Exhaust manifolds are generally simple cast iron units which collect engine exhaust and deliver it to the exhaust pipe. For many engines after market high performance exhaust headers (also known as extractors in Australia) are available. These headers consist of individual primary tubes for each cylinder, which then usually converege into one tube called a collector. Headers that do not have collectors are called zoomie headers, and are used exclusively on race cars.

The goal of performance exhaust headers is mainly to decrease flow resistance (also know as back pressure), and to increase the volumetric efficiency of an engine, resulting in a gain in power output. The mechanism by which a header does this is called exhaust scavenging. The processes occurring can be explained by the gas laws, specifically the ideal gas law and the combined gas law.

It is a common myth among drag racers and motor-enthusiasts that not enough back pressure in the exhaust will cause a loss of torque. This myth stems from the phenomena associated with a loss of low-end torque when using headers with large primary tubes. Most enthusiasts incorrectly conclude that their restrictive OEM exhaust provided more torque because of the back pressure it creates. The correct reason for the loss in torque is explained below.

The state of an amount of gas is determined by its pressure, volume, and temperature according to the equation:

where

is the absolute pressure,
is the volume of the vessel,
is the number of moles of gas,
is the universal gas constant,
is the absolute temperature.

If we analyze the formula for the ideal gas law, we can easily see that if the volume decreases, and all the variables on the right side of the equation remain constant, that P, pressure must increase to satisfy the equation. Thus, P is inversely proportionate to V. In layman’s terms, the pressure in the combustion chamber will increase if we decrease the volume in the chamber (move the piston towards the cylinder head). This concept is intuitive for most people, however it is important to understand the underlying processes involved.

When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. At some point during the exhaust stroke the exhaust valve will open. The high pressure exhaust gas escapes into the exhaust header, creating an exhaust pulse. An exhaust pulse is a release of exhaust gas, containing three main parts, a high pressure "head", a medium pressure "body" and a low pressure "tail". The high pressure "head" is created from the huge pressure difference between the exhaust in the combustion chamber and the atmospheric pressure outside of the exhaust system. As the exhaust gases equalize between the combustion chamber and the atmosphere, the difference in pressure decreases and the velocity at which the exhaust is leaving the engine decreases. This forms the medium pressure "body" component of the exhaust pulse. The remaining exhaust gases form the "tail" component. This tail component may initially match in pressure to that of the atmosphere, however, the pressure is further reduced by the siphoning effect created by the momentum of the high and medium pressure components. The end result may be a pressure at the low end of the exhaust pulse that is less than the atmospheric pressure. This creates a greater pressure difference between the intake manifold and the combustion chamber, which increases the velocity in which air is brought into the engine. This increase in intake air velocity leads to an increase in the amount of air in the combustion chamber, which allows the engine to add more fuel and thus make more power.

Modern naturally aspirated four-stroke engines usually feature valve-overlap where the benefit of exhaust scavenging is further increased by opening the intake valve while the exhaust valve is also open. This overlap helps purge the combustion chamber of any remaining exhaust gas, and may allow a small amount of intake air to escape out the exhaust port.

The magnitude of the exhaust scavenging effect is a direct function of the velocity of the high and medium pressure components of the exhaust pulse. Performance headers work to increase the exhaust velocity as much as possible. One technique is tuned length primary tubes. This technique attempts to time the occurrence of each exhaust pulse, to occur one after the other in succession while still in the exhaust system. The lower pressure tail of an exhaust pulse then serves to create a greater pressure difference between the high pressure head of the next exhaust pulse, thus increasing the velocity of that exhaust pulse. In V6 and V8 engines where there is more than one exhaust bank, Y-pipes and X-pipes work on the same principle of using the low pressure component of an exhaust pulse to increase the velocity of the next exhaust pulse.

Great care must be used when selecting the length and diameter of the primary tubes. Tubes that are too large will cause the exhaust gas to expand and slow down, decreasing the scavenging effect. Tubes that are too small will require additional force to expel the exhaust gas from the chamber, causing unneeded labor on the engine and ultimately a loss of power. This is true for all parts of the exhaust system. In competitive environments it’s often required to select the header based on the specific application of the engine. Since engines produce more exhaust gas at higher RPMs the header will respond differently across the RPM range. Typically, large primary tubes offer the best gains in power and torque at higher RPMs, while smaller tubes offer the best gains at lower RPMs. Many people who put race headers on their vehicle experience a noticeable low-end torque loss. This is a result of insufficient exhaust gas output at lower RPMs. The exhaust expands once it enters the primary tube and slows down, reducing the scavenging effect. Many automotive mechanics and enthusiasts erroneously conclude the loss in torque was due to a lack of back pressure, when in fact the real cause was the expansion of the exhaust and resulting decrease in velocity. Despite the low-end torque loss, at higher RPMs the engine will produce more power and in race situations, the vehicle should be faster.

Many headers are also resonance tuned, to utilize the low-pressure reflected wave rarefaction pulse which can help scavenging during valve overlap. This pulse is created in all exhaust systems each time a change in density occurs, such as when exhaust merges into the collector. For clarification, the rarefaction pulse is the technical term for the same process that was described above in the "head, body, tail" description. By tuning the length of the primary tubes, usually by means of resonance tuning, the rarefaction pulse can be timed to coincide with the exact moment valve overlap occurs.

Some modern exhaust headers are available with a ceramic coating. This coating serves to prohibit rust and to reduce the amount of heat radiated into the engine bay. The heat reduction will help prevent intake manifold heat soak, which will decrease the temperature of the air entering the engine.

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