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.