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Exhaust 101

Exhaust System Basics •
Basics of Combustion •
Controlling the Products of Combustion •
Exhaust System Components •
On Board Diagnostics •
Inspections

Exhaust System Components and Design

EXHAUST HEADER/MANIFOLD

Typically made of cast iron or fabricated tubing, the headers, which are connected to the engine at the exhaust ports with flange connectors, are designed to collect exhaust gases from each cylinder.

These gases are then funneled into a common outlet, which connects to the next pipe or component in the exhaust system. Headers may have additional connections for emission control components.

TYPES OF PIPE IN THE EXHAUST SYSTEM

The Exhaust pipe carries the collected gases and vapor from the exhaust manifold or header to another component further downstream in the exhaust system.
The "Y" pipe is an exhaust pipe, which connects both exhaust headers of a "V" engine to form a single exhaust system. It may also be used to split a single exhaust system into a dual exhaust system.
The "H" pipe consists of a right and left- hand exhaust pipe, attached to a header or catalytic converter and connected by a balance pipe to form a dual exhaust system component.
Balance pipes are used in many dual exhaust systems to merge sound pulses from left-hand and right-hand sources. This helps reduce harsh exhaust sounds, equalizes muffler and tail pipe life, and can aid in improving mid-range torque output of the engine.
A "Crossover" pipe connects one exhaust header to the other, thus creating the start of a single exhaust system combining multiple exhaust outlets into one.

The Intermediate pipe connects the exhaust pipe with the muffler or the resonator, whichever is first in the system. Its purpose is to carry the gases to the muffler for silencing or to the resonator for further silencing. Not all automobiles have intermediate pipes. This component may also be referred to as an extension pipe or connecting pipe.
The Tail pipe completes the design task of an exhaust system by directing the exhaust gases out of the vehicle to a point where they cannot enter the passenger compartment. A split tail pipe may be called a front and a rear tail pipe. Generally, a tail pipe is greater than one foot in length. The tail pipe is the last "exhaust pipe" in the exhaust system.
The tail spout serves the same purpose a the tail pipe except it is shorter in length, usually a foot or less. This component is most often found on vehicles with rear mounted mufflers.

MUFFLER

The muffler is the main source of silencing of exhaust gas noises. It is a combination of tuning chambers, formed by partitions and ventilated and solid tubes. It is designed to effectively contain, absorb and dissipate noise pulses while moving the exhaust gases and vapor smoothly through and ultimately out of the tail pipe. Rock (mineral), wool, fiber-mat, or fiberglass roving placed in the muffler cavities serves to further absorb and eliminate unwanted exhaust sounds.

The location of a muffler varies considerably depending on the vehicle model, but most mufflers are located toward the rear of the vehicle. Internal muffler design is determined by the "noises" in need of control. The muffler can assume many shapes from round to oval, to custom stamped.

To do its job correctly, a muffler must be specifically designed, both inside and out. The inside of the muffler must promote engine performance and sound control, while the outside of the muffler must fit a specific make, model, and year of vehicle. Also, both the inside and outside of the muffler must be able to stand up to the effects of corrosion.

As a vehicle's engine runs, it burns anywhere from 1,200 to 15,000 "charges" of air and gasoline per minute. Each time an air and gasoline charge is burned, the engine expels the residual gases into the exhaust system in the form of high-pressure gas. The sound waves created by the high-pressure gas are very powerful. In order to control the sound level of a running engine, the power of these sound waves must be reduced.

It is the muffler's responsibility to contain and control the force and noise created by a running engine. In order to do this, the muffler must effectively reduce the pulsations of the exhaust gases, while still permitting the gas to pass freely through and avoid excessive backpressure. Backpressure acts as a brake against the engine, reducing power and performance.

Inside a Muffler

Internally, a muffler is a combination of chambers, partitions, louvered tubes, and solid tubes. Together, these components are balanced to attenuate sound energy while the exhaust gases are moving efficiently through the muffler.

The number and arrangement of the tubes and partitions used in a muffler depends upon the sound frequencies produced by the engine. Some chambers within the muffler have no outlet at all. They are Hemholtz tuners that reduce the low- pitched sound frequencies by providing a cushion for the sound waves. Smaller chambers or pinch cans cancel the high pitched sound waves by channeling exhaust gas through their acoustic openings into larger chambers.

The internal structure of a muffler varies between different vehicles because a muffler can be "tuned" to an engine to provide the most effective sound deadening while maintaining performance. To match a muffler to an individual vehicle application may require 30 inches or more of tuning length. If there is only room on the vehicle for 10 inches of muffler, then this tubing must be divided into three 10-inch tubes, resulting in tri-flow routing. A point to note is the more the exhaust gases must be forced to turn and curve, the higher the backpressure created in the muffler. Therefore, the internal design of the muffler is critically important.

Outside the Muffler

Externally, a muffler must physically conform to the space restriction of the vehicle's underbody. The overall size and shape of a muffler is determined by the size and shape of the available space. Proper design and physical placement is important because of the following reasons:

Provides proper clearance to the vehicle underbody.
Avoids excessive heating of floor boards.
Assures trouble-free installation.
Assures proper clamping to prevent dangerous leaks.
Eliminates strained hanger mounts.

Muffler Construction

Mufflers must be rugged enough to withstand the pulsing vibrations of a high-powered engine, as well as the roughest road shock and the worst enemy of all, corrosion. Some of the major causes of premature muffler failure are the following:

Rupture from backfires
Internal corrosion from acid condensate
External corrosion from chemicals used on icy roads
Improper suspension support which causes bushing fatigue and fracture

To protect against rupture, muffler heads are spun-locked to the shell to provide the strongest resistance to backfires. The inner shell and outer cover are also installed 180 degrees apart with a mechanical lock seam. The spun-locked heads and shell lockseam also assure a gas-tight fit between the head and shell.

For every gallon of gasoline burned in the engine, a gallon of acid-bearing vapor passes into the exhaust system. These corrosive acids condense and collect in the bottom of the muffler casing, causing mufflers to rust from the inside out. If all driving was at high speeds on freeways and turnpikes, the internal muffler parts would be kept hot enough to evaporate these corrosive acids, and the inside of the muffler would remain dry. But with stop-and-go driving, the muffler does not get hot enough inside to prevent these acids from condensing and starting corrosion. Mufflers have a number of anti-corrosion features to combat this problem. These include:

Louvered Tubes - These tubes provide better gas flow to maintain a more uniform internal temperature. By avoiding cool spots within the muffler, most acid condensation can be prevented.
Internal Drainage System - A complete internal drainage system prevents any acids from collecting between partitions or at the muffler heads.
Muffler Bushings - Road shock and vibration can cause early muffler bushing breakage. Muffler bushings extend from the head into and through the first internal partition. This two-point bridge contact gives greater structural strength. This is referred to as "bridge construction" and is a good example of the effort that goes into designing a quality muffler.
Spot Welded Partitions - Spot welding is used to attach the partitions to the muffler shell. This process provides more strength and rigidity than other methods of attachment. Extra partitions are used where additional strength is required.
Mechanically Joined Internal Tubes - To ensure longer life, internal tubes are mechanically joined to the partition to allow free-floating expansion and contraction during temperature changes. This unique design eliminates the breaking of spot welds and subsequent part distortion or loose part noise problems.

RESONATOR

The resonator is a second silencing element that is used on some vehicles that have underbody space limitations. When a muffler required to eliminate exhaust noise is too large to easily fit under the vehicle; two smaller silencing elements will then be used. The resonator serves to level out any loudness or roughness, which is not adequately handled by the undersized muffler.

CATALYTIC CONVERTER

Since the 1950's, automotive manufacturers have been trying to reduce the amount of polluting chemicals produced by internal combustion engines. It wasn't until the middle 1960's that we saw emission control devices first installed on passenger cars. As the 1970's began to roll by, automobiles were continually modified so that they would satisfy increasingly stringent federal emission level limits.

In the early years of emission control systems, auto manufacturers focused their efforts on modifications designed to produce a cleaner running automobile. Unfortunately, most of these modifications resulted in decreased vehicle power, decreased fuel efficiency, and an overall deterioration of engine driveability characteristics.

Because of the fact that early model emission control devices made cars run cleaner at the expense of everything else, most of North America's drivers developed a very hostile attitude toward emission control devices. In fact, during the middle seventies, disabling and/or removing emission control devices became a widespread and profitable business; people felt that if they could do away with their vehicle's emission control components, the car would run more like their earlier cars. The truth is, however, that most cars run worse when the emission control devices are disabled.

In the midst of this unfavorable climate, catalytic converters were introduced. As you can probably imagine, the introduction of a new emission control device that required the use of the new, (and at that time, more expensive) "unleaded" fuel didn't do much to help the popularity of the converter. On top of that, many consumer advocate groups claimed that catalytic converters were a fire hazard. Other people complained that converters substantially reduced the fuel efficiency of the car.

The unfortunate twist to this whole story is that in almost every case, the negative claims against converters were not really true. The fact is, catalytic converters were, and still are, the best means of reducing engine pollutants. The catalytic converter reduces engine pollutants more efficiently than any other emission control device on the car, and does so in a way that barely affects the power of the engine or the fuel economy of the vehicle. In no way do converters hinder the desirable driveability characteristics of a car, and the concern about converters causing fires never became a problem.

Early in converter history people replaced their catalytic converters with substitute "test pipes." It's only been within the last few years that emission testing programs, mandatory vehicle inspections, and EPA enforcement of "tampering" laws have slowed the removal of catalytic converters and other emission control equipment. In fact, today, many aftermarket catalytic converters are sold to consumers who at one time removed their original equipment converter.

Converter Types

As you may already know, there are three types of catalytic converters:

oxidation converters
three-way converters
three-way-plus oxidation converters

For the most part, each of these different converters came about because the EPA stiffened the regulations that govern the amount of pollutants that come from a new car. Here's a quick summary of the events that spawned our three different types of catalytic converters.

In 1975, the EPA stiffened the regulations that limit the amount of carbon monoxide (CO) and hydrocarbons (HC) produced by new cars. As a result of the stiffer regulations, auto manufacturers began using oxidation catalytic converters.

In 1979, the EPA stiffened the regulations that limit the amount of oxides of nitrogen (NO_x) produced by new cars. As a result of the stiffer regulations, auto manufacturers began using three-way-plus-oxidation converters on most applications.

In 1981, the EPA again stiffened the regulations that limit the amount of oxides of nitrogen (NO_x) produced by new cars. As a result of the stiffer regulations, auto manufacturers began using the reduction, or three-way catalytic converter on many applications.

How Do Converters Work?

Within the stainless steel shell of an aftermarket catalytic converter lies a substrate that's coated with a combination of platinum, palladium, and sometimes rhodium. These three chemicals are frequently called precious, or noble, metals. Typically, oxidation converters are loaded with platinum and palladium. Three-way and three-way-plus converters are loaded with platinum, palladium, and rhodium.

Hot exhaust gases containing the "terrible trio" (carbon monoxide, oxides of nitrogen and hydrocarbons) of pollutants travel through the exhaust pipe and eventually come in contact with the precious metals that are loaded on the converter's substrate. The substrate is a honeycomb of small ceramic passageways.

When the exhaust gas comes in contact with the precious metals or catalyst, a chemical reaction takes place that weakens the bonds of the polluting chemicals and allows them to easily convert into the more desirable by-products of combustion, which were discussed earlier.

2-way converters were first introduced in the mid 1970's. They oxidize hydrocarbon and carbon monoxide emissions only. The name 2-way refers to the number of pollutants affected by the converter.

In this design, exhaust gases are directed to flow over the substrate where they come in contact with the catalyst. The exhaust gases increase in temperature and continue to oxidize. Hydrocarbon (HC) and carbon monoxide (CO) emissions are converted into water (H₂O) and carbon dioxide (CO₂) before they enter the muffler.

Because of the intense heat created by this process, exhaust gases leaving the converter should be hotter than the gasses entering the converter. This also explains why heat shields are required on most units.

3-way without air converters were introduced in the late 70's. These converters reduce NO_x emissions as well as oxidize hydrocarbon and carbon monoxide.

Inside, a 3-way without air converter looks like a 2-way converter. But, the 3-way without air substrate is coated with rhodium and palladium.

If an engine's exhaust is high in hydrocarbons and carbon monoxide, an air pump and tube feed extra oxygen directly to the converter. Inside a 3-way with air-injection, there are two substrates.

The front chamber is coated with rhodium and palladium. Rhodium reduces NO_x emissions into simple nitrogen (N₂) and oxygen (O₂). This process is most effective when there's little oxygen present. That's why this substrate is located upstream of the air tube inlet.

A second substrate of palladium and platinum is located downstream of the air inlet, so the increased oxygen enhances the oxidation of hydrocarbons and carbon monoxide.

Converter Failure

No matter what the converter design, they all fail for the same reasons. Since there are no moving parts in a converter, the most common cause of failure is contamination.

When overly rich fuel mixtures or raw fuel is introduced into a converter, its temperature can rise to the point that its substrate will simply meltdown. This is known as thermal failure. Thermal failure will deactivate the catalyst and, in extreme cases, block exhaust flow through the unit.

Thermal shock occurs when an overheated converter is fed cold raw fuel or comes in contact with winter elements. The ceramic substrate cools too rapidly and contracts unevenly. It starts to crack and break up. Normal exhaust system vibration will cause it to disintegrate even further.

Silicone that has leaked into the exhaust from antifreeze or sealants will similarly coat the catalysts, with the same result - a non-functioning converter. In cases of serious contamination or meltdown, the converter needs to be replaced.

Obviously, restrictions of any kind - inside the converter or elsewhere in the exhaust system - will also affect converter performance. If the restriction is internal, the converter needs to be replaced.

Converters can also sustain physical damage. Body punctures or weld cracks create leaks that require converter replacement.

EXHAUST SYSTEM ACCESSORIES

Clamps are devices used to provide gas- tight, leak-free connections at points where two pipes or a muffler and a pipe are joined together. Also used to attach the exhaust system hangers.
A Bracket is any metal protrusion, which is fastened to a component of the exhaust system that is used in suspending or reinforcing the exhaust system. These devices are generally made of metal stampings, and often use rubber components within their structure for vibration control.

Gaskets are sealing devices placed between two parts of that exhaust system. The gasket assumes the shape and size of the mating surfaces and often has provisions for securing via multiple bolt holes punched through the material. These devices may be made of composition materials, formed metal, or a combination of the two materials.
A flange is a cast or formed part used to join two units of an exhaust system to each other or to the exhaust header. A flange may be a loose piece, "trapped" on a pipe, or welded to the end of a pipe.

A Hanger is a device with flexibility and noise isolating qualities, which is used to attach exhaust system components to the vehicle's underbody.

AIR GAP PIPES

An air gap pipe is actually one pipe inside of another. There is a 1/16th of an inch to 3/16th of an inch air gap between the inner and outer pipe that acts as an insulator and eliminates the need for a separate heat shield. Air gap pipes have distinct advantages over heat shielded pipes. These include:

Heat protection all around the pipe rather than just on the side with the shield.
No worry of shields rusting off.
Easier to handle since there are no shields to get bent or torn off during shipping.
Better installation clearance.

Some applications use air gap on the Original Equipment (OE) exhaust systems. This is because air gap is actually part of the emission control package. The increased insulation offered by the air gap maintains higher temperature to the catalytic converter. This results in quicker light off on start up and more efficient operation thereafter.