Exhaust/Emissions Systems Overview

5 Steps to a Better Exhaust System

The Basics

Exhaust System Components

Exhaust System Components

5 Steps to a Better Exhaust System

1. Check your exhaust system for leaks and get these fixed immediately
2. Replace your exhaust manifolds with headers
3. Replace your stock exhaust pipes with 2 1/2" pipes with minimal bends
4. Replace your stock mufflers with performance mufflers
5. If you don't have a dual exhaust system, install one

The Basics

A well designed exhaust system is one of the cheapest ways of increasing engine efficiency, and therefore increasing engine power. Remember that on a four stroke engine, only one stroke out of the four does any work - the power stroke. The other three strokes - intake, compression and exhaust - all absorb some of the power that was made on the power stroke. If we can minimize the amount of power that is lost by these idle strokes, we will have more power available to drive the wheels, which is what the engine is supposed to be doing. To know why there is worthwile gains to be had from fitting a good performance exhaust system, we will start by carefully looking at the factory fitted system.

A V-8 engine requires two exhaust manifolds and either one or two mufflers and often accompanying resonators. If one muffler is used, the exhaust pipe from one manifold meets the other one in the form of a "Y". This is also known as a "Y-split" exhaust. Most V8s use what is called a Dual Exhaust system. A Dual Exhaust system requires two exhaust manifolds and two mufflers. Each side of the exhaust system is completely separate from the other. The advantage of a dual exhaust system is that the engine exhausts more freely, thereby lowering the back pressure which is inherent in an exhaust system. With a dual exhaust system, a sizable increase in engine horsepower can be obtained because the "breathing" capacity of the engine is improved, leaving less exhaust gases in the engine at the end of each exhaust stroke. This, in turn, leaves more room for an extra intake of the air-fuel mixture.

The purpose of the exhaust system is to control the emissions and exhaust produced by the engine. The idea is to turn the harmful pollutants your car produces into harmless ones that don't ruin the environment. These pollutants include hydrocarbons (unburned), carbon monoxide, carbon dioxide, nitrogen oxides, sulfur dioxide, phosphorus, lead and other metals. Although emissions control systems vary between manufacturers and vehicles, they all have the same goal and use many of the same methods.

As with all factory components, the stock exhaust system is a compromise between noise, cost, warranty, and space limitations. The stock type mufflers cause some back-pressure, which adversely affect performance. The pipe configurations and/or size also caused back-pressure in many applications. In addition, most of the standard exhaust manifolds are made of cast-iron, because it is vastly cheaper and much quicker to manufacture than a crafted branch exhaust manifold. The weight and the thermal characteristics of the cast-iron, however, limits the length of the individual runners, and its shape causes the gasses to follow some abrupt turns. The silencers are also mostly not built to enhance the gasflow out of the combustion chamber. This means that the engine has to force the exhaust gasses out of the combustion chamber on the exhaust stroke, with the result that the chamber still has some spent gas inside when the fresh charge of air/fuel mixture arrives. This residual gas (which has done it's work, and will not burn again) takes up space in the chamber which could otherwise be filled by a healthy, combustible mixture that can produce power. The situation worsens: as engine revolutions increase, so does the back pressure in the exhaust system, because the engine has to pump more gas through the restrictive outlet. It is not uncommon to see back pressure rise to 5psi on some engines at peak power. At the end of the exhaust stroke, the spent gas still inside the combustion chamber, remains at that pressure. Next the intake valve opens, and this pressurized exhaust gas pops out through the intake valve into the inlet tracts, pushing back the fresh charge of combustible mixture. When the piston has travelled down far enough to draw in the intake charge, we now have a very much diluted mixture, further reducing the efficiency of the engine.

It is clear that there are real gains to be had from making sure that the exhaust gasses are effectively removed from the combustion chamber. Smoothly bent pipes, relatively free flowing mufflers, headers, and a balance pipe will result in a more efficient exhaust system and thus greater performance. A well designed exhaust system can even "draw" the gasses out of the chamber, using the momentum of the gas travelling down the pipe to suck the residual gasses out of the combustion chamber. The gas travelling down the pipe creates an area of low pressure behind it. This not only purges the combustion chamber, but also draws more mixture into the chamber during the valve overlap period. So, instead of having high pressure exhaust gas popping into the inlet tracts, we now have a partial vacuum inside the combustion chamber, which pulls the fresh charge into the chamber when the intake valve opens. To optimize the exhaust system, let's look at each component of the exhaust system and how we can modify it for more power.


Exhaust Manifold and Header

The exhaust manifold, usually constructed of cast iron, is a pipe that conducts the exhaust gases from the combustion chambers to the exhaust pipe. It has smooth curves in it for improving the flow of exhaust. The exhaust manifold is bolted to the cylinder head, and has entrances for the air that is injected into it. It is usually located under the intake manifold.

A header is a different type of manifold; it is made of separate equal-length tubes. Four tube headers will slightly out perform three tube and Ram Air manifolds, and the three tube headers and RA units are a little better than stock manifolds. However, most standard exhaust manifolds found on Muscle cars are relatively efficient, and do not hurt performance as much as the aftermarket would have you believe. The headers should have 1-5/8" primaries for most vehicles, while higher performance street/strip cars may run slightly stronger with 1-3/4" primaries

Manifold to Exhaust Pipe Gaskets

There are several types of gaskets that connect the exhaust pipe to the manifold. One is a flat surface gasket. Another type uses a ball and socket with springs to maintain pressure. This type allows some flexibility without breakage of the seal or the manifold. A third type is the full ball connector type, which also allows a little flexibility.

Exhaust Pipe

The exhaust pipe is the bent-up or convoluted pipes that connect the entire exhaust system together. Some are shaped to go over the rear axle, allowing the rear axle to move up and down without bumping into the exhaust pipe; some are shaped to bend around under the floor of the car, connecting the catalytic converter with the muffler. Exhaust pipes are usually made out of stainless steel, since the high heat conditions involved with the muffler system will cause rust. Exhaust pipes are classified by their diameter, with wider diameters being perferred for increased performance. A 2-1/2" system is adequate for most street cars, and 3" exhaust pipes might help slightly when you begin to run in the mid 12's. Faster cars definitely benefit from 3" exhaust pipes. In addition to width, the actual design of the exhaust pipe has a tremendous effect on performance. The more bends, kinks, and rough edges inside the pipe, the greater the internal friction on the exhaust gasses and the less efficient the exhaust system. Therefore, performance exhaust systems usually feature exhaust pipes that are smoothly bent, smooth on the inside, and have as fewest bends as possible to reduce the friction inside the pipe. Uniform diameter head-pipes of adequate size will improve performance over carelessly bent or badly crimped pipes. Mandrel bent pipes provide the best available performance.

Exhaust Pipe Hangers

Hangers hold the exhaust system in place. They give the system flexibility and reduce the noise level. The hanger system consists of rubber rings, tubes and clamps.

The EGR Valve

The Exhaust Gas Recirculation (EGR) valve is used to send some of the exhaust gas back into the cylinders to reduce combustion temperature. Why would we want to do this?

Nitrous oxides (nasty pollutants) form when the combustion temperature gets above 2,500 degrees F. This happens, because at such temperatures, the nitrogen in the air mixes with the oxygen to create nitrous oxides. Nitrous oxide combines with the hydrocarbons in the air to create smog, definitely not a good thing. Therefore, the purpose of the EGR valve is to recirculating some of the exhaust gas back through the intake manifold to the cylinders, which lowers the combustion temperature. Lowering the combustion temperature lowers the amount of nitrous oxide produced. Consequently, less of it comes out the tail pipe.

There are two types of EGR valves. One operates through the use of a vacuum, and the other operated through the use of pressure. Both types allow the exhaust gas in to lower the combustion temperature when it gets too high.

PCV Valve

The process of combustion forms several gases and vapors; many of them quite corrosive. Some of these gases get past the piston rings and into the crankcase. If left in the crankcase, these substances would cause all kinds of bad things (rust, corrosion, and formation of sludge), so they have to be removed. Back in the old days, they used to be dumped out into the atmosphere through a tube. Once we realized what a problem pollution was in the sixties, the PCV (Positive Crankcase Ventilation) system was developed to take the place of the old "dump tube."

The PCV system uses a hose connected between the engine and the intake manifold to draw these gases out of the engine's crankcase and back into the cylinders to burn with the regular fuel. The only problem to solve is how to keep these gases from going willy-nilly into the manifold and upsetting the required air-fuel ratio. The solution to this problem is the PCV valve.

The PCV valve controls the release of crankcase gases and vapors to the intake manifold. The valve is kept closed by spring action when the engine is at rest. When the engine is running normally, the low vacuum it creates allows the valve to open and release crankcase vapors and gases into the intake manifold for burning. If the engine is idling or you are slowing down, the vacuum level rises and pulls the valve plunger into the valve opening. This partially blocks off the opening so that only a small amount of vapors and gases can be drawn into the intake manifold.

One really comforting feature of the PCV valve is its behavior in the event of a backfire. If your car backfires in the manifold, the pressure makes the spring close the valve completely. With the valve closed, there is no chance that the flame can move into the crankcase and cause an explosion.

The Air Pump

The air pump sends (or pumps) compressed air into the exhaust manifold and in some cases to the catalytic converter. The oxygen in the pressurized air helps to burn quite a bit of any unburned hydrocarbons (fuel) and therby converts the poisonous carbon monoxide into good old carbon dioxide.

A belt from the engine drives the air pump. It has little vanes (thin, flat, curved fins) that draw the air into the compression chamber. Here, the air is compressed and sent off to the exhaust manifold where it speeds up the emissions burning process. Stainless steel nozzles are used to shoot the air into the exhaust manifold, because they will not burn. Some engines use a pulse air injection system. This system uses pulses of exhaust gas to operate an air pump that delivers air into the exhaust system.

The Catalytic Converter

To further help reduce harmful emissions, modern cars (those built after 1977) have a catalytic converter in the exhaust system. The catalytic converter is installed in the exhaust line, between the exhaust manifold and the muffler. Basically, the harmful gases enter the catalytic converter, a kind of stainless steel container, which is lined with chemicals (catalysts - which are chemicals that cause a reaction between other chemicals without being affected itself) such as aluminum oxide, platinum and palladium. These chemicals cause the carbon monoxide and hydrocarbons to change into water vapor and carbon dioxide. Some converters have a third lining of chemicals, platinum and rhodium, that reduce nitrogen oxides (three-way, dualbed converter). Therefore, the pollutants are changed from harmful gases to harmless ones before they are let into the environment through the muffler and tail pipe.

The materials within a catalytic converter vary between cars. Catalytic converters are designed to do different things, depending on the design of the converter. Some catalytic converters use what is called an "oxidation" catalyst; this usually consists of ceramic beads coated with platinum to reduce hydrocarbons and carbon monoxide. Through the catalytic action, the hydrocarbons and carbon monoxide are "burned" to create water vapor and carbon dioxide. This type of catalytic converter needs an input of oxygen, so oxygen is usually injected into the cylinder head, or directly into the exhaust header or manifold.

Newer catalytic converters have a two part design. The front half is a "three-way" catalyst, which burns various pollutants, and reduces hydrocarbons, carbon monoxide, and oxides of nitrogen into water, carbon dioxide and nitrogen. These converters require exact fuel air mixtures in order to maintain efficient exhaust reduction. The rear section of these converters is the normal oxidation catalyst that further reduces hydrocarbons and carbon monoxide. Air from the air pump is injected into the center of these converters. Here the air is allowed to mix with the exhaust before it passes into the oxidation catalyst, where it burns off its toxic chemicals and reduces emissions.

The reason that leaded gas cannot be used in an engine with a catalytic converter is that the lead coats the chemicals in the converter. This makes them unable to do the job anymore, since the chemical lining can't come in contact with the pollutants. At first, this was a big disappointment, because lead acted as a lubricant and helped to reduce wear on some of the engine parts. Luckily for our engines and the environment (not to mention us), car manufacturers soon got around the problem by making tougher parts and coating them with special metal.


Exhaust gases leave the engine under extremely high pressure. If these gases escaped directly from the engine the noise would be tremendous. For this reason, the exhaust manifold sends the gases to a muffler, which is located between the catalytic converter (if present) or exhaust manifold and the tail pipe. The muffler quiets the noise of the exhaust by "muffling" the sound waves created by the opening and closing of the exhaust valves. When an exhaust valve opens, it discharges the burned gases at high pressures into the exhaust pipe, which is at low pressure. This type of action creates sound waves that travel through the flowing gas, moving much faster than the gas itself (up to 1400 m.p.h.), that the muffler must silence. It generally does this by converting the sound wave energy into heat by passing the exhaust gas and its accompanying wave pattern, through perforated chambers of varied sizes. Passing into the perforations and reflectors within the chamber forces the sound waves to dissipate their energy. The pressure of the gases is reduced when they pass through the muffler, so they go out of the tail pipe quietly.

There are two types of muffler design. A Reverse-Flow muffler is oval-shaped and has multiple pipes. Four chambers and a double jacket are used to accomplish muffling of the exhaust noise. Exhaust gases are directed to the third chamber, forced forward to the first chamber, from where they travel the length of the muffler and are exhausted into the tail pipe. A Straight Through Muffler has a central tube, perforated with several openings which lead into an outside chamber packed with a sound absorbing (or insulating) material. As the exhaust gases expand from the perforated inner pipe into the outer chamber, they come in contact with the insulator and escape to the atmosphere under constant pressure. Because of this, the expanding chamber tends to equalize or spread the pressure peaks throughout the exhaust from each individual cylinder of the engine. This type of muffler is designed for the purpose of reducing back pressure and, consequently, makes slightly more noise.
Since a muffler cannot reduce the noise of the engine by itself, some exhaust systems also have a resonator. Resonators are like little mufflers, and are usually the "straight through" type. They are added at the end of the exhaust system to take care of any noise that has made it through the muffler.

Most "performance" mufflers provide some degree of quieting, all provide improved airflow, and many look good enough to display to bystanders! However, in order to obtain or maintain a reasonable market share, some makers make claims of "increased airflow", "flows better than a straight pipe", or "10% better flow than competitor X". We have conducted extensive testing of mufflers on actual strip performance and found that with a single 2-1/2" muffler on a 12.50 vehicle, there is very little performance difference between all brands of performance mufflers. When tested with a good dual system with some type of crossover, there is almost no measurable difference between brands, providing similar sizes are compared. Static airflow will be affected by test pressure and the methods in which air is directed into and out of the muffler. Accordingly we feel that bench tested airflow on mufflers is misleading and should carry very little weight when evaluating muffler performance.

There are distinct differences in pricing, noise levels, build quality, and packaging of "performance" mufflers. Here are some general observations about mufflers:
  1. The larger the volume in a muffler case, the quieter the muffler.
  2. Exhaust noise is the result of two factors - the noise escaping out the tailpipes, and the noise generated by the muffler case. Round or oval mufflers produce very little case noise because there are no flat sides (other than the ends which are typically reinforced), and thus no areas to vibrate in harmony with the exhaust noise passing within the muffler. Any vibration of a flat surface will generate noise. Rectangular mufflers can generate noise which will be heard inside the car as a resonance. The use of heavy metal cases, extensive welding of the flat metal to internal components, and internal insulation can help reduce the noise produced by the case in this style of muffler.
  3. The use of stainless steel will increase the expected life, add to the cost, and produce a better looking muffler.
  4. Larger pass tubes, or larger passageways within the muffler case will always make the exhaust noisier, and may reduce back-pressure.
  5. Excessive noise is not an indication of improved performance, and reasonably quiet mufflers are not necessarily restrictive.

To summarize, when a muffler of a given pass-through size produces reasonable flow capability, slight additional flow capability probably will not be recognized by our street/strip vehicles. When shopping for a performance muffler, determine how much noise and/or tone you want, how long you expect the muffler to last, the package size you can use, whether appearance is of significant value, and finally how much money you want to spend. We feel that the average street or street/strip vehicle will not be able to measure any performance differences between the top 8 or 10 mufflers available if similar sizes are used on a good dual system.

Unfortunately not all free flow performance exhaust systems are created equal. We regularly see aftermarket systems producing less power than the standard systems they replace. Some are unpleasantly noisy, and their gasflow potantial are dismal.

The Tailpipe

The tailpipe is a long metal tube attached to the muffler. It sticks out from under the body of your car, generally at the rear, in order to discharge the exhaust gases from the muffler of your engine into the air outside the car. Tailpipes are not as critical of irregularities in construction due to the cooling of the exhaust and resulting lower gas volume. Also, 3" tailpipes and mufflers are not necessary until you get well into the 11's. However, severe crimps or wrinkles should be avoided, and mandrel bent pipes should be used when practical and possible.

Final Adjustments

Changing to a more efficient exhaust system can cause the engine to require different settings for optimum performance. It is likely that the engine will be able to run with a leaner part-throttle or cruising mixture, often resulting in a significant gain in fuel economy. If the fuel system isn't matched to it's new environment, you will never enjoy the full benefit of your new exhaust system. And the best way to optimize your engine, is on a loading type dynamometer with an accurate exhaust gas analyser.

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