You need the right plan, exhaust system design, and component selection. You cannot simply select a random collection of exhaust parts and expect to realize top performance. You must take into account your engine displacement, head size, cam timing, intake design, carb size, and other aspects of the engine package. You also need to consider intended operating RPM, application, transmission type, and other factors. This chapter examines specific exhaust system components, including cast exhaust manifolds, tubular exhaust headers, exhaust piping, mufflers, sound-tuning resonators, and catalytic converters, along with the role that each plays.
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In simple terms, an exhaust manifold is somewhat similar in function as an intake manifold. An intake manifold receives incoming feed air from a central point, whether that involves a carburetor or throttle body. The air charge is then distributed to the individual cylinders. An exhaust manifold allows individual cylinder exhaust gases to exit the cylinder head, immediately into a single collective path, or chamber. An exhaust manifold is not the most efficient design, but it is the most simple, most direct, and least expensive method of routing the engine’s exhaust pulses out of the engine. Most exhaust manifolds do not provide an equal length of exhaust flow cylinder-to-cylinder before the individual cylinder gases combine and enter the exhaust piping.
A typical exhaust manifold, because of the commonly used cast-iron construction, is also heavy. Although weight isn’t a consideration for a typical street engine, it’s certainly a factor for a competition engine. Compared to exhaust headers, which feature either a steel or stainless steel flange and tubes, a cast-iron exhaust manifold can be prone to stress cracking resulting from thermal changes and/ or mechanical stress. These cracks can be caused by improper fastener tightening or as a result of operating vibrations in the remainder of the exhaust system, since an iron casting is more brittle than steel. While a cracked cast-iron manifold can be repaired, welding cast iron properly is fairly tricky and requires an experienced welder, often using a powder-based welding procedure.
With this said, you can use cast-iron exhaust manifolds if you’re not concerned about maximizing engine power. However, tuned (tube diameter and length) tubular exhaust headers are preferred for enhancing horsepower and torque. Properly functioning manifolds “get the job done” and are certainly appropriate for anyone on a tight budget, or for those interested in maintaining a factory-original appearance from a restoration standpoint. Basically, cast-iron exhaust manifolds are relatively compact, durable, and provide engine function, but are limited in terms of maximizing engine breathing.
The use of tubular headers, rather than cast-iron exhaust manifolds, offers two advantages: reducing vehicle weight and, more important, extracting additional horsepower and torque by providing individual exhaust paths for each cylinder. Individual primary tubes are less restrictive, in contrast to the unequal exhaust paths found in manifolds. Because exhaust manifolds typically feature very sharp flow paths that result from packing a cylinder head’s exhaust bank into one compact unit, the use of tubular headers provide the distinct advantage of a much less restrictive flow for each individual cylinder.
Although tubular headers are available in a range of configurations, some with unequal-length tubes (for ease of fitment) and others with equal-length tubes, the option of using equal-length primary tubes offers the advantage of more efficient balancing of exhaust pressures and flow. Tubular header construction also allows increasing primary tube length before the cylinder gases combine at or after the collector. Longer primary tubes tend to result in increased torque at lower RPM, and tuning by altering primary tube length provides a distinct advantage over using a short-path exhaust manifold. While a cast-iron manifold typically features sharp and more-restrictive paths for the exhaust, tubular header primary tubes that are mandrel-bent provide drastically improved flow by eliminating sharp and restrictive bends.
Potential disadvantages of tubular exhaust headers include a typically higher price tag, especially when moving into stainless steel construction. However, the increased cost is fully justified when considering the performance advantage over manifolds. Depending on the specific vehicle, the installation of tubular headers presents an added challenge due to space constraints in the engine bay, but this is often unavoidable simply because of the available clearances. Again, depending on the vehicle and engine, installation can range from an easy drop-in to a knuckle-busting ordeal. However, those of us who insist on maximizing engine performance are willing to accept this challenge.
Compared to cast-iron exhaust manifolds, tubular headers tend to produce more noise due to the thinner wall construction of the tubes. The thick cast-iron construction has better sound-insulating properties. These potential issues are of little concern when you consider the performance improvements that tubular headers offer.
Exhaust Manifold and Header Coatings
Regardless of your choice between cast-iron exhaust manifolds and tubular headers, from a standpoint of appearance and longevity, either style is easily installed. If you’re dealing with cast-iron exhaust manifolds, a coating can be applied with either a heat-resistant paint or a ceramic-based coating.
Heat paints have been around for decades, and while formulations may have improved over the years, maintenance is very likely needed in order to keep a fresh appearance. This means removing the manifolds, cleaning and stripping back down to bare metal, and re-coating. Depending on the specific application, a coating of heat paint may retain an acceptable appearance for as long as several years, or as short as a few weeks. A much-preferred approach is to take advantage of a ceramic thermal barrier coating as offered by Jet Hot, Swain, Polydyn, Calico, and others. Today’s ceramic coatings come in a variety of colors and finishes (flat, satin, and gloss), and if properly applied, typically last for the life of the manifolds.
If you’re contemplating tubular exhaust headers, you have three basic options: heat paint, ceramic coatings, or stainless steel construction. Heat paint is better than nothing, but it’s simply not the best long-term solution. For steel headers, a ceramic coating is by far the better way to go. However, keep in mind that while the outer surfaces are coated, the inside walls of the tubes are likely not coated. Some coating services may attempt to also apply the coating to the inner walls; this is often difficult if not impossible to achieve.
An option is to choose tubular headers that are made with high-quality stainless steel, which resist corrosion both inside and out. Stainless headers are offered in a natural satin finish or can be fully polished. Also consider that even though stainless steel headers do not require a coating in order to prevent corrosion, applying a thermal barrier ceramic coating improves heat management, lowers underhood temperatures, and improves exhaust heat scavenging. A thermal barrier ceramic coating provides the same benefits to any material (cast iron, steel, or stainless steel).
Exhaust pipes route exhaust gases from the manifold or header collector to a planned termination point, which may involve routing through mufflers and converters. Any exhaust pipe that is shaped to fit the specific vehicle routes the exhaust gases, but in order to maximize engine performance, you need to pay attention to pipe construction from a restriction, or backpressure, standpoint.
Shaping the pipe bends can be accomplished by crush bending, wrinkle bending, or mandrel bending (all three approaches are explained in Chapter 3). While crush or wrinkle bending results in restrictions and/or deviations in exhaust flow, mandrel bending maintains a uniform diameter and smoother flow path. For any performance exhaust system, mandrel bending is the only acceptable shaping method. For the majority of common V-8 engine applications, the exhaust pipe diameters to consider are in the 2.5- to 3.5-inch range. If the pipe diameter is too small, the exhaust gas is restricted at higher engine RPM. If too large, it can reduce exhaust flow velocity and in turn this can reduce the scavenging effect. This can result in reduced low-end torque as well as a potential for an annoying “droning” noise at cruising speeds.
Exhaust pipe is most commonly available in either steel or stainless steel. Steel pipe is commonly available with zinc or aluminized surface treatments that resist corrosion, but if you’re concerned with both appearance and longer-term corrosion resistance, pipes can be ceramic coated with the same process utilized for header coating. For the ultimate in durability, high-grade stainless steel exhaust pipe is available from performance exhaust manufacturers. For turbocharged systems, hightemperature silicone coupler sleeves are commonly used to connect pipes to intercoolers or at any pipe connection where flexing and/or thermal expansion is a concern. These couplers are commonly attached to exhaust pipes with broad band clamps, usually of the T-bolt type.
Thermal expansion is an issue that many people are unaware of. As exhaust temperature rises, the metal heats and expands. As a result, the exhaust pipe tends to grow in length. This may not be visually evident, but small changes in pipe length can create stress when this growth experiences any resistance. For example, if solid support hangers rigidly secure the entire exhaust system to the chassis, thermal growth stresses might be applied to pipe connections, flanges, flange bolts, etc. In extreme cases, this can result in cracked exhaust manifolds, distorted flanges, etc. The exhaust system that follows the exhaust manifold or headers should be allowed to move slightly as needed, to prevent stress-related damage and to help isolate the exhaust system from the chassis. This will help to minimize bothersome harmonic vibration that is otherwise felt and heard in the passenger compartment; rubber or other damping material is used on OEM exhaust hangers are used for this reason.
If custom aftermarket hangers (such as those made from billet aluminum or stainless steel) are desired, some form of damping isolation should be featured, such as high-temperature silicone or elastomer bushings at the hanger-to-chassis bolt location.
Exhaust pipes must terminate outside of the vehicle to prevent toxic fumes from entering the passenger compartment. Pipes must exit at the rear or at the sides of the body. While some drag-race vehicles might feature long-tube open headers that terminate under the chassis (where the car builder has determined the best exhaust plan for maximum power and torque at the track), consider that the vehicle is only being operated for a 1/8- or 1/4-mile run at a time.
For any vehicle that will be driven for any extended amount of time (at idle or at any vehicle speed), it is critical to route the exhaust gases away from the vehicle’s underside to avoid the potential for deadly carbon monoxide to migrate into the vehicle interior.
When connecting an exhaust pipe to a muffler, lap and butt joints are the two types of connections to consider. A lap joint features one pipe inserted into the mating pipe. This connection can then be secured by welding or with the use of a clamp. A saddle clamp (also known as a U-bolt clamp) pinches the lap joint together. This creates a solid connection but makes it difficult to disassemble the joint in the future. A band clamp provides a wide footprint over the joint, with one side of the clamp diameter accommodating the exhaust pipe and the other side of the clamp fitting the muffler neck side. This is referred to as a lap joint band clamp. A butt joint is used when the OD of the exhaust pipe and muffler neck are identical. This type of joint can be connected by either welding or by using a wide band clamp that features the same diameter on each end of the clamp.
When sizing exhaust pipe for a naturally aspirated engine the exhaust system should be able to flow about 2.2 cfm for every unit of horsepower. For example, if the engine produces 425 hp, the exhaust system should be able to flow about 935 cfm. For a single exhaust system, exhaust pipe diameter should be around 3.25 to 3.5 inches. For a dual-exhaust system, pipe diameter should be in the 2.5-inch range. This is only theory. Actual real-world sizing can depend on what RPM range the engine is expected to make top torque.
A slightly smaller diameter can move torque to a lower RPM and a slightly larger diameter can move torque to a higher RPM range. Enlarging the system too far can decrease exhaust gas velocity and can be detrimental to gas scavenging. Here is a simple formula that provides a rough estimate for exhaust pipe diameter:
Exhaust Pipe Diameter = RPM ÷ 1,000 x engine displacement ÷ 2
For example, if you expect the maximum torque to hit at around 4,800 rpm and the engine displacement is 408 ci, the formula gives you a diameter of 979.2 cfm (4,800 ÷ 1,000 x 408 ÷ 2). This requires an exhaust pipe diameter (for a dual system) to be around 3.5 inches.
The purpose of a muffler is to aid in managing the engine’s exhaust sound level, and to “tune” exhaust tone. While some view the use of mufflers as simply a necessary evil, in terms of controlling engine exhaust noise level, mufflers provide an additional engine tuning aid in terms of horsepower and torque, primarily for street-driven vehicles that must operate and perform in a wide range of vehicle and engine speeds. For the sake of illustrative comparison, consider a firearm’s silencer. This device attaches to the gun-barrel exit in order to decrease the audible noise created by the explosive discharge. Contrary to the popular term, this device does not “silence” the discharge. Rather, it suppresses the noise, so the proper term for this device would be a suppressor, not a silencer. The same holds true for an automotive exhaust system’s muffler. Instead of silencing or eliminating the exhaust noise, the muffler suppresses the exhaust note. The design of the muffler has a direct effect on both the level of noise and the tone of the engine’s exhaust.
The combustion process that takes place in the engine’s cylinders creates sound waves that travel from the cylinder head through the exhaust system. Along the way, these sound waves run through the muffler. Depending on the design of the muffler, these sound and pressure waves are altered by sound-absorbing insulation (such as fiberglass) or by interruptions as the waves interact by baffles and/or tubes that may feature perforations, any or all of which serve to cause the sound waves to lose energy. In a high-performance muffler, the objective is to alter these sound waves while minimizing flow resistance and backpressure. Muffler designs vary from simple straight flow-through tubes with perforations where the sound waves are dampened by sound-absorbing matting that’s wrapped around the perforated tube, to designs that use a complex series of angled baffle walls and carefully designed internal tube bends that divert the waves while promoting both efficient and low-resistance flow and a specific noise level and “tone flavor.” Achieving efficient flow and developing a specific sound is a carefully orchestrated balancing act.
The total “recipe” of a muffler design, including internal volume of the muffler body, the thickness (or gauge) of metal, and the type and shape of internal baffling, can result in a range of sounds. The sound can be from quiet to throaty to raspy. The results can vary and can be tuned for different engine speed and load conditions. The muffler sound, or exhaust note, varies widely depending on the style, size, design, and manufacturer. Generally speaking, it should come as no surprise that a muffler that features fewer exhaust path deviations, sound-absorbing chambers, and frequency-altering aspects produces a louder, or higher-decibel, exhaust note.
In addition to choosing a muffler based on low restriction, for a street application, you need to consider the type of sound that you desire. Apart from meeting acceptable decibel levels appropriate for street use, the muffler’s effect on engine exhaust note influences the buying decision for a particular muffler brand and style. Some drivers may prefer a quiet sound output at idle and cruising, and a throaty, louder note during acceleration, while others may prefer a louder, more notable sound at all engine RPM. Without having the luxury of trying a variety of muffler designs, the only practical approach is to consider how the muffler manufacturer describes a muffler’s sound, listen to the exhaust sounds of vehicles and engines similar to yours at local car shows, and search vehicle-specific Internet forums for the opinions of others. As a result, you are better equipped to make your selection decision.
Some manufacturers offer directreplacement mufflers; some offer mufflers specifically designed for power and sound enhancement for performance-minded applications, such as Borla, Corsa, Flowmaster, SuperTrapp, Spiral Turbo, and Magnaflow. Manufacturers such as these specialize in developing mufflers that maximize engine power while providing the preferred type of sound.
Keep in mind that a competition muffler’s requirements involve maximizing engine power while meeting a particular race sanctioning body’s sound decibel requirements, where applicable. For a street application, the design parameters are more restrictive. Performance muffler manufacturers must design mufflers that not only enhance power, but also provide the type of sound desired, while avoiding annoying droning or booming sounds that may be objectionable to the driver and passengers.
There is no such thing as the ideal muffler for every application because personal tastes vary. Some drivers prefer a raspy sound, while others prefer a low-frequency or “mellow” sound, while others may prefer a very quiet exhaust note at idle and cruising speed, with a notable increase in sound under hard acceleration. A muffler’s sound differs depending on the engine type, displacement, and powerband. With so many variables to consider, the reality in terms of selecting a muffler for a given application is to read each manufacturer’s descriptions of its models, and gather input from others who own vehicles similar to yours.
While the Internet is full of information both useful and false, vehicle-specific forums offer a good starting point to gauge the opinions of others who have tried a certain muffler brand and model with a specific vehicle and engine combination. Leading performance muffler manufacturers invest an extensive amount of research, development, and testing to provide just the right sound that various customers desire. Muffler technology is much more complicated than most people realize. Although different-diameter pipeto-muffler sizes can be adapted using step-up or step-down adapters (or by swedging a small neck to a larger diameter), it’s best to match diameters at all connections in order to maintain a constant diameter. Slip-together connections require a pipe OD to match the muffler ID neck.
Catalytic converters are installed in the exhaust stream in order to lower harmful emissions. They’re designed to change three harmful pollutants, hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), into water vapor (H2O), carbon dioxide (CO2 ), and reduced levels of oxides of nitrogen. Hydrocarbons and carbon monoxide are formed as the result of incomplete combustion of the fuel. Ambient air (the air that we breathe) that is drawn into the engine as part of the air/fuel mix contains a high level of nitrogen. When the nitrogen burns, it results in oxides of nitrogen. Both hydrocarbons and oxides of nitrogen are considered pollutants and contribute to smog. Carbon monoxide contaminates the body’s bloodstream, reducing delivery of oxygen to vital organs.
The air we breathe consists of a mixture of several gases, including 78-percent nitrogen, 20.99- percent oxygen, and .03-percent carbon dioxide, in addition to .94-percent argon, .01-percent hydrogen, .00123-percent neon, .004- percent helium, .00005-percent krypton, and .000006-percent xenon.
How They Work
In the simplest terms, the engine takes in the harmless nitrogen, oxygen, and carbon dioxide; burns air with fuel to create harmful hydrocarbons, carbon monoxide, and oxides of nitrogen; then the catalytic converter “scrubs” the exhaust in an attempt to convert emissions back into harmless nitrogen gas and water vapor. To summarize, you can view this as a chain effect of “in clean, burn dirty, exit clean.”
Catalytic converters act as chemical conversion units that change these harmful emissions in order to lower oxides of nitrogen levels and to transform hydrocarbons and carbon monoxide into harmless emissions. Today’s catalytic converter features a ceramic or metal core called the substrate, which is washcoated with a catalyst of precious metals that provide chemical reactions to convert the harmful emissions.
Scientists discovered that certain precious-metal materials provide the reactions needed to accomplish this feat, including how hydrocarbons and carbon monoxide react with oxygen and how carbon monoxide reacts with oxides of nitrogen. The precious metals commonly in use include platinum (Pt), palladium (Pd), and rhodium (Rh). Platinum offers good oxidation characteristics in terms of helping to convert hydrocarbons and carbon monoxide into water vapor and harmless carbon dioxide, but not so great at reducing oxides of nitrogen. Palladium offers good conversion for all three harmful emissions. Rhodium is the most expensive of the washcoat materials but provides the best performance for removal of hydrocarbons, carbon monoxide, and oxides of nitrogen, but again, it’s very expensive.
To achieve the best emissions conversion performance, it’s common for today’s catalytic converters to feature a combination of rhodium and palladium. The chemical reaction that occurs inside the catalytic converter must take place under high temperatures. The process of the chemical reaction, along with exhaust heat, results in increased temperatures within the converter to around 1,300 degrees F. As a result, converters tend to run very hot. This is the reason that heat shields are often required for vehicle installations, in order to protect surrounding panels, fluid lines, and wiring harnesses, and to reduce the chances of the hot converter igniting any flammable materials on the road surface. A quick and easy way to tell if a converter is working is to measure surface heat before and after the converter, since the level of heat should be higher at the exit of the converter.
There are three types of catalytic converters: two-way, three-way, and three-way with air. Two-way converters were employed in the early days of converter installation, until about 1981. The two-way converter, so-named because it handled only two combustion by-products, was relatively good at converting carbon monoxide and hydrocarbons, but not oxides of nitrogen. This early style usually featured precious-metal-coated pellets. They were replaced with three-way converters around 1981.
Three-way converters feature honeycombed substrates washcoated with specific precious metals and handled carbon monoxide and hydrocarbons as well, but also tackled the issue of oxides of nitrogen reduction. The term “three-way” refers to the handling of all three types of emissions. Three-way converters are used on electronic fuel management, computer-controlled engines in conjunction with oxygen sensors to control air/fuel ratio more precisely.
Three-way with air converters are three-way converters that also feature an air injection tube located between the two separate brick cells that feed extra air to help reduce oxides of nitrogen emissions. These are referred to as three-way-plus converters, not four-way converters. They feature two chamber sections of substrate. The front chamber substrate is likely coated with rhodium and palladium to handle oxides of nitrogen reduction, and the second chamber substrate is likely coated with a combination of platinum and palladium. The air injection tube is located between the two chambers, providing added oxygen for the second chamber to handle conversion of hydrocarbons and carbon monoxide.
Performance Catalytic Converters
Catalytic converters are required by law for street-driven vehicles that were originally equipped with these emissions-control devices. Even though your vehicle may require one or more converters, that doesn’t mean that you’re stuck with the OEM factory designs that in many cases feature undesirable exhaust flow restriction. It’s obvious that for a performance application, you want to reduce backpressure when and where it makes sense. In order to provide a better-breathing converter for the go-fast crowd, several aftermarket manufacturers now offer fully functioning catalytic converters that feature reduced backpressure compared to OEM converters. Reduced backpressure converters feature larger cells (bigger honeycomb holes) or a greater number of cells in order to lower flow resistance.
Eastern Catalytic is one good example of a manufacturer that caters to the performance market; it offers two types of performance converters. The Tru Performance model features either polished 304 stainless steel or satin-finish 409 stainless steel bodies with 300-cells-per-inch ceramic catalyst and steel mesh V-ring retainers to resist blowout under hard throttle. The Bullet Cat converter features all-tubular stainless steel construction (looks like a really short glasspack) and is available with either a 200-cell metallic foil or 400-cell ceramic substrate. Either model accommodates engines up to 8.0L displacement. A high-strength heat-resistant corrugated foil is wound into a cylindrical shape, then wrapped with a layer of non-corrugated foil that’s brazed to the cylinder. The foil structure is washcoated with a proprietary NANO coating that meets OBD-II requirements. The foil construction is better able to withstand extreme pressure and vibration compared to ceramic substrates, which is ideal for forced-induction and high-horsepower applications.
Converters such as the Tru Performance model are appropriate for any naturally aspirated performance application, while the Bullet model provides a higher level of durability suitable for forced-induction applications. I’ve cited Eastern’s products as good examples. High-performance catalytic converters are also available from manufacturers including Magnaflow, Flowmaster, Dynatech, SLP, Walker, Vibrant, and DynoMax. In essence, a catalytic converter that is referred to as a performance converter should feature a more-robust construction, along with reduced flow restriction, whether this is achieved by larger cells in the bricks or a larger number of cells for increased surface area. If your vehicle is street driven and is required to maintain one or more catalytic converters, you definitely want to move up to a quality-built high-performance converter design that offers as little flow restriction as possible. If your engine features forced induction, especially turbocharging, and you must run a converter, it is imperative to upgrade to a sturdily built converter that provides the durability needed in a forced-induction application.
Even though you may be forced to retain a catalytic converter, be aware that less restrictive and more-durable converters are readily available. Providing that the converter is designed for high performance with reduced restriction, choosing a performance catalytic converter is easy. Simply order the selected high-performance converter that features the appropriate size inlet and outlet to mate to your exhaust piping in order to maintain a uniform diameter.
Written by Mike Mavrigian and Posted with Permission of CarTechBooks