-another was that KB had access to a brand-new Shelby GT500 courtesy of Earl's Automotive. In addition to the Tangy Tangerine (our color code-not Ford's) GT500, Kenne Bell had a new prototype twin-screw supercharger upgrade for the 5.4L, but that test will have to wait until next month, as we were able to test only the minor bolt-ons for this issue. Rest assured that we will have a complete dyno flog of the new Kenne Bell kit as soon as it is ready for installation. In the meantime, we simply took advantage of what will certainly be the two most effective GT500 mods (at least those retaining the stock blower), namely blower pulleys and ECU programming.

After the performance tune, it was time to change the factory supercharger pulley.

Despite the potential difficulties associated with establishing a baseline, the guys at Kenne Bell ran the motor with full datalogging. It is also nice that they had full control over the timing and fuel curves, which we will discuss here shortly. For now, know that for all of the testing, the timing and fuel curves were identical except as noted. In stock trim, the supercharged 5.4L GT500 motor produced 443 hp and 428 lb-ft of torque. Given the 500hp flywheel rating, we'd say every precious pony offered by Ford was present and accounted for.

As expected of the long-stroke 5.4L, the torque curve was impressive, exceeding 400 lb-ft from 3,000 rpm to 5,400 rpm. The torque production exceeded 300 lb-ft at just 1,500 rpm, so despite the hefty curb weight, even the stock GT500 should have no trouble getting underway. Though the dyno runs were terminated at 6,200 rpm, it appears that additional engine speed would yield additional peak power, as the curve showed every sign of continuing to rise. It should be noted that the long stroke of the 5.4L will produce tremendous piston speeds, so it will not have the ultimate rpm capability of the smaller 4.6L. On the plus side, the added torque offered by the increased displacement (over a 4.6L) means you don't have to rev it to enjoy the extra power.

Metco supplied three different pulleys, ranging from a stock replacement down to a 2.59-inch pulley that allowed the supercharger to produce 11.0 psi of boost.

After running the motor in stock trim, we ventured into the land of modifications. With no cold-air or MAF systems available at the time, we simply removed the stock filter element. There were few changes in power, but just as with the '05-'06 Three-Valve GT motors, the mass air meter built into the top of the factory airbox was sensitive to changes. Even removing the filter had an effect on the timing and fuel curves. By locking the timing and generating a steady air/fuel curve (duplicating the factory curve), we discovered that the factory airbox worked well; removing the filter unearthed just 5-6 hp. We suspect that a revised intake system, MAF system, or better yet, a true cold-air intake will release additional power, especially at higher boost levels.

The key to the Metco system was the press-fit adapter hub that allowed easy installation of the supplied pulleys.

Next on the list was the exhaust system. Once again, all we could do was remove the factory after-cat system (with a true x pipe) to see if the stock exhaust represented any type of restriction. The exhaust modification was actually worth some power (10-12 hp, but the gains were most pronounced in the midrange), so it looks like manufacturers such as MagnaFlow, Bassani, and Borla will be able to improve not only the sound quality, but also the power output of the GT500.

After the induction and exhaust testing, we were free to start making serious power. To ensure the safety of the new GT500 motor, we added race fuel to the mix to eliminate any chance of detonation when increasing the ignition timing and boost. A change in the factory programming was first. As you may suspect, the factory timing and fuel curves on the GT500 motor were a tad on the conservative side, and for good reason. Believe it or not,some mindless GT500 owner may actually try to run regular unleaded fuel in their 500hp supercharged mod motor while simultaneously lugging it up a hill. To keep the motor idiot-proof, Ford left some power on the table for those of us who understand that any supercharged motor deserves nothing but the good stuff. With our tank topped off with race fuel (103 octane), we were safe to explore the limits of timing and fuel without ruining a perfectly good motor.

Equipped with the smallest Metco blower pulley, the Shelby produced 517 hp and 521 lb-ft of torque. All totaled, we improved the power output by 74 hp and nearly 100 lb-ft of torque.

Running the total ignition timing up to 23 degrees but keeping the air/fuel curve down near a safe 11.0:1, the power output of the 5.4L jumped from 443 hp to 478 hp (pay no attention to the 484 hp listed on the dyno sheet-it was a spike at the end of the run). The additional timing improved power everywhere, from 1,500 to 6,200 rpm. It also had a positive effect on the torque curve, as the motor exceeded 400 lb-ft from 2,300 to 6,200 rpm and never dipped below 350 lb-ft. It looked like the new GT500 motor was going to follow in the footsteps of the '03-'04 Cobra motors, as one simple modification had improved the power output by a solid 35 hp.

The next modification was the one we were most excited about-more boost. Not surprisingly, Ford saw fit to equip the GT500 motor with a press-fit blower pulley. Removal is difficult, especially if eventual reuse of the stock pulley is in mind. It was difficult to remove without damaging it. Luckily for us, Metco supplied a series of pulleys designed to work with an adapter hub. The hub system allowed us to easily interchange pulley sizes after the installation of the press-fit hub. The pulley selection ranged from a stock replacement (roughly 3.00 inches) to a slightly smaller 2.77-inch version, down to the surprisingly thin 2.59-inch pulley. In stock trim, the Eaton supercharger mustered a peak boost pressure of 8.5 psi. Stepping down to the 2.77-inch Metco pulley resulted in a jump in boost of roughly 1 psi (to 9.5 psi). Stepping down even further to the 2.59-inch pulley brought the peak boost to 11 psi and the peak power numbers to an impressive 517 hp and 521 lb-ft of torque. Torque production from the supercharged Shelby exceeded 500 lb-ft from 2,900 to 5,000 rpm, making for one sweet powerband. Even down at 2,000 rpm, the 5.4L pumped out 450 lb-ft of torque.

It looks like the GT500 motor is every bit as receptive to power mods as the original '03-'04 Cobra motors, but here are a few things to ponder about the 5.4L motor in the new wonder Stang. While the Metco pulleys offered boost and power gains, there is a limit to the size of the blower pulley given the physical size of the Eaton blower snout. It is unlikely blower pulleys will go much smaller than the Metco 2.59-inch. When you combine this with the fact that a larger crank pulley may not be a reality on this motor (interference from front cover bosses much like the Ford GT motor), you have a situation that ultimately limits how much boost you can run on your mod motor.

Also note that the horsepower output exceeded the torque output on the stock run and the run made with the additional ignition timing. This situation changed once we installed the Metco blower pulleys, especially the smallest version. The torque production exceeding the horsepower production is a sign that the improvements offered by the additional boost were more pronounced at engine speeds below the power peak. This means the supercharger is nearing its flow limit. Of course, there is more power to be had with cams and a better inlet and exhaust system, but the smart money for big boost is not with the Eaton Roots-style blower. Rather, it's found in the upcoming Kenne Bell twin screw. See ya next month.

Kenne Bell Custom Tuning
As expected, changes to the factory timing and air/fuel curve produced positive effects. The additional 6 degrees of timing combined with a steady air/fuel curve of 11.0:1 improved the power output of the GT500 motor from 443 to 478 hp. Torque production jumped from 428 to 456 lb-ft, with more sizable gains realized at the lower rev ranges. There appeared to be more power with additional engine speed, but it should be noted that the long stroke of the 5.4L will produce excess piston speed. After the custom tuning, torque production from the supercharged 5.4L exceeded 400 lb-ft from 2,300 to 6,200 rpm.

Metco 2.59-inch Blower Pulley
Having a factory supercharged motor is a blessing indeed. Just check out the power gains offered by a somewhat simple pulley swap. The Metco pulley system included an adapter hub that made pulley changes a snap, though removing the stock pulley was a bit of a chore. The smallest 2.59-inch Metco pulley raised the peak boost pressure to 11 psi and the power output to an impressive 517 hp. Torque production exceeded 500 lb-ft from 2,900 to 5,000 rpm, peaking at 521 lb-ft. Gaining additional boost pressure from the Eaton Roots-style blower may be difficult, but the cure for the more-boost blues will come from Kenne Bell in the form of a twin-screw blower upgrade. Look for full testing next month (or as soon as KB has it ready).