In 1986, when the Mustang got its first fuel-injection system, nay-sayers were quick to predict that Mustang performance was dead. However, within two years, the 5.0-liter Mustang was the hottest thing on the street after modifiers figured out that the EFI V-8 responded well to typical hot-rodding tricks. Ten years later, in 1996, the Mustang GT switched from the tried-and-true pushrod small-block to the new 4.6 modular V-8. Once again, hot-rodders figured out ways around the more complicated computer system to install better-flowing heads and superchargers.
Now another ten years have passed and we're well into the second year of the new S197 Mustang. Tuners have had over a year to work with the '05-'06 Mustang's three-valve modular V-8, with its ever-more-advanced Spanish Oak processor, and new, stouter chassis, so we figured it's time to see where we stand when it comes to modifying the latest Mustang. To get the scoop, we interrogated Dario Orlando at Steeda Autosports, a Ford-related aftermarket performance company tied closely to Ford through the SEMA/Ford Technology Transfer initiative, giving Steeda a head start on '05 Mustang parts development.
Engine Enhancements
You can't talk about '05-'06 horsepower enhancements without discussing the new, much-advanced, Spanish Oak processor, which is a bit of a double-edged sword for Mustang performance fans. On one hand, the new computer is the reason the 281ci V-8 produces 300 hp while meeting emissions and fuel economy standards. As Orlando points out, a stock '05-'06 GT with five-speed manual transmission makes 273 rear-wheel horsepower, or about the same as a naturally-aspirated '87-'93 5.0 Mustang with all the tricks, such as aftermarket intake, heads, and exhaust. On the other hand, the Spanish Oak computer is so advanced that small changes, like altering the intake air flow with a performance filter, can throw it out of calibration, even sending the engine into "limp-home" mode.
If you're planning to modify an '05-'06 Mustang GT engine, you might as well go ahead and spring for a hand-held flash tuner like this XCalibrator from Superchips Custom Tuning. Without altering the Spanish Oak computer's calibrations, modifications won't work like they should, in some cases even sending the computer into "limp-home" mode. Tuners from Steeda come with calibrations loaded in for Steeda modifications. | 
A cold air kit in combination with revised computer calibrations is the "best bang for the buck," providing up to a 28hp increase. |
Underdrive pulleys free up approximately 11hp on the '05 GT's three-valve 4.6. | |
"It's infinitely more advanced than previous Mustang computers," Orlando told us. "It has many more capabilities as far as what it monitors, how fast it monitors, and how often it monitors."
What this means is that the average hot-rodder, the guy who installs cold air kits and exhaust systems in his driveway, is going to need a hand-held flash tuner, like the XCalibrator from Superchips Custom Tuning or the DiabloSport Predator, to modify the calibrations. "It's now a package deal," Orlando says.
"The average guy can install anything we sell. But unlike in the past, now we sell a cold-air kit with a computer flash to go with it. There's not a lot anyone can do without reflashing the computer." Bottom line: Any modification that's big enough to make a noticeable difference in performance will need a retune.
While the more advanced Spanish Oak computer requires tuning with modifications, some performance enhancements, like these Steeda after-axle mufflers, are easier to install than on previous Mustangs with under-car mufflers and long tailpipes. | 
Extensive testing on Steeda's in-house Dynojet shows that the '05 Mustang GT produces 273 rear-wheel horsepower in stock form; up to 310 hp with a cold-air kit, underdrive pulleys, and performance exhaust; and a "safe" 400 hp with a centrifugal supercharger. |
As you can see with this Vortech installation, the packaging of a centrifugal supercharger on the '05 GT creates a twisting weave of inlet pipes. Horsepower and torque gains remain impressive with the right calibrations and an intercooler. | |
One example is cold-air kits. Steeda's version increases air flow by over 12 percent, which makes the engine flatten out with a 15:1 air/fuel ratio. "That's deadly anyway," Orlando points out. "But any detonation on the '05's three-valve engine could cause engine damage."
But with the computer flash, the engine is recalibrated for the proper air-fuel ratio, allowing the cold-air kit to safely add up to 28 hp. (Editor's note: We documented a 16hp increase in our Steeda cold-air installation article in the Oct. '05 issue). "You also pick up about 30 lb-ft of torque. And anytime you can pick up free torque, that translates into better gas mileage too."
According to Steeda, performance after-axle exhaust systems are worth around 7 hp and underdrive pulleys gain 11 hp. But the key is the combination of mods. "With a combination of cold air, pulleys, and exhaust, you're basically going to peg out at around 307 to 310 hp at the tires. That's about all you can do to these engines without getting into the heads or adding a supercharger."
Superchargers have traditionally performed well on modular engines, and the '05's three-valve is no exception. According to Orlando, "It still comes down to intercooling, whether it's a centrifugal or screw-type supercharger. You can't put hot air into an engine or you'll have detonation issues. When we dyno-test '05 GTs cold, they make 400 hp. But after that first run and it's warm, we have to program it to where it's limited to 370-375 hp. Otherwise, it could detonate."
Although packaging was somewhat more difficult than on previous Mustangs, both Vortech and Paxton were quick to market with ever-popular centrifugal superchargers, with available intercoolers. As for screw-types, Whipple, Magnuson, and Eaton are all working on kits. Steeda plans to produce its own intercooler system once they decide which version for their Steeda packages.