Auto Exhaust - Blog

Your car’s exhaust system carries away the gases created when the fuel and air are burned in the combustion chamber. These gases are harmful to humans and our environment. Frequent checks of your exhaust system is a must to provide for you and your family’s safety. Make sure there are no holes in the exhaust system or in the passenger compartment where exhaust fumes could enter. Let’s begin by listing the parts of the exhaust system and their functions.

Exhaust manifold:
The exhaust manifold attaches to the cylinder head and takes each cylinders exhaust and combines it into one pipe. The manifold can be made of steel, aluminum, stainless steel, or more commonly cast iron.

Oxygen sensor:
All modern fuel injected cars utilize an oxygen sensor to measure how much oxygen is present in the exhaust. From this the computer can add or subtract fuel to obtain the correct mixture for maximum fuel economy. The oxygen sensor is mounted in the exhaust manifold or close to it in the exhaust pipe.

Catalytic converter: This muffler like part converts harmful carbon monoxide and hydrocarbons to water vapor and carbon dioxide. Some converters also reduce harmful nitrogen oxides. The converter is mounted between the exhaust manifold and the muffler.

Muffler:
The muffler serves to quiet the exhaust down to acceptable levels. Remember that the combustion process is a series of explosions that create allot of noise. Most mufflers use baffles to bounce the exhaust around dissipating the energy and quieting the noise. Some mufflers also use fiberglass packing which absorbs the sound energy as the gases flow through.

Exhaust pipe: Between all of the above mention parts is the exhaust pipe which carries the gas through it’s journey out your tail pipe. Exhaust tubing is usually made of steel but can be stainless steel (which lasts longer due to it’s corrosion resistance) or aluminized steel tubing. Aluminized steel has better corrosion resistance than plain steel but not better than stainless steel. It is however cheaper than stainless steel.

The primary function of the intake manifold is to evenly distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s). Even distribution is important to optimize the efficiency and performance of the engine. It may also serve as a mount for the carburetor, throttle body, fuel injectors and other components of the engine.

Due to the downward movement of the pistons and the restriction caused by the throttle valve, in a reciprocating spark ignition piston engine, a partial vacuum (lower than atmospheric pressure) exists in the intake manifold. This manifold vacuum can be substantial, and can be used as a source of automobile ancillary power to drive auxiliary systems: ignition advance, power assisted brakes, cruise control, windshield wipers, power windows, ventilation system valves, etc.

This vacuum can also be used to draw any piston blow-by gases from the engine’s crankcase. This is known as a closed crankcase ventilation or positive crankcase ventilation (PCV) system. This way the gases are burned with the fuel/air mixture.

The intake manifold has historically been manufactured from aluminum or cast iron but use of composite plastic materials is gaining popularity (e.g. most Chrysler 4 cylinders, Ford Zetec 2.0, Duratec 2.0 and 2.3, and GM’s Ecotec series).

Turbulence
The carburetor or the fuel injectors spray fuel droplets into the air in the manifold. Due to electrostatic forces some of the fuel will form into pools along the walls of the manifold, or may converge into larger droplets in the air. Both actions are undesirable because they create inconsistencies in the air-fuel ratio. Turbulence in the intake causes forces of uneven proportions in varying vectors to be applied to the fuel, aiding in atomization. Better atomization allows for a more complete burn of all the fuel and helps reduce engine knock by enlarging the flame front. To achieve this turbulence it is a common practice to leave the surfaces of the intake and intake ports in the cylinder head rough and unpolished.

Only a certain degree of turbulence is useful in the intake. Once the fuel is sufficiently atomized additional turbulence causes unneeded pressure drops and a drop in engine performance.

Volumetric efficiency
The design and orientation of the intake manifold is a major factor in the volumetric efficiency of an engine. Abrupt contour changes provoke pressure drops, resulting in less air (and/or fuel) entering the combustion chamber; high-performance manifolds have smooth contours and gradual transitions between adjacent segments.

Modern intake manifolds usually employ runners, individual tubes extending to each intake port on the cylinder head. The purpose of the runner is to take advantage of the Helmholtz resonance property of air. Air flows at considerable speed through the open valve. When the valve closes, the air that has not yet entered the valve still has a lot of momentum and compresses against the valve, creating a pocket of high pressure. This high-pressure air begins to equalize with lower-pressure air in the manifold. Due to the air’s inertia, the equalization will tend to oscillate: At first the air in the runner will be at a lower pressure than the manifold. The air in the manifold then tries to equalize back into the runner, and the oscillation repeats. This process occurs at the speed of sound, and in most manifold travels up and down the runner many times before the valve opens again.

The smaller the cross-sectional area of the runner, the higher the pressure changes on resonance for a given airflow. This aspect of Helmholz resonance reproduces one result of the Venturi effect. When the piston accelerates downwards, the pressure at the output of the intake runner is reduced. This low pressure pulse runs to the input end, where it is converted into an over-pressure pulse. This pulse travels back through the runner and rams air through the valve. The valve then closes.

To harness the full power of the Helmholtz resonance effect, the opening of the intake valve must be timed correctly, otherwise the pulse could have a negative effect. This poses a very difficult problem for engines, since valve timing is dynamic and based on engine RPM, whereas the pulse timing is static and dependent on the length of the intake runner and the speed of sound. The traditional solution has been to tune the length of the intake runner for a specific RPM where maximum performance is desired. However, modern technology has given rise to a number of solutions involving electronically-controlled valve timing (for example Valvetronic), and dynamic intake geometry (see below).

Some naturally-aspirated intake systems operate at a volumetric efficiency above 100%: the air pressure in the combustion chamber before the compression stroke is greater than the atmospheric pressure. The additional energy required to compress the air above atmospheric pressure comes from the momentum of the piston. In combination with the exhaust manifold[vague] the valve opening time can be prolonged and friction losses reduced. The exhaust manifolds achieves a vacuum in the cylinder just before the piston reaches top dead center.[citation needed] The opening inlet valve can then—at typical compression ratios—fill 10% of the cylinder before beginning downward travel.[citation needed] Instead of achieving higher pressure in the cylinder, the inlet valve can stay open after the piston reaches bottom dead center while the air still flows in.[citation needed][vague]

In some engines the intake runners are straight for minimal resistance in some other engines the intake runners are have turns. Turns allow for a denser packaging of the whole engine, are needed for some variable length designs, and allow to reduce the size of the plenum. In an engine with at least 6 cylinders the averaged intake flow is nearly constant and the plenum volume can be smaller. To avoid standing waves within the plenum it is made as compact as possible. The intake runner each use a smaller part of the plenum surface than the inlet, which supplies air to the plenum, for aerodynamic reasons. Each runner is placed to have nearly the same distance to the main inlet. Runners, whose cylinders fire close after each other, are not placed as neighbors.

Variable length intake manifold
Variable Length Intake Manifold (VLIM) is an internal combustion engine manifold technology. Four common implementations exist. First, two discrete intake runners with different length are employed, and a butterfly valve can close the short path. Second the intake runners can be bent around a common plenum, and a sliding valve separates them from the plenum with a variable length. Straight high RPM runners can receive plugs, which contain small long runner extensions. The plenum of a 6 or 8 cylinder engine can be parted into halves, with the even firing cylinders in one half and the odd firing cylinders in the other part. Both sub-plenums and the air intake are connected to an Y (sort of main plenum). The air oscillates between both sub-plenums, with a large pressure oscillation there, but a constant pressure at the main plenum. Each runner from a sub plenum to the main plenum can be changed in length. For V engines this can be implemented by parting a single large plenum (at max RPM) by means of sliding valves into it when RPM is reduced.

As the name implies, VLIM can vary the length of the intake tract in order to optimize power and torque, as well as provide better fuel efficiency.

Lower intake manifold on a 1999 Mazda Miata engine, showing components of a variable length intake system.There are two main effects of variable intake geometry:

Venturi effect - At low rpm, the speed of the airflow is increased by directing the air through a path with limited capacity (cross-sectional area). The larger path opens when the load increases so that a greater amount of air can enter the chamber. In dual overhead cam designs, the air paths are often connected to separate intake valves so the shorter path can be excluded by deactivating the intake valve itself.
Pressurization - A tuned intake path can have a light pressurizing effect similar to a low-pressure supercharger due to Helmholtz resonance. However, this effect occurs only over a narrow RPM range which is directly influenced by intake length. A variable intake can create two or more pressurized “hot spots.” When the intake air speed is higher, the dynamic pressure pushing the air (and/or mixture) inside the engine is increased. The dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the speed/dynamic pressure is increased.
Many automobile manufacturers use similar technology with different names. Another common term for this technology is Variable Resonance Induction System (VRIS).

In automotive engineering, an intake manifold or inlet manifold is the part of an engine that supplies the fuel/air mixture to the cylinders. An exhaust manifold or header collects the exhaust gases from multiple cylinders into one pipe. The word manifold may come from the Old English word manigfeald (from the Anglo-Saxon manig and feald ) and refers to the folding together of multiple inputs and outputs.

Variable Length Intake Manifold (VLIM) is an internal combustion engine manifold technology. Four common implementations exist. First, two discrete intake runners with different length are employed, and a butterfly valve can close the short path. Second the intake runners can be bent around a common plenum, and a sliding valve separates them from the plenum with a variable length. Straight high RPM runners can receive plugs, which contain small long runner extensions. The plenum of a 6 or 8 cylinder engine can be parted into halves, with the even firing cylinders in one half and the odd firing cylinders in the other part. Both sub-plenums and the air intake are connected to an Y (sort of main plenum). The air oscillates between both sub-plenums, with a large pressure oscillation there, but a constant pressure at the main plenum. Each runner from a sub plenum to the main plenum can be changed in length. For V engines this can be implemented by parting a single large plenum (at max RPM) by means of sliding valves into it when RPM is reduced.

As the name implies, VLIM can vary the length of the intake tract in order to optimize power and torque, as well as provide better fuel efficiency.

Lower intake manifold on a 1999 Mazda Miata engine, showing components of a variable length intake system.There are two main effects of variable intake geometry:

Venturi effect - At low rpm, the speed of the airflow is increased by directing the air through a path with limited capacity (cross-sectional area). The larger path opens when the load increases so that a greater amount of air can enter the chamber. In dual overhead cam designs, the air paths are often connected to separate intake valves so the shorter path can be excluded by deactivating the intake valve itself.
Pressurization - A tuned intake path can have a light pressurizing effect similar to a low-pressure supercharger due to Helmholtz resonance. However, this effect occurs only over a narrow RPM range which is directly influenced by intake length. A variable intake can create two or more pressurized “hot spots.” When the intake air speed is higher, the dynamic pressure pushing the air (and/or mixture) inside the engine is increased. The dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the speed/dynamic pressure is increased.
Many automobile manufacturers use similar technology with different names. Another common term for this technology is Variable Resonance Induction System (VRIS).

Baoshan Iron & Steel Co., China’s biggest steelmaker, said tumbling demand and prices may force it to write off inventories and incur a fourth-quarter loss.

Full-year profit in 2009 for the Shanghai-based mill will also likely fall from this year, Chief Financial Officer Chen Ying said on Oct. 30 in an online conference with investors.

The global economic slowdown since June has curbed demand by builders and carmakers, damped prices and turned Chinese mills unprofitable in October, the China Iron & Steel Association said. Baoshan Steel said yesterday market conditions will be more severe in the next five months.

“The worst time for the steel industry hasn’t arrive,” Liu Yuanrui, a Shanghai-based analyst with Chang Jiang Securities Co., said by phone. “The profitability of the steel industry is set to be lower in the fourth quarter and the first three months of next year.”

Baosteel rose 2.8 percent to close at 4.73 yuan, reversing a 4.4 percent decline as Chinese stocks rallied after the central bank cut borrowing costs to bolster the world’s fourth- largest economy. The benchmark CSI 300 Index rose 2.4 percent.

The company yesterday posted third-quarter profit of 2.85 billion yuan ($417 million), 26 percent below analysts’ estimates.

Inventory Write-Offs

The company had already set aside 990 million yuan as provision for inventory losses for its stainless steel business, the second largest in the nation, it said yesterday. It may need to write off inventories of steel products, Chen said today, without giving specifics.

Steel demand is dropping in China as carmakers and builders face declining orders. China’s economy grew 9 percent in the third quarter, the slowest pace in five years.

China’s economic growth will slow further in 2009, Baoshan’s Chen said.

Chinese steel product exports fell by half in October, the China Iron & Steel Association said today in a statement. China, the world’s biggest steel producer, will have a sharp decline in exports in the months to March as the global financial crisis worsens, it said.

About 32 percent of China’s 71 biggest steelmakers incurred losses in September, the association said.

‘Serious Threat’

“We believe steel industry’s profitability will continue to be under serious threat in the first half of 2009 given the deterioration in the property sector and weak external demand outlook,” Goldman Sachs Group Inc. analysts led by Song Shen wrote in a note today.

Baosteel Group Corp., the state-owned parent of Baoshan, is studying a plan to buy back shares after the stock market tumbled, Baoshan’s Chen said. China is encouraging state-owned companies to increase holdings of publicly traded units to bolster the market.

Angang Steel Co., China’s second-biggest mill, has said its parent Anshan Iron & Steel Group would buy back as much as 4.99 percent of its shares over a 12-month period.

Baoshan is hedging against an increase in the U.S. dollar as the company buys iron ore in the currency, Chen said. The company imports all of its iron ore through long-term contracts

Had the dealer install a new head gasket and exhaust manifold gasket last week.now have a slow oil drip on the number one exhaust manifold bolt, the one in the very front of the engine, that attaches the exhaust to the head.Seems like the oil is getting through the threads somehow, any ideas?Will take it in on Monday. Thanks ED. ‘89 325I

You sure it’s coming from the exhaust manifold bolt or stud? My guess is either the valve cover is leaking. Can you take a picture?

It’s leaking from the stud, like it’s blowing thru the threads.Valve cover gasket and rubber half moons just replaced as well.Don’t have digital camera. Def. leaking at the stud tho, any ideas?

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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.