Steve Turner
Former Editor, 5.0 Mustang & Super Fords
November 2, 2007
Photos By: Anderson Ford Motorsport
What do you do with a spankin'-new Shelby GT 500 with only 50 miles on it? Well, strap it to the chassis dyno and find more power, of course. That's just what Jeff Reasons had in mind when he left his car in the capable hands of Rick Anderson at Anderson Ford Motorsport. Rick cooked up a combination that delivered 129 more horsepower to make sure Jeff's GT 500 made more power than his supercharged Fox convertible.

Horse Sense: Don't think you're out of the bolt-on game if you don't have a GT 500. Most companies that make parts for the GT 500 already have a robust lineup for 4.6 and 5.0 Mustangs as well.

Easily the most hyped, most lusted-after Mustang in recent memory, the '07 Shelby GT 500 is the most powerful factory Mustang yet. For most people, 500 hp at the flywheel and all that exclusivity of Shelby and SVT would be plenty. But those would probably be the same folks who lock the car in the garage, hoping for dollar signs in 20 years. Those people probably don't read this magazine. The horsepower-crazed members of the 5.0&SF army know this car was born for bolt-ons, much like its Terminator predecessor.

Sure, we can't all have a new one, but someday we can all hope to eventually own a Shelby GT 500. For those who joined the game early, the bolt-on world is their oyster. Just a few parts are all that's needed to see huge gains in performance. We've covered many of the pulley and cold-air combinations for these cars, and they work well, but we wanted to get deeper into the car and check out some more serious gear, particularly PMAS' new Velocity mass air meter and Bassani's long-tube headers and full 3-inch exhaust. Fortunately, Anderson Ford Motorsport was more than willing to experiment on our behalf.

Of course, to do so, we needed a willing participant to let us tear into a new GT 500. Great minds think alike, as new '07 Shelby GT 500 owner, Jeff Reasons, couldn't wait to tear into his car. Jeff also owns a Fox convertible with an AFM Stage II S-Trim supercharger kit. It lays down more than 518 hp at the wheels. All Jeff wanted was his GT 500 to make more power than his Fox. In a demonstration of how far base horsepower and easy bolt-ons have come, it took only the aforementioned mass air and exhaust, as well as a pulley and tune, to move Jeff's GT 500 to the king of the hill in his garage.

Here's Jeff's GT 500 after the full treatment. Visible are the AFM Power Pipe, PMAS Velocity mass air meter, Metco 2.6-inch blower pulley, and Metco idler pulley. Due to the wide engine, you can't see the Bassani long-tubes from this angle, and the Predator tune is hiding in the Silver Oak processor. All told, this simple combination of parts provided huge gains across the entire rpm range.

Here's the complete Bassani Xhaust system for the GT 500. This system is serious business, replacing the solid factory system with massive 1 3/4-inch to 1 7/8-inch stepped, long-tube headers (PN BA-S5407R; $1,460); a 3-inch X-shape-crossover with high-flow cats (PN BA-5407R-3; $399); and 3-inch Aft-Cat with mufflers (PN BA-5407R-5; $799). This system is a thing of beauty and should give plenty of exhaust to support even more power in the future.

Always wanting to learn what the limits are and how to work around them for more power, Rick had long suspected the stock airbox on the GT 500 would limit power.

One sure way to find out is strapping it to a flow bench. AFM has one in the form of a Super Flow SF-1200, which Rick uses to develop his Power Pipes, among other things. With the stock air filter in place, the stock box flows 826 cfm at 25 inches of water. Removing the filter bumped the flow slightly to 898 cfm, but we know the factory mass air meters don't like the wild air blasting through. The filter straightens and smoothes the airflow.

Obviously 826 or so cfm isn't enough air to make the kind of power even a bolt-on GT 500 is capable of producing. Moreover, Rick learned that the stock mass air calibration is set up to max out its voltage before the injectors max out their flow, causing the air/fuel ratio to richen considerably. While this keeps things nice and safe, it severely limits power production. Rather than simply tuning around this, Rick opted to use PMAS' new Velocity mass air meter in conjunction with a new AFM Power Pipe and a custom Predator tune (PK-GT500PVP; $785), which is calibrated not to prematurely max out the voltage. PMAS says the design of the meter reduces signal noise and improves airflow over the factory meter design, and it will support up to 1,500 hp. At the same 25 inches of water, the Velocity meter flowed 1,422 cfm on the AFM flow bench, nearly doubling the flow of the stock meter/airbox combination.

AFM's Danny Biggs crafted a special tune to make the most of the Velocity mass air and Power Pipe combination. To see what the Velocity and Power Pipe were worth on their own, Danny installed the parts with a tune containing the stock fuel and timing parameters. Then he moved up to the Performance tune for the remainder of the testing.

After opening the inlet, it was time for more boost. It's still hard to believe that factory supercharging has made power so easy to find. After pulling the stock blower pulley, the AFM crew installed one of Metco Motorsports' 2.6-inch pulleys (PN MSP2.60; $159.95) and accompanying idler pulley (PN MSI-90; $189.95). The pulley increases boost output, and the idler makes sure it stays increased by reducing belt slip. Adding the pulley resulted in 3 more psi for a total of 11 pounds of boost.

Since muffler swaps are so easy and common on the S197 platform, Rick elected to see what a Bassani after-axle muffler kit (BA-5407-5; $449) was worth on its own. The gains were modest, but the GT 500's personality greatly improved, thanks to more exhaust bark to go with all that bite.

The stock cast-iron manifolds are designed for leak-free, quiet performance in a compact form. The manifolds have to be compact because of the wide engine, so you're probably thinking it's going to be a challenge to get stepped long-tube headers in the same spot. Guess what-you're right.

The Bassani long-tubes are shipped as separate pipes to facilitate the installation on the car. Even with the separate pipes, the installation is time consuming and tedious. Reports of AFM tech Chad Kolakowski perched atop the engine tugging on pipes and speaking in the tongue of a sailor on leave were unconfirmed.

As you can see, the headers fit. As Rick said, they're hard to install because they work. In other words, they don't sacrifice performance (check the On the Dyno sidebar) for an easy installation.

Between the stepped long-tube and the Aft-Axle mufflers are a full 3-inch X-shape crossover with high-flow ceramic catalytic converters. Everything in the Bassani system uses slip-fit connectors and band clamps, similar to the factory parts. That makes for easier adjustment.

How's this for climbing right up the horsepower ladder? The AFM crew took Jeff Reasons' '07 GT 500 from 415.25 hp at the tires all the way up to 544.93 to the ground. That's impressive stuff. As you can see, a huge gain (53.89 hp peak to peak) came from replacing the restrictive factory airbox with the PMAS Velocity meter and Power Pipe. Just the Predator Performance tune tacked on another 27.11 peak horsepower, and the Metco pulley made good with 23.57 peak horsepower. Meanwhile, the Bassani mufflers clocked in with 6.31 peak horsepower, and the rest of the exhaust added 18.8 for a total of 25.11 peak horsepower. We know the stock systems on the latest Mustang are good, so when Rick described this as the most power he's seen out of an exhaust system, that's high praise indeed.

Of course, it's sexy to talk about the peak numbers, but it's the area under the curve that you really feel. As you can see in the column showing the difference between the baseline and this package of parts, that the gains are impressive for the duration of the pull, that's power you can feel. Bolt-ons have really come a long way.

RPMBaselinePower Pipe
& Mass Air
Predator Tune2.60 Pulley
RPMBassani MufflersBassani Headers & XDifference