Basic concepts you should know when considering a new set of headers or evaluating your existing headers.

HOW HEADERS WORK
Headers are much more than just suitably sized exhaust pipes attached to your engine. Headers are an integral part of your engine. The significant energy remaining in the exhaust gas in the cylinder after the power stroke can be used to increase engine power and efficiency by:
Minimizing the contamination of the intake charge with exhaust gases.
Starting the flow of induction during the valve overlap period.
Minimizing pumping losses on the exhaust stroke.

The header performs its function by:
Utilizing the compression wave that is created when exhaust pressure in the cylinder is blown down after the exhaust valve opens. This compression wave travels through the exhaust gas remaining in the primary header pipe, and accelerates the exhaust gas toward the collector.
Passing this strong compression wave through an expansion chamber (collector) located at the end of the primary header pipe. This produces a strong suction wave. This suction wave travels back up the header pipe toward the open exhaust valve, accelerating the fresh exhaust gas in the pipe toward the collector.
Timing this reflected suction wave (scavenging wave) to arrive back at the exhaust valve during the valve overlap period, when both the exhaust and intake valves are off their seats.

For the header to function properly it must have the following characteristics:
Primary header pipes must be small enough to maintain the strength of the compression waves and suction waves in the pipes.
Primary header pipes must be large enough to allow optimal flow of gases past the exhaust valve, and down the header pipes.
Primary header pipes must be of proper length to ensure correct arrival time of the scavenging wave that will be reflected back to the exhaust valve.
Collector must be of proper diameter and length to provide the correct duration and intensity of the scavenging wave.
Primary header pipes and collectors must produce a scavenging wave that is present at the exhaust valve during the valve overlap period over an established operating RPM range for the engine.
The basic design philosophy is to create a collected header that produces the best torque and horsepower from your engine over its operating RPM range. The primary header pipes and collector dimensions can be fine-tuned to optimize the portion of the operating RPM range that is most important to you. The following paragraphs will help you understand how headers influence engine performance at various RPM levels.

At very low RPM the first returning scavenging wave in the header pipe will arrive far too early to provide scavenging during valve overlap. In fact the first scavenging wave will be followed by a compression wave, and possibly another scavenging wave and compression wave before the valve overlap period. Since low RPM engine operation tends to be dominated by very light throttle application, these waves in the exhaust are weak and have only minor influence on engine operation. Low RPM performance is influenced primarily by intake valve closing time, compression ratio, valve overlap duration, and to a lesser extent exhaust valve opening time.

A performance engine’s operating range begins when the engine first starts to make good torque under full-throttle application. This is usually at an RPM that is about half of your peak horsepower RPM, and marks the beginning of the midrange torque band. At this RPM the scavenging waves in the header pipes may still be arriving a little prematurely. This will cause the piston to be literally sucked up the bore on the exhaust stroke, and will reduce the engine’s pumping losses to essentially zero. But the compression wave that follows the first scavenging wave in the header pipe may arrive at the exhaust valve during valve overlap. This compression wave has the potential to spoil the intake charge by injecting exhaust gas during the valve overlap period.

The combination of properly sized primary header pipes and collector work together to bring the torque curve in as early as possible, dampen the strength of the following compression wave, and keep exhaust velocity high to eliminate reverse flow.

As your engine is accelerated through the middle of its midrange torque band, the header will fully “tune-in” and provide strong scavenging before and during the valve overlap period. The scavenging wave from the header may actually drop the cylinder pressure down to half of atmospheric pressure. Intake charge can be drawn past the intake valve during valve overlap even though the piston is still moving up the bore on the exhaust stroke. The combustion chamber is thoroughly scavenged of exhaust, which increases engine efficiency and power. Flow of induction down the intake manifold runner is maintained by the powerful scavenging wave from the header during the valve overlap period. As the exhaust valve closes, the piston begins to speed down the bore on the intake stroke, continuing the induction process. Use a Performance Factor of 4 through 7 to optimize the tune of the header in your engine’s midrange.

Torque will decrease as the engine reaches its peak horsepower RPM. The scavenging wave in the header will also arrive back at the exhaust valve with little time to spare before intake valve opening. The strength of the scavenging wave will be at its maximum just before the exhaust valve closes. The scavenging wave is very strong at high RPM, and helps draw the intake charge across the intake valve. A Performance Factor of 9 or 10 can be selected to allow the engine to make good horsepower well past the peak horsepower RPM. These settings are for all-out drag racing, and can be used to intentionally kill-off mid-range torque to a small degree.

Regardless of the performance level of your engine, you should calculate the size and length of both your primary header pipes and collector. You should never guess at header sizes. Your engine will only operate well when it is equipped with a properly sized header and collector combination.