Switching from stock to performance headers:
Switching from a stock exhaust manifold to performance headers (sometimes called extractors) helps to optimize exhaust gas flow and in turn increases the high-end power of the engine. By using an individual pipe for each exhaust port that has smoother bends with a larger turn radius, airflow resistance is decreased and a calculated pipe diameter is also used to cause a “scavenging” effect for the specific engine performance requirements. The pipes merge together into a collector and then flow into a larger down pipe, just before the catalytic converter.
Stock exhaust manifolds are generally made of simple cast iron or stainless steel. These units collect engine exhaust from multiple cylinders and deliver it to the exhaust pipe. Aftermarket high performance exhaust headers are available for most makes and models. These differ because instead of depositing exhaust gases directly into the collector they are constructed from individual exhaust head pipes for each cylinder. These tubes then converge into one tube called a collector. Headers are sometimes built without a collector for racing or supercharged applications. These headers without collectors are called “zoomie headers”.
How do headers work and what are they for?
The ultimate goal of performance exhaust headers is decreasing flow resistance or back pressure, and increasing the volumetric efficiency of an engine. This results in increased power output. As each exhaust valve in the head of the engine is opened and exhaust gas is forced out, it passes down one of the header pipes and through to the down pipe, where the exhaust gas velocity causes a slight vacuum in another header pipe. This effect pulls exhaust gasses from the cylinder under vacuum, therefore increasing the efficiency that gases can enter the exhaust system ultimately aiding engine performance. This effect is known as scavenging. When coupled with a cam upgrade that includes some overlap, the scavenging effect pulls air through from the intake manifold as well, further increasing performance. Using this effect to its best potential for the driver’s habits or desired peak performance rpm range, headers can be “tuned”. This means that the length of exhaust headers are optimized and tested to provide the best performance gains under a given set of conditions. Stock exhaust manifolds are typically made of cast iron and are not tuned.
What is the benefit of header coatings?
Headers can also be ceramic coated to reduce heat radiation into the engine compartment, and to increase the heat inside the downpipes. This retained heat increases the velocity of the exhaust gases through the tubes.
Three Common types of thermal insulation:
- Ceramic Paint - sprayed or brushed onto the manifold and then cured in an oven. These coatings are usually thin therefore little insulation is provided, however they significantly reduce engine bay heating by lessening the heat output via radiation.
- Ceramic Mixture Coating - bonded to the manifold via thermal spraying to give a thicker and tougher ceramic coating with very good thermal insulation. This is often used on performance production cars and track-only racers.
- Exhaust Wrap - wrapped completely around the manifold. Although this is cheap and fairly simple, it can lead to premature degradation of the manifold due to trapped moisture or abrasions caused by wrap slippage.
What are headers made of?
The most common types of aftermarket headers are made of either ceramic, or stainless steel. Ceramic headers are lighter in weight than stainless steel; however, under extreme temperatures or off road driving they can crack. Some headers offer a ceramic coating over stainless steel construction. This coating is heavier than either alone but offers increased performance due to higher temperatures inside of the downpipe. Another option offered is insulation. This can be added to a standard or aftermarket manifold or a set of headers. Much like the ceramic coating, this decreases the amount of heat given off into the engine bay, therefore reducing under hood temperatures.
Will performance headers make my exhaust system louder?
Headers may or may not increase the dB level of the exhaust. This depends on the original exhaust manifold being replaced and the engine. The most common sound effect of header pipes is "header ring". When the exhaust pulses, a revving sound can sometime occur increasing under hood noise.
Header Tuning and Exhaust Scavenging:
When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. When the exhaust valve opens, the high pressure exhaust gas escapes into the exhaust manifold or header, creating an exhaust pulse comprised of three main parts:
- The high-pressure head is created by the large pressure difference between the exhaust in the combustion chamber and the atmospheric pressure outside of the exhaust system.
- The medium-pressure body component of the exhaust pulse. As the exhaust gases equalize between the combustion chamber and the atmosphere, the difference in pressure decreases and the exhaust velocity decreases.
- The remaining exhaust gas forms the low-pressure tail component. This tail component may initially match ambient atmospheric pressure, but the momentum of the high- and medium- pressure components reduces the pressure in the combustion chamber to a lower-than-atmospheric level. This relatively low pressure helps to extract all the combustion products from the cylinder and pull the intake charge during the overlap period when both intake and exhaust valves are partially open.
The effect created when the three parts are optimized with the use of a “tuned Header” is known as scavenging. Length, cross-sectional area, and shaping of the exhaust ports and pipes influences the degree of scavenging effect, and the engine speed range over which scavenging occurs.
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 employed to scavenge power from an exhaust system 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 creates a greater pressure difference between the high pressure head of the next exhaust pulse 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.
When selecting the length and diameter of the primary tubes you must know the application well. 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 create exhaust flow resistance which the engine must work to expel the exhaust gas from the chamber, reducing power and leaving exhaust in the chamber to dilute the incoming intake charge. Since engines produce more exhaust gas at higher speeds, the header(s) are tuned to a particular engine speed or rpm range according to the intended application. Typically, wider or larger diameter primary tubes offer the best gains in power and torque at higher engine speeds, while narrower or smaller diameter tubes offer the best gains at lower speeds.
Exhaust system tuning:
Many headers are also resonance or length tuned. Typically, longer primary tubes resonate at a lower engine speeds than shorter primary tubes. This means that shorter headers produce more top-end horsepower while longer tubes produce increased low-end torque gains. This type of tuning utilizes the low-pressure pulse which can help scavenging the combustion chamber during valve opening overlap. This pulse is created in all exhaust systems each time a change in air density occurs, such as when exhaust merges into the collector. By tuning the length of the primary tubes, the low pressure pulse can be timed to coincide with the exact moment valve overlap occurs.
Many automotive companies offer aftermarket exhaust system upgrades as a subcategory of engine tuning. These upgrades can significantly improve engine performance and do this through means of two main principles:
- By reducing the exhaust back pressure, engine power is increased.
- By reducing the amount of heat from the exhaust being lost into the under hood area. This reduces the under hood temperature and consequently lowers the intake manifold temperature, increasing power. This also has positive side effect of preventing heat-sensitive components from being damaged. Furthermore, keeping the heat in the exhaust gases speeds these up, therefore reducing back pressure as well.
Tuning the pipe:
Because a tuned pipe cannot be effective over the full spectrum of the RPM, it has to be "tuned" for a certain RPM range. Usually, high performance headers are tuned for the lower RPM because that's where most engines have weakest horse power and torque. By adjusting the exhaust pipe's total length the engine can be "tuned” at the pipe. During a testing or development phase most tuned headers are cut at the coupler that connects exhaust manifold and tuned pipe little by little and test driven until improvements are seen at desirable RPM band. Generally longer pipe moves the effective band of tuned pipe to lower RPM range, and shorter length moves the effectiveness band to higher RPM.
Definitions and Terminology:
- 4-2-1 Headers or Tri-Y’s - On V-8 engines where 4 pipes merge into 2 then merge into 1. Different opinions as to which cylinders in the firing order are paired to provide desired performance. In a V8 the pairing tends to join cylinders with similar firing intervals, and this will differ between left and right cylinder banks.
- 4 into 1 Headers - 4 pipes merge directly into 1
- Shorty Headers - This is basically an exhaust manifold that is fabricated from tubes instead of stamped or cast iron.
- Equal Length Headers - Each downpipe is the same length from the exhaust port to the collector cup. These are usually used when the cat-back exhaust system is resonance tuned.
- Zoomie Headers - Built without a collector for supercharged and racing applications.