The real reason for the huge...
The real reason for the huge power gains offered by the headers was the wild cam timing. The combination of aggressive specs (lift and duration) and cam settings (significantly advanced) resulted in plenty of cam overlap. It was the overlap that produced dilution of the intake charge with the stock exhaust manifolds. Since there was no overlapping of the ports with the long-tube headers, the power was up significantly.
While the hike in compression was a surefire benefit, the GT1000 motor needed even more power (compared to the standard GT500 motor). With maximizing efficiency in mind, the cylinder heads were treated to full CNC porting. Impressive flowing right off the shelf, the GT500 heads were further improved with a serious complement of valves, springs and retainers to go along with the full CNC porting. Next up came the custom cam profiles that were responsible for the huge gains offered by the headers on this normally aspirated motor. Not only did they sport over 240 degrees of duration (Dynatek Racing was naturally hush-hush about the actual specs), but also more than their fair share of centerline advance.
The cams for the GT1000 motor were installed well advanced of the typical straight-up position. It was this combination of aggressive cam timing and placement (advance) that played havoc with the stock exhaust manifolds. Topping off the normally aspirated version of the GT1000 motor was (appropriately enough) the big daddy of all factory 5.4L intake manifolds, the Cobra R. Truth be told, we had only the bottom half of the Cobra R intake (borrowed from Accufab), which was topped by a fabricated aluminum upper plenum and flange to mount the inlet and throttle body from an '03 supercharged Cobra. It's not nearly as cool as a complete Cobra R, but we knew it had plenty of power potential.
Feeding the heads was a custom...
Feeding the heads was a custom intake combination consisting of a Cobra R lower intake and fabricated upper plenum. The reason for all the monkey motion was that we didn't have access to the upper portion of the Cobra R intake assembly. (They're as rare as hen's teeth.)
Before getting to the impressive dyno results, we need to take a look at header theory, as headers do much more than just provide a flow path from the heads to the exhaust system. The first notion that needs to be dispelled is that power gains offered by headers come from improvements in the flow rate. The reality is that headers improve the power output of a motor through scavenging. We will go into detail on the different types of scavenging, but it's possible for long-tube headers to actually flow less than a set of stock exhaust manifolds and still offer substantial power improvements. From a flow standpoint, the longer the tube, the lower the flow rate. The increase in flow resistance or drop in flow rate comes from the increase in surface area exposed to the air stream.
Based solely on length, the short stock exhaust manifolds may offer improvements in airflow over a long-tube header. As it turns out, the absolute flow rate is also determined by the exit orifice and any irregularities in the internal flow passages. This means that if the exit of the stock exhaust manifold measures just two inches and the long-tube header has a 2½-inch collector, the smaller-diameter exit on the stock exhaust manifold may well be the restriction. This holds true for changes in direction and pinch points in the stock exhaust manifold designed to provide bolt access or gain clearance for some component in the engine compartment.
While absolute flow is important, the real power behind a long-tube header comes from the scavenging effect that not only improves exhaust flow out, but also enhances the flow of the intake tract into the combustion chamber. For our needs, we focus on two distinct forms of scavenging: the kinetic energy of outgoing gases and reflected pressure waves.
 The custom upper intake was...  The custom upper intake was fed by an Accufab inlet and throttle body originally designed for the supercharged '03 Cobra motor. All testing was run using the FAST engine management system and MSD ignition coils. All combinations were optimized in terms of ignition timing and air/fuel ratio. |  To establish a baseline on...  To establish a baseline on this normally aspirated 5.4L, we first ran the motor with the stock GT500 exhaust manifolds breathing through a set of 2.5-inch open pipes. |  Run on the dyno, the 5.4L...  Run on the dyno, the 5.4L produced a disappointing 366 hp and 364 lb-ft of torque. Thinking that there was a problem somewhere with the motor, we performed a compression test, changed the oil (after bleeding down all the lash adjusters--thanks, John Mihovetz), and even ran the motor without the upper plenum on the intake. We ran it as an individual-runner intake. Ultimately, the only thing that cured the low-power blues was swapping on a set of long-tube headers. |
 Rather than just run one set...  Rather than just run one set of headers, we decided to hit the guys at American Racing headers up for three different sizes. In the end, they supplied a set of 1, 1 7/8, and even a set of 2.0-inch headers designed for the 5.4L in the GT500. (The company will not offer the 2.0-inch headers). |  Once installed, the headers...  Once installed, the headers really woke up the 5.4L. By eliminating the reversion and improving scavenging, the 5.4L produced 492 hp at 6,500 rpm and 442 lb-ft at 5,200 rpm. |  Each of the three headers...  Each of the three headers was run with a collector extension. In the case of the 1-inch headers, the merge collector measured 3.0 inches. |