The Complete Performance Exhaust Pipe Guide

In this article, I discuss the pipe that routes exhaust gases from the headers to the termination of the exhaust path, as well as pipe sizing, materials, bending methods, pipe clamping, and pipe support.

This tech tip is from the full book, PERFORMANCE EXHAUST SYSTEMS: HOW TO DESIGN, FABRICATE, AND INSTALL.

For a comprehensive guide on this entire subject you can visit this link: LEARN MORE ABOUT THIS BOOK HERE

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Materials

To select exhaust pipe for a given application, first consider your budget. Expense factors include materials, quantity of pipe (single versus dual system), the application of a specialty coating, and pipe-bending labor. Stainless steel is more expensive than mild steel. The advantages of using stainless steel are longer life, in terms of corrosion resistance, and superior heat transfer. If your budget is tight, if you don’t plan to operate the vehicle in a variety of climates, and you’re not overly concerned with surface corrosion, mild steel is certainly an option. Pipes made of mild steel are available as bare steel, but are also commonly offered with corrosion-resistant aluminized or galvanized surface treatments. If your budget allows, stainless steel is preferred, especially from standpoints of longevity and long-term appearance.

Fit and Clearance Considerations

In terms of fitment to a vehicle, you can cobble a system together by purchasing a selection of pipe shapes (straight sections, pre-bent 45-degree and 90-degree sections, etc.) and trimming to length. Pipe-to-pipe sections must be either welded or clamped together. Another option is to purchase a pre-formed kit that is already shaped to fit your specific vehicle. Many manufacturers offer complete systems for popular vehicles that include pipe and mufflers that generally require only assembly and installation.

Yet another option is to have your entire exhaust pipe system custom-bent for your specific application. The custom approach is often required for unique applications, such as custom street rods, where no pre-designed system may be available. The choice of your pipe system is based on both cost and how much, if any, of the work you prefer to do on your own.

Your selection of pipe diameter should be based on engine size and existing or planned horsepower level. Typically, a performance-oriented V-8 engine application requires pipe diameter in the 2.5- to 3.5-inch range. Keep in mind that for any given application, pipe diameter should be slightly larger for a single exhaust system than for a dual system. However, bigger is not always better. If the pipe is too large in diameter, you lose exhaust flow velocity, which can actually hurt engine performance. Pay attention to the entire undercarriage for clearance issues. Placing pipes too close to the floor can lead to excessive heat transfer to the floor and carpet. Also, if the pipes are close enough to the floor, frame, or subframe they can make contact, resulting in annoying banging or irritating resonance noise. Remember that the exhaust system is connected to the engine and moves in relation to the engine’s movement on its mounts. Unlike some race cars that have a solid-mounted engine, street vehicles feature engine and transmission mounts that provide a bit of compliance.

Precise mandrel bending allows a header manufacturer to achieve a precision< fit. The example shown here is for a Cobra replica.

Make sure that the pipes clear the transmission and driveshaft properly. If the pipes are too close to the transmission, the transmission fluid temperature could elevate, which could potentially result in transmission damage. Generally speaking, I maintain at least a 1-inch clearance between any exhaust pipe and the transmission or the transmission cooling lines. Pay attention to driveshaft clearance to make sure that the pipes cannot contact the driveshaft. When trial-fitting the pipes, support the vehicle on a lift or stands and allow the rear suspension to hang unloaded. If you plan to use a crossover pipe with a dual-pipe system, make sure that the crossover clears the driveshaft even with the rear suspension unloaded.

Unless you’re using a side-exhaust layout, your pipes route to the rear bumper area. As a result, the pipes need to route over or under the rear axle. If the pipes run over the axle, sufficient clearance between the pipes and the axle housing is critical. With the vehicle’s suspension loaded, compress the suspension by having a helper bounce the rear of the vehicle up and down while observing body-to-axle clearance. If the pipes route under the axle, check clearance with the rear suspension unloaded. Any vehicle’s exhaust system must exit beyond the body to avoid exhaust fumes from entering the vehicle interior. If you plan to run a full-length exhaust system with the pipes exiting the rear, the tips should be flush with or slightly beyond the rear bumper. If the pipe(s) is not long enough, or if you want to enhance appearance, add a stainless steel or chrome exhaust tip to the exit of each pipe. Since the exhaust tips are visible, stainless steel is preferred because of its corrosion resistance.

When planning the exhaust pipe routing locations, always be aware of pipe-hanger requirements. The system must be properly supported to minimize movement and to avoid stresses. If the system is not properly supported, the weight of the mufflers and pipes can easily induce excessive strain at a number of locations, including at the header collector and even all the way to the header flange attachment at the cylinder heads. Avoid solid hangers for street use. All support hangers should provide some degree of compliance to accommodate exhaust system vibration, engine-rocking movement during acceleration and deceleration, and to allow the exhaust pipes to expand and contract under heat and cool-down. If you choose to use attractive billet hangers they should at least feature temperature-resistant bushings.

Exhaust Pipe Sizing

As with carburetors or camshafts, bigger is not necessarily better. The exhaust pipe diameter needs to be matched to the engine displacement and level of horsepower output. Although a larger diameter pipe may look cool, if the diameter is too large for the application, you lose exhaust velocity and low- to mid-range power suffers as a result. In the world of pipe, published diameters always refer to the pipe’s OD. As a result, the ID is slightly smaller, the extent of which is determined by the pipe wall thickness/gauge.

Pipe CFM

You need to determine engine intake volume and then approximately match this volume for the exhaust. The formula for finding this is:

Engine Intake Volume = RPM x .001 x displacement ÷ 2

As you might imagine, exhaust pipe starts as straight lengths. This is one of the stock inventory racks of stainless steel pipe at the Corsa factory.

Crush bending leaves a telltale depression along the throat of the bend. This deformation results in a non-round cross section in the bend area. While certainly functional, from a high-performance standpoint, this does not provide optimal flow.

For instance, if your target is to obtain optimum performance at, say, 4,000 rpm, and the engine displacement is 350 ci, the volume is 700 cfm (4,000 x .001 x 350 ÷ 2). The exhaust pipes’ total CFM should be in the same range. To determine the ideal exhaust pipe diameter, refer to “Exhaust Pipe Diameter” on page 138. To develop an accurate estimate of pipe flow capacity, refer to “Pipe CFM” on page 139.

Pipe Bending

Three types of pipe bending approaches are available for automotive exhaust pipe: crush, wrinkle, and mandrel. The bending process may be handled manually or with hydraulic assistance.

Crush Bending

Also called press, ram, or compression bending, crush bending refers to tube bending for which only a radius die and backing shoes are employed, with no supporting mandrel placed inside the tube. The result, although functional, is a bend on the inside of the radius. As the pipe is drawn across the radius die, the outside of the tube bend (called the heel) is stretched, while the inside of the bend (called the throat) is contracted, leaving a slight depression along the inside of the bend.

Exhaust pipes that are intended for specific production vehicle installations include the appropriate hanger pegs already welded in position.

Exhaust pipe manufacturers generally produce pipes for a specific vehicle application in large quantities on a given day, so that tooling and machine calibrations don’t require changing during that particular day.

Although pipes can be bent using a wrinkle- or crush-bending process for OEM and OEM-replacement applications, the majority of high-performance pipes are mandrel-bent to maximize flow and to obtain a pleasing appearance.

Exhaust pipe makers often store their prototype pipes to keep as a reference if any future changes are required. These are a few of the hundreds of design patterns at Stainless Works.

Wrinkle bending results in a series of wrinkles along the throat of the bend. The pipe is heated to soften the metal and then bent. As the pipe is bent, the outside of the pipe’s bend stretches and the throat area contracts, forcing the inside of the bend to fold in a wrinkle pattern. This is commonly found on high-production OEM or OEM-replacement pipes.

This is one of the most common, and equipment-wise, the most economical type of bending. The typical exhaust shop likely uses this type of bender. While mandrel bending can result in a smooth, consistent surface on both the outside and inside of the bend radius, a crush bender achieves, for lack of a better term, a factory-original appearance. Crush bending results in a non-round, somewhat elliptical, shape that slightly restricts flow.

Wrinkle Bending

This bending process, which is also referred to as crinkle bending, does not use a mandrel. Heat is applied to the pipe at the area to be bent until the pipe is red-hot. As the pipe is forced along the die, the inside throat of the bend collapses in a series of accordion-shaped wrinkles. This type of bend is commonly found in many OEM and direct-replacement aftermarket pipes.

Wrinkle bends slightly reduce the inside diameter, along with creating a series of humps that can affect both flow and sound. This type of bending that results in inside-radius wrinkles is typically achieved during a high-production process. Wrinkle bending is not common for do-it-yourself bending or at any custom shop.

Mandrel Bending

Both crush and wrinkle bending distort the pipe diameter at the bend area. Mandrel bending can provide a smoother, virtually uninterrupted final product, free of kinks and creases. Mandrel bending differs from crush bending because of the mandrel that’s placed inside the pipe. The mandrel supports the inner walls of the pipe as the pipe sweeps through the die and backing shoes, resulting in a bend that is virtually free of diameter or profile changes. The mandrel is also lubricated, usually with a lithium grease, to provide a smooth gliding action inside the pipe. However, just because a mandrel is placed inside the pipe doesn’t automatically mean that you won’t have a slight deformation inside the bend radius.

Mandrels are comprised of a series of rounded radius-faced balls that are secured together by a flexible cable. The series of balls (usually made of brass) is able to flex and follow the bend radius while supporting the inside walls of the pipe during the bending process. The width and the number of balls have an effect on the final appearance. In addition, in order to achieve a super-smooth inside bend, a wiper die is used on the bending machine that smooths out any irregularities as the pipe passes through the dies, resulting in a smooth bend that retains the original diameter and enhances appearance at the same time.

Mandrel bending, because of the inside support of the pipe during the bending process, maintains a round and consistent ID and is void of exterior deformation.

Here’s a comparison between a mandrel bend (left) and a crush bend (right). Notice the deformation in the inner radius (arrow) of the crush bend.

Mandrel pipe bending uses a mandrel inserted into the pipe at the bend area in order to support the pipe and to prevent it from collapsing or deforming. The mandrel consists of a series of “balls” that are held together by a cable. Ball diameter, width, and quantity are dictated by pipe diameter, bend length, and bend angles to be accomplished.

Mandrel balls are able to pivot along a band path. As the pipe is bent within the radius die and its backing shoes, the series of balls follow the bend, providing pipe wall support.

The mandrel is installed based on the pipe diameter to be bent. A lubricant is applied to the mandrel balls to permit a smooth glide through the pipe.

A wiper die is positioned in-line with the radius die. This serves to smooth out any minor pipe surface deformations created by the mandrel bend.

Crush-bending machines are the most common at service garages and local pipe-bender shops. A manual or semi-automatic machine requires the operator to position the pipe for each bend.

Mandrel-bending equipment is expensive and is generally found only in high-performance exhaust manufacturing facilities and high-level custom exhaust shops. If you purchase headers from any of the leading manufacturers, the pipes are mandrel bent, as are exhaust pipes from performance exhaust makers, such as Stainless Works, Corsa, Borla, Flowmaster, etc. Many independent custom shops offer mandrel bending if you need to have custom exhaust components made to order to fit your vehicle and off-the-shelf systems are not available. Compared to crush or wrinkle bending, mandrel bending is the preferred method because it produces smooth consistent bends. In fact, it produces no abrupt changes in volume through the bend area: Optimum engine performance can be achieved. Pre-made mandrel-bent pipe or having custom pipe made with the use of mandrel bending is more costly, but in terms of both performance optimization and appearance, it’s worth it. You get what you pay for.

Manual Bending Machines

A hand-operated kit that features a selection of dies and backing shoes can manually bend exhaust pipe. Sheer brute force via a lever that pulls the pipe across the bending die does the bending. It’s the cheap but labor-intensive route. While manual exhaust pipe benders are adequate for accomplishing the basic function of bending a pipe at a given angle, the results are less than ideal for optimum results. Because there is no internal support for the pipe, manual bending results in kinks and crush areas on the inside of the bend. If you don’t care about pipe diameter uniformity or appearance, manual benders are certainly an option. The machines range widely in both cost and quality. Inexpensive imported tools can be obtained for as little as $200 to $700, but quality can vary widely. Be aware that there is a difference between tubing benders and pipe benders. Inexpensive tubing benders that do an adequate job are generally suited to small-diameter tubes up to around the 1/2-inch-diameter size. Cheap pipe benders available at discount hardware outlets are commonly intended for bending thin-wall pipes up to 2 inches in diameter.

To accommodate exhaust pipes, the bender must be able to handle your planned pipe diameter, which is likely in the 2.5- to 3.5-inch range. The least-expensive tools typically include a bending fixture (often designed to be secured on a bench vise) and a selection of “shoes” that accommodate various tube or pipe diameters. Manual benders are useful for the basic function of creating a bend, but you do not achieve a distortion-free inside radius. The inside of the bend features a crush where the inside of the bend tries to collapse slightly without being able to maintain a consistent diameter throughout the bend area. These relatively inexpensive benders are available from a variety of retail sources, such as discount tool suppliers. Quality varies greatly, and many are made overseas.

This tech tip is from the full book, PERFORMANCE EXHAUST SYSTEMS: HOW TO DESIGN, FABRICATE, AND INSTALL.

For a comprehensive guide on this entire subject you can visit this link: LEARN MORE ABOUT THIS BOOK HERE

Using a crush-type bender machine, the pipe is placed between the radius die and a pair of backing shoes. The radius die generally features a 180-degree throat, while the backing shoes are straight.

As the pipe is bent using hydraulic force, the backing shoes (placed on adjacent die arms, spread apart) create the desired angle.

During crush bending, the outside wall of the pipe is stretched, while the inside wall (or throat of the pipe) compresses slightly.

On a manual-bending machine, an index pointer is manually positioned along a degree scale for the desired bend. Once the pointer reaches the desired angle, the operator manually stops the bending process.

Other types of manual bending machines require manual setting of all centerline radius points and pipe angles. They provide hydraulic force to actually perform the bends. In this case, the term “manual” simply refers to the need for the operator to place the pipe in the correct position (lengthwise and angle) for the intended bend. Depending on the size, number of features, and quality this type of bender can range in price from $800 to well over $4,000. Again, without a mandrel to provide internal pipe support, you end up with a slight deformation on the inside radius of the bend. Some owners can accept a slight deformation or wrinkle to the pipes, but many high-performance engine builders and racers find this unacceptable. For manual bending, the radius die is the fixture that provides the bend angle. The backing shoes (sometimes also called wiper dies) provide a contact surface at the outside of the bend, opposite of the radius die. The pipe is captured between the radius die and the backing shoes. The backing shoes are located on adjacent pivots, which allows them to turn while following the bend provided by the radius die. While the operator must manually position the pipe for each bend and for each bend angle, a hydraulic ram provides the bending force.

Bending machines are capable of more than simply bending the pipe. Depending on the available dies, you can perform a number of other tasks, including pipe flaring, expansion, reducing, ball flaring, and more. When using dies and backing shoes without inside supportive mandrels to perform a bend, the inside of the bend distorts somewhat, slightly reducing the diameter along the sweep of the bend, with a slight bulge at each end of the bend. This is where the advantage of a mandrel bend really shines. Since a mandrel (located inside the pipe) supports the pipe walls during the bend, you avoid the slight collapse of the inside of the bend. If you want maximum performance and optimum appearance to maintain a consistent pipe diameter free of kinks, mandrel bending is the only way to go, and you’re simply not going to achieve this with a manual bender. If your goal is to obtain a smooth-flowing and visually appealing exhaust system, you have three choices: Buy an existing pre-made kit from a leading performance exhaust maker that is already designed to fit your vehicle, take the vehicle to a custom pipe bender who has a mandrel bending machine, or if you insist on fabricating your own, purchase pipe sections in a variety of shapes (straight, 45-degree, 90-degree, etc.) so you can piece your system together by trimming and welding. The use of ball-type flange connectors at the header collector exits help to adjust pipe angles.

CNC Bending Machines

A CNC pipe bender is highly prized for accuracy, repeatability, and production speed. Once a software program has been written for a given length of pipe, a straight section of the selected-diameter pipe is placed into the machine. Once the machine is set up, achieving the final pipe shape essentially involves pressing a button. The machine automatically feeds the pipe to a specified distance, performs a bend, turns the pipe in the required direction, makes the next bend, etc. The machine does the work while the technician monitors the job from start to finish.

CNC (computer numeric control) bending is accomplished by creating a profile program for the length of pipe, which then controls the bending machine. Once a pattern pipe has been developed, that pattern is digitally mapped in order to create the software program. Here, a technician at Corsa plots a pattern pipe using a coordinate-measuring machine (CMM). The pattern information is then sent to the CNC bending machine, where multiple pipes can be produced that precisely match the original pattern.

For high-production pipe designs (primarily for OEM applications), dedicated dies are sometimes used on CNC benders instead of selected-size radius dies. This allows the CNC operator to feed straight pipe into the machine, with all bends created automatically using computer-selected die forms as the pipe travels through the workpath. (Photo Courtesy Addison-McKee)

Here, custom application-specific dies are being used on a CNC bender. (Photo Courtesy Addison-McKee)

The exhaust pipe’s design is programmed for the CNC bender. A CNC technician verifies that the correct program has been uploaded and monitors the bending process both at the dies and on the monitor screen.

Once the straight section of pipe is positioned in the CNC bender machine, the automated bender is set to run.

The CNC bender feeds the pipe through the dies, automatically rotating the pipe for multiple bends according to the software program.

CNC bending machines are expensive and are primarily used by exhaust system manufacturers. The nice feature of CNC bending is its repeatability. The last pipe in a production run that’s formed according to a specific program is identical to the first pipe that was formed.

Pipe Shape

There’s no rule that says a pipe must be round. Specialty dies are available that allow an oval profile to be created. Why would you want an oval pipe? This shape is favored primarily when you have clearance issues. While an oval pipe can be made with the same cross-section volume as a round pipe, an oval pipe is narrower on one plane, which provides additional ground clearance for vehicles that have extremely low ground clearance (such as many NASCAR applications). This allows you to flow the same volume while providing a more compact installation (relative to undercarriage and ground). Oval pipes are also used on street rods where either ground clearance or appearance (or both) is an issue.

Exhaust pipe materials include mild steel that may or may not be coated and various grades of stainless steel. Some OEM systems that are advertised as stainless feature very low levels of alloys, and can rust, as seen here with a removed OEM “stainless steel” system. If you want a stainless steel system, you must turn to a quality aftermarket manufacturer.

There’s no law that says that all exhaust pipe must be round. Oval pipe that provides added ground clearance is readily available from race exhaust pipe makers. (Photo Courtesy Burns Stainless)

Oval exhaust pipe provides added ground clearance while maintaining exhaust volume. Oval-profile pipe is available in straight and angled sections. (Photo Courtesy Vibrant Performance)

In terms of appearance, oval pipes provide a more unique look to the exhaust system. If you choose oval pipe, you won’t be able to make your own bends, but as long as you can lay the system out using a combination of straight, 45-degree, and/ or 90-degree pipe, raw sections are available for you to trim to length and weld together.

Pipe Material and Coatings

Common materials include mild steel and various grades of stainless steel. Mild steel is the easiest material to bend, while stainless materials are a bit stiffer but still conform well when formed in a precision bending machine operated by a skilled technician. As you might suspect, material prices vary as well; mild steel is the least expensive and higher grades of stainless steel are the most expensive.

Mild Steel

Mild steel pipe requires a coating to resist corrosion. This can include aluminized pipe (where a molten spray of aluminum is applied to the outside of the pipe during manufacturing), galvanized pipe treated during manufacturing, or by the application of specialty coatings. These coatings can include high-temperature paint, high-temperature powdercoat paint, or specialized ceramic coatings. Of the many choices, ceramic coatings are the most durable. However, be aware that most ceramic coatings (or any coatings for that matter) are applied to the outer surfaces only. Although mild steel pipe lasts longer when coated, both in terms of function and appearance, mild steel can still degrade from the inside. All exhaust systems are exposed to moisture on the inside walls, simply because of the thermal changes that result from engine cold starts to warm up to cool-down.

Stainless Steel

Stainless steel, while still exposed to those same conditions, is much more resistant to both external conditions as well as internal sweating. However, there are varying grades of stainless steel. Many carmakers offer what they call stainless steel exhaust systems, when in fact this may be a very inexpensive “mystery mix” steel formula that features a very small percentage of alloy. As a case in point, in the late 1980s when my road race team ran Ford Mustang GTs in showroom stock endurance road races, the cars came from the factory equipped with shorty tubular exhaust headers that were advertised as being made from stainless steel. After one race, the headers turned brown and had quite a bit of surface rust. Although likely more durable than mild steel, the stainless material certainly didn’t live up to its name.

Aftermarket performance sources tend to use much higher grades of stainless, such as 304 or 321. These have much higher alloy content and do not rust. Some exhaust makers offer a choice of stainless grades, priced accordingly. A higher grade, 321 for example, provides higher fatigue resistance and is better suited for extreme temperature and vibration conditions such as turbocharging. In terms of function, even stainless steel can benefit from aftermarket ceramic coatings. Although stainless steel captures heat with less outward heat radiation than mild steel, applying a ceramic thermal barrier coating improves this even further. The less heat radiation, the more efficient the exhaust system becomes, in terms of thermal management and (at least in theory) engine performance.

High-quality stainless steel provides excellent heat management and resistance from rust/corrosion. In addition, a high-grade stainless steel can be fully polished to a chrome-like appearance.

To avoid appearance issues and to increase system longevity, either ceramic-coated system components or a 100-percent stainless steel system is required. The only real difference to consider is that a ceramic coating protects only the exterior of the pipes so long-term corrosion on the inside of the pipes can take place. A high-grade stainless steel pipe provides anti-corrosion protection on both inside and outside walls.

Naturally, the highest levels of heat exist at the exhaust headers, with exhaust temperature diminishing as exhaust travels through the rest of the system. For maximum performance, ceramic coating the headers is more beneficial than coating the remainder of the system. If you’re using a high grade of stainless steel in the entire system, you probably don’t need the rest of the system (after the headers) to be coated. If your application is a street toy and you’re using stainless steel headers, pipes, and mufflers the only reason to justify an additional ceramic coating is for the sake of color appearance. If the headers and the remaining system is mild steel, then coating (of any type) is definitely needed. At the very least, a combination of ceramic-coated headers, aluminized pipe, and aluminized or powdercoated mufflers make sense. From the standpoint of function, either mild steel or stainless steel are viable choices for just about any application, including street, drag racing, road racing, and off-road racing. The advantages of stainless steel include increased durability, reduced radiant heat, slightly lighter weight, and appearance.

Iconel

For a street application where long-term rust prevention is a concern, stainless is a better choice. For very extreme applications where exhaust heat and exhaust pipe stress is incredibly high, such as in Formula 1, Inconel is widely used because of its high-temperature fatigue resistance and resulting ability to withstand extreme heat levels. However, Inconel is very difficult to work with and generally costs about four or five times that of comparable-size stainless steel.

Titanium

Another exotic metal used in extreme applications is titanium, favored for its light weight and higher corrosion resistance compared to stainless steel. Again, cost is a factor; it is usually as much as ten times the cost of stainless steel.

Crossover Pipes

A crossover exhaust system is used with a V-type engine that has two opposing cylinder banks and a dual-exhaust system. A number of methods can be employed to improve engine exhaust efficiency (and to gain power) that involve connecting each bank of a dual-exhaust system.

An exhaust pipe crossover connects the driver-side and passenger-side pipes at an intersection point between the headers and the mufflers. This connection of the right- and left-bank exhaust can help to even out the engine’s exhaust pulses and can improve exhaust scavenging. During scavenging, the flow of each pipe helps to pull the exhaust through the adjacent pipe, which is much like two branches of a stream that join to form a river, and in this case the current velocity increases.

This system features an X-pattern crossover. It aids in balancing engine combustion pulses and improves exhaust flow scavenging. As the dual pipes merge, each engine bank’s pulses help to collectively pull exhaust through each pipe.

Here’s an example of an H-pipe crossover. Although it helps balance the engine pulses between the two engine banks, it does little to improve exhaust scavenging.

When designing a dual system to accept an H-pipe, or any crossover style, make sure that the crossover location doesn’t interfere with the vertical movement of the driveshaft. Notice that this H-pipe is located forward of the transmission output.

This X-pipe assembly was made from four separate pieces to achieve straight parallel inlets and outlets.

This X-pipe was designed with opposing entry and outlet angles. (Photo Courtesy Burns Stainless)

Merged crossovers are precision-cut for flow, and then carefully TIG welded to seal all seams. This results in a smooth flow transition with minimal exhaust flow turbulence, as opposed to a straight balance pipe that’s welded at 90 degrees to the main pipes.

Bending the two pipes together can form a crossover and an elliptical cutout on each pipe where they mate, merge, and are welded together. This can also be achieved by connecting the two pipes together by means of additional pipes in an H or X pattern. An H-pipe design simply features a horizontal pipe that runs left to right and connects the two pipes. An X-pipe design connects the two sides together with an added X-shaped pipe arrangement. A merged, or X-pipe, design provides a smoother flow than an H-pipe design because it has a higher degree of exhaust flow scavenging.

In some cases, the vehicle’s chassis and clearance issues may dictate the choice of crossover design. Generally speaking, any type of crossover merge (rather than two separate engine bank pipes) should improve horsepower and torque. Depending on the specific engine and the chosen exhaust design, a crossover connection can also have an effect on the exhaust tone.

This tech tip is from the full book, PERFORMANCE EXHAUST SYSTEMS: HOW TO DESIGN, FABRICATE, AND INSTALL.

For a comprehensive guide on this entire subject you can visit this link: LEARN MORE ABOUT THIS BOOK HERE

Converting to Dual Exhaust

Converting a single-exhaust system to a dual system only makes sense when dealing with an engine that features two cylinder banks (a V-style engine). Converting to a dual system provides increased breathing rather than choking both banks into a single exhaust path. This reduces exhaust restriction and greatly enhances bank-to-bank exhaust-pulse balancing. Beyond the performance advantage, switching to a dual system just plain makes the engine sound better. The reality is that, for a street machine, the exhaust sound is an important factor. Switching from a single to a dual can provide a performance advantage, depending on a host of variables including engine displacement and power output. In some cases, let’s say with engines under 300 hp, you may not gain any additional power, but you enhance appearance by having two exhaust tips instead of only one. That aside, you need to consider the following areas before jumping in.

Manifold Cap

The original single-exhaust system likely features cast-iron exhaust manifolds that cross over to each other via a downpipe, in which case the crossover from one of the manifolds must be capped off. Depending on your make, model, and year, these caps may be available in the aftermarket, or you need to fabricate one. In addition, one manifold’s outlet may be of a different diameter than the other manifold’s outlet. This can be addressed by either having your exhaust shop reduce or enlarge the pipe diameter of either side pipe where it mates to the manifold. On the other hand, you can purchase a pair of exhaust manifolds that feature the same outlet diameter, or you can move to a pair of tubular headers.

Material

In the majority of cases where someone is converting from a single- to a dual-exhaust system, an upgrade from cast exhaust manifolds to tubular headers is common. After all, the reason to switch from single to dual exhaust is to increase performance, so the decision to switch to headers is pretty much a foregone conclusion.

Pipe Hangers

Depending on the particular vehicle and the factory options that were available for that make, model, and year, the installation of a dual system may or may not have been planned by the automaker. This means that you need to determine your own locations for pipe hangers and you need to pay close attention to the space available for muffler installation. This can dictate the shape and size of mufflers that fit without interfering with the driveshaft, fuel lines, fuel tank, rear suspension, and rear axle, in addition to ground clearance considerations.

Aluminum and stainless steel aftermarket pipe hangers are available in a wide variety of configurations. This example from Stainless Works is attached to the pipe with a T-bolt clamp.

Fitment

In terms of muffler shape, an oval profile generally provides more clearance than a round profile. Again, this varies depending on the diameter of pipe that you’re running and the type of sound that you’re after. Another area of possible concern is the transmission crossmember. If the vehicle was originally equipped with a single exhaust, the transmission crossmember may feature only one hump for pipe clearance, in which case you need to install a double-hump crossmember to accommodate the dual system.

Make sure that you have adequate room for dual pipes; they must properly clear the fuel tank. If the fuel tank is slightly offset, you may not have enough pipe clearance on one side. This might require replacing the fuel tank with one that allows left-to-right centered mounting. In general, exhaust pipes should be at least 1.5 inches away from the fuel tank and additional clearance is preferable. If your vehicle model was never offered with a dual-exhaust system, you need to have the pipes custom bent at a competent exhaust shop. Also pay attention to the rear valance. If the rear valance panel features a single cutout for the exhaust tip, converting to a dual system likely requires notching the opposite side in order to achieve a proper fit and a balanced appearance.

If the original single-exhaust system featured a transverse muffler (positioned laterally under the car), you need to carefully select your mufflers to suit the new longitudinal positioning. If you simply do not have enough ground clearance to accommodate two mufflers mounted in-line with the pipes, an alternative might be to consider baffles installed inside straight sections of the twin pipes. The starting point begins with the headers because the termination of the headers at the collectors dictates where the pipes begin. The collector typically features a mounting flange that provides the attachment point for the pipe. Although a fixed mating flange at the front of the pipe may work in many cases, the angle provided by the installed header collector may shoot straight back, or it may angle slightly upward or downward, or left or right.

Connectors

To provide a degree of angle adjustment, consider the use of a ball-style connector. This provides a pivot point, allowing you to adjust the initial angle of your pipe to best fit your application. Tighten the bolts at the ball-style connector lightly; snug enough to hold position but loose enough to allow movement while you continue routing your pipes rearward. Again, depending on the specific vehicle, you should be able to run straight pipe sections from the collector rearward, keeping them somewhat parallel to the vehicle floor. Locate the best locations for your mufflers, in terms of fit and clearance, between the belly and driveshaft.

In most cases, you locate the mufflers forward of the rear axle and fuel tank. Make sure that the placement provides ample room between the rear of the mufflers and any routing obstacles, such as the rear axle and the fuel tank, to continue most conveniently with any required pipe bends. If at all possible, temporarily suspend the mufflers in the desired locations. This provides an easy target for routing the front pipes from the collectors to the mufflers, so you can determine pipe length and any required bends. At the very least, position each muffler in the desired location and place chalk marks on the belly to create matchmarks for muffler length reference.

Examine the required path from the collector to the muffler inlet. If you get lucky, you’ll be able to run straight pipe between the collectors and mufflers. If the path requires that the pipes angle inboard toward the center of the belly, the front pipes may require a slight bend inboard and a subsequent bend that regains a parallel run back to the mufflers. This is where the use of mufflers that feature offset inlet and outlet necks come in handy, as this can minimize or even eliminate the need to create any bends in the front pipes.

A stainless steel braided flex coupler with internal sleeve allows a pipe-to-pipe connection that reduces or eliminates stress between the two pipe ends. A flex coupler is useful in high-movement or high-vibration areas where a degree of “give” is required. (Photo Courtesy Vibrant Performance)

A V-band clamp assembly, with coupler flanges, is a three-piece arrangement. The coupler flanges are welded to each pipe end. With the two pipe flanges butted against each other, the groove inside the V-band clamp captures the pair of flanges. When tightened, the clamp provides a leak-proof and solid connection. (Photo Courtesy Vibrant Performance)

Rubber hanger isolators and a short flex coupler are being used on this 2008 Pontiac G8 GT. Use of a flex coupler eliminates suspected exhaust pipe stress points.

This style of clamp can be welded to the pipe. If you can imagine the style and shape of hanger desired, chances are someone in the aftermarket offers it.

If you’re piecing the system together on your own, keep in mind that ball-style connectors can often aid in achieving desired angles, although they add a bit of bulk along the path. As you put the sections together consider the use of band-style pipe clamps rather than U-bolt-style clamps. The band clamps don’t deform the pipe and provide added wiggle room while adjusting your system. In addition, band-style clamps provide superior joint sealing and make it easier to disassemble the system later.

Connecting each pipe to a muffler requires one of two approaches: welding or clamping. The pipe may slip into the muffler neck, requiring a pipe OD that matches the muffler neck ID. This is known as a slip joint. For instance, if the muffler features a 3-inch ID, the pipe must feature a 3-inch OD. The firm connection can then be made by either welding or with the use of a pipe clamp (again, I prefer band clamps). If the pipe OD and muffler neck OD are equal, a slip joint isn’t practical unless you use a pipe-expander tool to slightly enlarge the muffler neck diameter.

However, there are other ways to accomplish the task. With the two ends butted up against each other, a wide band clamp can be used to secure the connection, or if you prefer, the ends can be welded together. If you’re faced with two different outside diameters, a stepped band clamp can be used that features different diameters at each end.

Written by Mike Mavrigian and posted with permission of CarTech Books