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Exhaust manifold
Exhaust manifold
Exhaust manifold
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Diagram of an exhaust manifold from a Kia Rio. 1. manifold; 2. gasket; 3. nut; 4. heat shield; 5. heat shield bolt
Ceramic-coated exhaust manifold on the side of a performance car

In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald (from the Anglo-Saxon manig [many] and feald [fold])[1] and refers to the folding together of multiple inputs and outputs (in contrast, an inlet or intake manifold supplies air to the cylinders).

Exhaust manifolds are generally simple cast iron or stainless steel[2] units which collect engine exhaust gas from multiple cylinders and deliver it to the exhaust pipe. For many engines, there are aftermarket tubular exhaust manifolds known as headers in American English, as extractor manifolds in British and Australian English,[3] and simply as "tubular manifolds" in British English.[citation needed] These consist of individual exhaust headpipes for each cylinder, which then usually converge into one tube called a collector. Headers that do not have collectors are called zoomie headers.

The most common types of aftermarket headers are made of mild steel or stainless steel tubing for the primary tubes along with flat flanges and possibly a larger diameter collector made of a similar material as the primaries. They may be coated with a ceramic-type finish (sometimes both inside and outside), or painted with a heat-resistant finish, or bare. Chrome plated headers are available but these tend to blue after use. Polished stainless steel will also color (usually a yellow tint), but less than chrome in most cases.

Another form of modification used is to insulate a standard or aftermarket manifold. This decreases the amount of heat given off into the engine bay, therefore reducing the intake manifold temperature. There are a few types of thermal insulation but three are particularly common:

  • Ceramic paint is sprayed or brushed onto the manifold and then cured in an oven. These are usually thin, so have little insulatory properties; however, they reduce engine bay heating by lessening the heat output via radiation.
  • A ceramic mixture is bonded to the manifold via thermal spraying to give a tough ceramic coating with very good thermal insulation. This is often used on performance production cars and track-only racers.
  • Exhaust wrap is wrapped completely around the manifold. Although this is cheap and fairly simple, it can lead to premature degradation of the manifold.

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

Exhaust scavenging

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Cut through a 2-1 junction in an exhaust manifold showing pressure, which is nonhomogeneous due to centripetal forces, and flow.

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" comprising three main parts:

  1. 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
  2. As the exhaust gases equalize between the combustion chamber and the atmosphere, the difference in pressure decreases and the exhaust velocity decreases. This forms the medium-pressure body component of the exhaust pulse
  3. 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 induct the intake charge during the overlap period when both intake and exhaust valves are partially open. The effect is known as "scavenging". Length, cross-sectional area, and shaping of the exhaust ports and pipeworks influences the degree of scavenging effect, and the engine speed range over which scavenging occurs.[4]

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.[4] 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 range according to the intended application. Typically, wide primary tubes offer the best gains in power and torque at higher engine speeds, while narrow tubes offer the best gains at lower speeds.

Many headers are also resonance tuned, to utilize the low-pressure reflected wave rarefaction pulse which can help scavenging the combustion chamber 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. Typically, long primary tubes resonate at a lower engine speed than short primary tubes.

Why a cross plane V8 needs an H or X exhaust pipe

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Crossplane V8 engines have a left and right bank each containing 4 cylinders. When the engine is running, pistons are firing according to the engine firing order. If a bank has two consecutive piston firings it will create a high pressure area in the exhaust pipe, because two exhaust pulses are moving through it close in time. As the two pulses move in the exhaust pipe they should encounter either an X or H pipe. When they encounter the pipe, part of the pulse diverts into the X-H pipe which lowers the total pressure by a small amount. The reason for this decrease in pressure is that the fluid (liquid, air or gas) will travel along a pipe and when it comes at a crossing the fluid will take the path of least resistance and some will bleed off, thus lowering the pressure slightly. Without an X-H pipe the flow of exhaust would be jerky or inconsistent, and the engine would not run at its highest efficiency. The double exhaust pulse would cause part of the next exhaust pulse in that bank to not exit that cylinder completely and cause either a detonation (because of a lean air–fuel ratio (AFR)), or a misfire due to a rich AFR, depending on how much of the double pulse was left and what the mixture of that pulse was.[5]

Dynamic exhaust geometry

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Today's understanding of exhaust systems and fluid dynamics has given rise to a number of mechanical improvements. One such improvement can be seen in the exhaust ultimate power valve ("EXUP") fitted to some Yamaha motorcycles. It constantly adjusts the back pressure within the collector of the exhaust system to enhance pressure wave formation as a function of engine speed. This ensures good low to mid-range performance.

At low engine speeds the wave pressure within the pipe network is low. A full oscillation of the Helmholtz resonance occurs before the exhaust valve is closed, and to increase low-speed torque, large amplitude exhaust pressure waves are artificially induced. This is achieved by partial closing of an internal valve within the exhaust—the EXUP valve—at the point where the four primary pipes from the cylinders join. This junction point essentially behaves as an artificial atmosphere, hence the alteration of the pressure at this point controls the behavior of reflected waves at this sudden increase in area discontinuity. Closing the valve increases the local pressure, thus inducing the formation of larger amplitude negative reflected expansion waves. This enhances low speed torque up to a speed at which the loss due to increased back pressure outweighs the EXUP tuning effect. At higher speeds the EXUP valve is fully opened and the exhaust is allowed to flow freely.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Exhaust manifold

View on Grokipedia
from Grokipedia
An exhaust manifold is a critical component of an internal combustion engine's , designed to collect hot exhaust gases expelled from multiple cylinders and channel them into a single outlet pipe for expulsion from the vehicle. This process helps manage backpressure, ensuring efficient engine operation while directing gases toward downstream components like the . Typically constructed from durable, heat-resistant materials such as or , exhaust manifolds must withstand extreme temperatures up to 800°C (1,472°F) and thermal cycling without warping or cracking. variants, common in stock automotive applications, offer robust durability and cost-effectiveness but may contribute to higher backpressure due to their integrated, siamesed runner design. In contrast, tubular headers—made from stainless or mild —feature separate, equal-length tubes that merge into a collector, reducing exhaust interference and improving scavenging for enhanced performance, particularly in high-output or setups. These designs play a key role in optimizing , emissions control, and across and diesel engines. Beyond basic functionality, exhaust manifolds influence overall acoustics and efficiency; for instance, their geometry affects pulse tuning, where overlapping exhaust pulses aid in drawing out residual gases from cylinders. Common failure modes include leaks from mismatches or , which can lead to reduced power, increased emissions, and audible ticking sounds under load. Modern advancements, such as integrated exhaust manifold designs in turbocharged engines, further compact the system while improving heat retention for faster turbo spooling.

Fundamentals

Definition and Purpose

An exhaust manifold is a pipe-like structure attached to the that gathers exhaust gases from multiple cylinders and channels them into a single outlet for the . This component serves as the initial segment of the vehicle's , funneling byproducts away from the engine cylinders. The primary purposes of the exhaust manifold include the efficient expulsion of byproducts to prevent into the cylinders, thereby enabling proper by reducing backpressure. It also acts as the critical interface between the and downstream exhaust components, such as catalytic converters, directing gases for further treatment and expulsion. Exhaust manifolds were first used in early internal combustion engines around the late , coinciding with the development of multi-cylinder designs to simplify exhaust routing compared to individual cylinder vents. Key benefits include reducing engine heat buildup in the by promptly removing hot gases and minimizing space requirements under the hood through consolidated piping. Manifold design can further enable performance enhancements like exhaust scavenging to improve gas evacuation.

Basic Operation

During the exhaust stroke of a four-stroke internal combustion engine, the piston moves upward toward the cylinder head, compressing the residual combustion gases and forcing them out through the open exhaust valve. This valve opening, timed by the camshaft, allows high-pressure exhaust gases from each cylinder to enter the corresponding runner of the exhaust manifold, which is directly bolted to the cylinder head's exhaust ports. The runners, individual passages for each cylinder, initially channel these hot, pressurized gases away from the combustion chamber to prevent backflow into the cylinder. The runners of the exhaust manifold converge at a central collector, where exhaust pulses from multiple merge into a unified flow. This merging creates initial backpressure within the manifold, which helps seal the against the cylinder pressure differential and supports optimal valve overlap timing during the engine cycle transition to . Pulse tuning in the manifold design influences how these gas pulses combine, promoting smoother overall expulsion without delving into advanced effects. From the collector, the combined exhaust gases flow downstream through the outlet into the exhaust pipe, carrying significant heat (up to 650°C) and particulates toward subsequent components like the for emission treatment. Throughout this process, pressure dynamics in the manifold start at typical operating levels of 1-2 bar during the exhaust stroke, gradually dropping as the gases expand, cool, and encounter system resistance. This ensures efficient evacuation while minimizing energy loss from the engine.

Design Types

Log-style Manifolds

The log-style exhaust manifold features a simple, compact design consisting of individual short runners from each that merge into a single, undivided collector pipe, often curved to resemble a log in shape. This configuration collects exhaust gases from all cylinders into a common plenum, prioritizing ease of integration over complex flow paths. Key advantages of log-style manifolds include their low , compact that fits tightly within bays, and straightforward installation, making them ideal for applications emphasizing reliability and space efficiency. They also provide adequate low-RPM characteristics in stock setups by maintaining sufficient backpressure for everyday operation. However, these manifolds suffer from disadvantages at higher engine speeds, where uneven exhaust pulse lengths lead to turbulence and backflow in the shared collector, restricting overall flow efficiency. Additionally, the retains more in the common pipe, which can exacerbate and limit performance in demanding conditions compared to more advanced tubular alternatives. Log-style manifolds have been widely applied in production vehicles and economy cars since the early , serving as the standard for factory engines where cost and durability outweighed high-performance needs. They remain common in OEM turbocharged systems for their robustness in everyday automotive use.

Tubular Manifolds

Tubular manifolds consist of a series of individual, equal-length tubes that connect each exhaust port to a common collector, typically formed using mandrel-bending techniques on tubing to maintain smooth, unrestricted flow paths and allow isolated travel of exhaust pulses from each . This design contrasts with more compact log-style manifolds by prioritizing separate runner paths, which minimize interference between exhaust pulses and promote efficient gas evacuation. The primary advantages of tubular manifolds include enhanced high-RPM power output due to improved separation, which reduces backflow and interference, allowing for better scavenging and higher at elevated engine speeds. They are particularly favored in applications and aftermarket upgrades, where dyno testing has shown gains of up to 30 ft-lbs in the (2000-4500 RPM) on engines like the 3.0L, alongside overall horsepower increases from optimized flow. Despite these benefits, tubular manifolds present disadvantages such as higher manufacturing costs from specialized fabrication processes like bending and , larger physical size that complicates packaging under the , and increased installation complexity compared to cast alternatives. In applications, tubular manifolds are commonly employed in modern sports cars and performance engines, including models from the 1980s onward, where equal-length designs like those from SSI have been adapted for both competition and street use to support the flat-six engine's high-revving characteristics. They also contribute briefly to pulse tuning in V8 configurations by enabling tuned runner lengths for effects.

Cast versus Fabricated Construction

Exhaust manifolds can be produced through construction, where molten metal is poured into a mold to form a single-piece component. This method typically involves , which uses a for creating complex shapes, or , which employs a reusable metal mold under pressure for higher precision and volume production. In contrast, fabricated construction assembles the manifold from pre-formed components, such as tubes and collectors, joined by or bolting. This approach often utilizes computer (CNC) bending to shape tubes into custom configurations, enabling greater design flexibility for specific engine layouts. Cast manifolds provide enhanced durability and resistance to leaks due to their monolithic structure, which minimizes joints and potential failure points, though they are constrained by the mold's in achieving intricate or optimized shapes. Fabricated manifolds, however, allow for lighter weight and tailored flow paths through precise tube routing, but they are susceptible to weld failures under thermal cycling and . Historically, construction dominated exhaust manifold production through the mid-20th century, particularly until the , owing to its suitability for mass manufacturing. Fabricated methods gained prominence in the with the rise of aftermarket tuning and stricter emissions regulations, which demanded adaptable designs for integration and mounting. This evolution reflects a broader industry shift toward performance-oriented and compliant systems, with designs still common for log-style manifolds due to their straightforward molding.

Performance Considerations

Exhaust Scavenging

Exhaust scavenging in the context of exhaust manifolds refers to the dynamic process in multi-cylinder internal combustion engines where high-velocity exhaust pulses from one cylinder generate low-pressure regions that assist in evacuating residual gases from adjacent cylinders and enhancing the of fresh air-fuel mixture. This phenomenon is particularly prominent in four-stroke engines during the valve overlap period, when both and exhaust are open, allowing pressure differences to influence efficiency. The mechanism relies on the and pressure waves propagating through the exhaust manifold. As exhaust gases exit a at high during the blowdown phase—shortly after exhaust valve opening—they create a wave or negative pressure pulse that travels along the manifold runners. In tuned designs, such as those with equal-length tubular runners, this pulse arrives at the exhaust port of a neighboring at the optimal time, reducing backpressure and promoting outflow of exhaust gases while drawing in charge. For instance, in a four- with a of 1-3-4-2, 1 and 4 (or 2 and 3) can be paired in the manifold to synchronize pulses, amplifying the scavenging effect at specific speeds. Effective scavenging significantly improves by minimizing residual gas fractions in the cylinder, which can otherwise dilute the fresh charge and reduce combustion temperatures. Studies on supercharged four-cylinder engines demonstrate that tuned two-pulse exhaust systems—pairing cylinders to exploit pulse interference—can halve residual gas levels compared to a single common manifold, leading to up to 5-10% higher at mid-range speeds and reduced pumping losses. This enhancement supports engine downsizing in turbocharged applications, where positive intake-to-exhaust pressure differences further boost scavenging, contributing to 2-6% improvements in fuel economy when integrated with direct injection. Manifold design plays a critical role in optimizing scavenging, with trade-offs between broadband performance and narrow-band tuning. Log-style manifolds provide averaging of pulses for consistent low-speed but limit peak scavenging benefits, while fabricated tubular manifolds with precisely calculated runner lengths (often 0.5-1.0 meters for mid-range RPM) maximize pulse tuning, though they may increase backpressure at off-design speeds. In turbocharged setups, scavenging also aids spool-up by maintaining high exhaust velocities, but excessive tuning can elevate emissions due to leaner mixtures from improved charge quality. Overall, scavenging underscores the exhaust manifold's role beyond mere collection, transforming it into an active component for breathing efficiency.

Pulse Tuning and Backpressure

Pulse tuning in exhaust manifolds involves the strategic arrangement of runner lengths to synchronize exhaust waves, enabling their reflection back toward the cylinders at specific speeds to create effects that enhance gas evacuation. By timing these waves to coincide with valve overlap periods, the negative from reflected expansion waves assists in drawing out residual exhaust gases and drawing in fresh charge, optimizing across targeted RPM ranges. For instance, in inline-four engines, a 4-2-1 merging configuration pairs cylinders according to the (typically 1-3-4-2), where initial primaries from cylinders 1 and 4 (or similar pairings) merge into secondaries, and then into a single collector; this design promotes pulse separation and recombination to amplify mid-range performance without excessive interference. Backpressure plays a critical role in balancing exhaust flow dynamics, where moderate resistance—typically 0.01 to 0.1 bar—helps prevent over-scavenging at low RPM by maintaining sufficient velocity to avoid excessive fresh charge loss during valve overlap, thereby preserving low-end torque. This controlled restriction ensures that the exhaust system's inherent impedance supports pulse propagation without allowing unrestricted flow that could dilute cylinder filling at or part-throttle conditions. The propagation speed of these pressure waves in the , approximating the in an , is given by the equation: c=γRTMc = \sqrt{\frac{\gamma R T}{M}}
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