2005 Mustang GT American Racing Headers Install

In truth, this test involved both headers and an after-cat exhaust, but the significant midrange power gains came from the scavenging effect of the long-tube headers. Unlike air filters or throttle bodies that allow enough air past or not, cams, intakes, and headers have a decided tuning effect on the power curve. Camshaft timing dictates at which engine speed the motor will be most efficient, with higher-duration cams dictating higher engine speeds. Intake manifolds work much like long-tube headers in that longer runner lengths (primary lengths on the header) are optimized for lower engine speeds, while shorter lengths promote power higher in the rev range.

The same can be said of runner diameter (or cross section), as larger runners (or primary diameter pipes for headers) will increase the optimum engine speed. That is to say, a 2-inch by 34-inch primary header pipe will be optimized at a higher engine speed than a pipe that measures 1.75 inches by 34 inches. This tuning effect has nothing to do with the actual flow rate, as the diameter and length determine the travel speed of the resonance waves. Primary pipe length and diameter are but two of the many variables that can affect the performance of a set of headers.

Before getting to the test, perhaps a brief explanation of how this resonance occurs will shed some light on how difficult it is to produce the proverbial "ideal" or "best" set of headers for any given combination.

More than simple exhaust flow, true headers promote power production through two effective means of scavenging. In simplified terms, scavenging occurs when both the intake and exhaust valves are open (a position referred to as camshaft overlap). The outgoing exhaust flow helps draw in intake mixture by creating a low-pressure zone in the combustion chamber. This scavenging effect helps introduce more intake air and fuel mix, which allows the motor to make more power.

And this relatively simple scavenging effect is accomplished through two somewhat sophisticated mechanisms-the first being the kinetic energy of the outgoing gases. Since we lack the space for a detailed description of exhaust theory, we will have to revert to the Reader's Digest version. The opening of the exhaust valve produces a compression pulse. The release of this compression pulse creates high pressure in front of the wave but a depression on the backside of the wave. Since the speed of this wave exceeds that of the exhaust gas flow through the pipe, the depression or low-pressure zone produces a scavenging effect. This helps rid the combustion chamber of residual exhaust gases and, in turn, helps pull a fresh air/fuel charge in from the induction system.

The second method of scavenging produced by the long-tube header is called reflected wave scavenging. Once the pressure pulse has been released by the opening of the exhaust valve, the wave travels the length of the exhaust pipe. Upon reaching the end of the pipe (typically the collector), something magical happens. The positive pressure wave is allowed to expand into the relatively larger collector. This expansion causes a momentary drop in density of the air surrounding the end of the primary pipe. The elasticity of the air causes it to rebound toward the pipe exit. This creates a new negative pressure wave that then travels back up the primary pipe to the awaiting exhaust port. This reflection of the positive and negative pressure waves continues indefinitely, though the waves decrease in amplitude (or effective strength). For optimum performance, the exhaust-pipe length should be selected to produce the primary (first order) reflected (negative) pressure wave (at its lowest pressure) when the piston just passes TDC at the end of the exhaust stroke.