The First order of business...
The First order of business was to run the supercharged stang on the Dynojet chassis dyno. Equipped with the stock exhaust manifolds, cat pipes (minus cats), and after-cat (with Jardine mufflers), the supercharged Three-Valve motor produced 779 hp and 691 lb-ft of torque.
Since these waves travel at the speed of sound (which is pressure and temperature dependent), tuning this event for a specific engine speed requires changing the length of the primary pipes. Short primary runners employed on stock exhaust manifolds don't allow sufficient time for the compression wave to leave behind a depression capable of improving scavenging. The short primary lengths also promote early arrival of the reflected wave, which minimizes effective intake and exhaust scavenging.
It's important to note that no header yet produced is optimum for all combinations. The laws of physics dictate otherwise, as the scavenging effect of the headers is initiated by the opening of the exhaust valve, which is also dependent upon the overlap-which is also dependent on the reflected waves produced by the intake design. You can see that this is one fairly complex dynamic system, and that header choice comes down to not only your particular combination, but the point in the rpm curve that you'd like to optimize power production. On any given combination, it's possible to design the headers to improve power down low, in the midrange, and even at high rpm.
Off Came the stock exhaust...
Off Came the stock exhaust manifolds. Though not terrible from a flow standpoint, the cast-iron logs don't provide the scavenging offered by a true, long-tube header design.
The possible exception is the installation of headers in place of stock exhaust manifolds, as the stock manifolds provide almost no scavenging effect due to their excessively short primary length. Even by replacing the stock manifolds with a quality set of long-tube headers, the gains offered have as much to do with the exact combination tested as the design of the headers themselves.
That said, we can now better understand the sizable power gains offered by not only a quality set of headers, such as the ones chosen for this article from American Racing, but also a 3-inch after-cat exhaust system from the exhaust experts at MagnaFlow. While such an exhaust upgrade would be a welcome addition to any modern Mustang, we applied these to what can best be described as a wildly powerful, supercharged '05 GT.
The Baseline testing was run...
The Baseline testing was run using a stock cat pipe with the cats removed (stock pipe shown).
The GT in question was equipped with a 4.6L Three-Valve motor that had been upgraded with a forged reciprocating assembly from Sean Hyland Motorsports, a set of Stage 3 ported heads from Livernois, and matching Stage 3 cams from the Comp Cams catalog. The highlight of the mod motor was the Kenne Bell 2.8L H-series blower. Feeding the blower was a custom 4 1/2-inch MAF and air inlet, a dual 75mm throttle body, and a prototype of the new Mammoth intake manifold from Kenne Bell.
By minimizing inlet restrictions, we were able to coax just over 25 psi from the twin-screw blower. That the '05 GT ripped off plenty of nine-second quarter-mile times after the exhaust upgrade is a testament to the work put into the impressive Three-Valve motor.
With the induction system well sorted, we decided it was time the supercharged beast was allowed to exhale. This provided an excellent opportunity to test the effectiveness of exhaust upgrades on such a high-horsepower Three-Valve combination.
For our test, the supercharged mod motor was equipped with the stock exhaust manifolds feeding the stock cat pipes, after-cat exhaust, and aftermarket mufflers. The cats had long been removed from the cat pipe, while the stock mufflers had been replaced by a set from Jardine. In this configuration, the supercharged mill produced 779 hp and 691 lb-ft of torque.
The stock components were then replaced by the 1 3/4-inch stainless steel headers (with 3-inch merge collectors) from American Racing Headers, along with the 3-inch after-cat system from MagnaFlow.
 Though The factory after-cat...  Though The factory after-cat pipe section flowed well, there was no comparison between the stock exhaust and the MagnaFlow 3-inch system. |  Baseline Runs were made with...  Baseline Runs were made with a set of Jardine mufflers in place of the stockers. |  The Exhaust upgrades included...  The Exhaust upgrades included a set of 1 3/4-inch stainless steel headers equipped with merge collectors from American Racing Headers. The design of the merge collector has a decided effect on power production at specific engine speeds. |