Exhaust Pipe Fundamentals - Exhausting Fundamentals | Baggers

Exhaust Pipe Fundamentals - Exhausting Fundamentals

More Than Hot Air...

More Than Hot Air...

The engine's exhaust system plays not only a vital part in the looks and sound of your V-twin but also its performance. Although a free flowing exhaust system and air cleaner are typically the first two components replaced when improving performance, no component arouses more passion from a bike's owner than the exhaust system because the exhaust makes a dual statement through its sound and visual identity. Riders want a throaty-sounding exhaust note and outstanding attention-grabbing looks along with high performance over a wide rpm range.

Nevertheless, buying an exhaust system can be an intimidating experience because tradeoffs exist between looks and performance and because there is a plethora of designs. Pipe length, diameter, radius of bends, muffler volume and internal baffle design affect performance. If a pipe is too long, too short, too small or too big in diameter for a given engine size, cam timing and rpm range, performance will suffer. Bigger engines generate greater exhaust volume and require a higher-flowing system. Moreover, engineering an exhaust system for the street is more difficult than for racing due to the requirements of a wider power band and aesthetics. Furthermore, there is a staggering array of pipe designs, from traditional 2-into-2s to 2-into-1s. Header pipes come in both big and small diameters, while some are even stepped with multiple diameters. Then there are long and short pipe lengths. And large-displacement engines need a high-flowing muffler for stout high-rpm power.

Clearly, pipe choice can be confusing and time-consuming. Although many riders select an exhaust system based on only sound and looks, it is important to note that for optimum performance, a pipe must be matched to the engine combination and most importantly rpm range. Therefore, if you are into performance, a fundamental understanding of the exhaust process will provide valuable information for selecting your next exhaust system.

2-into-2 exhaust system

Staggered duals or 2-into-2 exhaust systems work well, are easily tunable and relatively low cost.

D. William Denish

System Fundamentals

For optimum performance, the exhaust system must be matched to the engine's induction system, cylinder heads and camshaft timing. These components should be tuned together as an integral system for maximum performance within a specific rpm range. If one component is changed or modified, the entire group of components must be retuned for maximum performance.

An optimized exhaust system achieves a balance of pressure between the engine's intake and exhaust tracts within a given rpm range. For a street engine, you want optimized torque in the low and midrange (2,500-4,500 rpm) for excellent acceleration and highway cruising along with decent power on the top end. However, every pipe design is a compromise. For example, if a pipe is designed for only bottom end torque, it will give up top-end horsepower and vice versa. Racers building large-displacement high-horsepower engines often design a pipe for top-end power to reduce low-end torque so the bike will launch easier, resulting in faster acceleration. An exhaust system is only efficient through a narrow range of the engine's entire rpm band, so priorities must be set and compromises made to achieve the desired performance characteristics. Major exhaust system components include a mounting flange, head pipe, muffler and sometimes a merge collector. The diameter, length and overall design of these components will have a major impact on engine performance.

crossover pipe

Some Harley models use a crossover pipe that, essentially, increases muffler volume for each cylinder because the cylinders share both mufflers. Moreover, a crossover can improve scavenging due to the pulsing in the opposite pipe.

D. William Denish

Pipe Diameter

Pipe diameter is critical for optimized performance because diameter has a major effect on exhaust gas velocity and cannot be changed once the pipe is fabricated. Together, engine displacement, compression ratio, valve diameter, camshaft specifications and rpm band determine the optimum diameter. Exhaust backpressure will increase if the pipe diameter is too small. Backpressure is the flow resistance created in the exhaust system. High backpressure increases the engine's pumping losses, resulting in increased pressure on the piston during the exhaust cycle.

Additionally, high backpressure reduces low-lift exhaust flow during the "blowdown" period. Blowdown is the phenomena of expanding exhaust gases helping expel combustion residue from the cylinder and begins when the exhaust valve opens. Blowdown refers to how efficiently combustion residue is expelled from the cylinder by expanding exhaust gases. Blowdown begins when the exhaust valve opens and ends when cylinder pressure and exhaust system pressure are equalized. Using blowdown to help remove exhaust gases reduces the engine's pumping losses because less physical demands are placed on the piston during the exhaust cycle. The ideal situation is to have a balance between backpressure and exhaust gas velocity. An excessively large pipe diameter will decrease backpressure but also decreases velocity, resulting in poor bottom-end-torque.

rear cylinder header

The rear cylinder header on pre-2009 touring models drops down the right side and dumps into the front-cylinder header. It also Ys off, making a hard radius toward the left side under the seat.

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Pipe Length

Pipe length is determined by the engine's application (touring, hot street, race, etc.) and rpm range. Pipe length regulates inertia and wave tuning, which establish the effect scavenging has on power production. Scavenging uses a column of fast moving exhaust gases (inertia scavenging) or a supersonic energy pulse (wave scavenging) to help cleanse combustion residue from the cylinder. Inertia and wave scavenging also can assist the intake charge into the cylinder. During engine operation, positive and negative waves are created in the exhaust system and travel back and forth throughout the length of the pipe. If pipe length is optimized, the negative wave will be timed to arrive at the exhaust valve during the valve overlap period. A properly timed negative wave will reduce pressure at the valve and help scavenge combustion gases from the chamber. The engine's most important rpm band must be identified so pipe length can be matched to the proper rpm because pressure waves can only be timed to help exhaust scavenging over a narrow rpm range. A longer pipe length optimizes power at low rpm while a shorter length improves upper-end performance.

Because an exhaust system can only be optimized over a narrow 1,500 to 2,000 rpm range, it is important to tune pipe diameter and length to the engine's most critical rpm band. For a street engine, it is recommended to tune pipe dimensions to peak torque rpm (usually 3,000-4,000 rpm). For racing, engine builders often tune the pipe between peak torque rpm and peak horsepower rpm or up-shift rpm, which often occurs between 4,000 and 5,800 rpm. Larger pipe diameters and shorter lengths improve high rpm operation, while smaller diameters and longer lengths favor bottom-end power.

rear cylinder header

This photo shows how the rear cylinder header on pre-2009 Touring models travels under the seat and downward to the muffler, engulfing the rider and passenger with exhaust heat.

D. William Denish

Stepped Pipes

Header pipes can be stepped for improved performance. Stepped headers are divided into two or more pipe sections (usually two or three) with each successive section at least 1/8-inch larger than the previous section. A stepped header generates more low-pressure waves than a non-stepped design, but the waves are weaker. Steps help maintain a higher average gas velocity over the total length of header pipe. A stepped header doesn't necessarily make more power than a non-stepped pipe, but it can broaden the engine's torque curve by widening the scavenging wave's effect. This can result in high torque at low rpm while maintaining high horsepower at high rpm.

right-side pipe

Notice how carboned the right-side pipe is on this pre-2009 Touring bike. That shows that most of the exhaust flow is being routed through the right-side muffler, resulting in uneven flow through the exhaust system and reduced performance.

D. William Denish

Mufflers

An exhaust system must have sufficient muffler volume to keep backpressure low at high rpm. Engine displacement, compression ratio, rpm, and horsepower are all factors determining adequate muffler volume. Typically, muffler volume should be roughly 10 times the cylinder volume to make adequate high-rpm power. But keep in mind that as horsepower increases, exhaust gas volume also increases. With increased exhaust gas volume, muffler airflow, and volume must also be increased. That means a 96ci engine producing 100 horsepower generates more exhaust gases than a similar engine producing only 90 horsepower and requires greater muffler capacity for optimized top-end power. Unfortunately, large mufflers are not aesthetically pleasing on the V-twin, so it is challenging to design an exhaust system for large-displacement engines that satisfies both aesthetics and performance.

Two-into-two exhaust systems use two mufflers, offering the potential for increased muffler volume. Such designs also are usually tunable through modifications of the internal baffles. Increasing the number and/or size of the holes in a baffle or shortening the baffles reduces backpressure and can help top-end power. Still, remember that increasing flow too much can kill bottom-end torque. Additionally, a tunable 2-into-1 system offers a big advantage over a non-tunable collector system, especially if the engine is large.

2009 Touring model

Notice on this 2009 Touring model that the rear header sweeps forward then curves back, finally merging with the front pipe near the transmission.

D. William Denish

Collectors

A collector system is a 2-into-1 exhaust that terminates or merges both the front and rear cylinder header pipes into a tapered collector. Collector systems typically increase torque below peak torque rpm, thereby improving low and midrange power. Generally, the longer and/or the smaller the diameter of the merge collector, the lower the rpm where peak torque occurs. Additionally, a long slow tapering collector effectively spreads the reflected wave over a wider rpm band. Essentially, it fools the engine into thinking the header pipes are longer than they actually are. On the other hand, a short and wide-angle collector generates the strongest return pressure wave, but works over a smaller rpm range. The merge angles of the header pipes and the internal design of the collector can also have an affect on power output.

2009 Touring Model

Shown is the left side of a 2009 Touring model. Notice how the exhaust crosses over under the frame before making a 90-degree bend and connecting to the muffler. That translates into less heat on the rider and passenger, not to mention fewer pipe connections to leak.

D. William Denish

2009 Touring Models Exhaust

The 2009 Harley Big Twin Touring models have a much-improved two-piece frame, beefier swing arm, fatter tire out back and a redesigned exhaust system. The factory noted complaints about the rider and passenger being subjected to excessive engine heat on pre-2009 Touring models. On these models, the rear cylinder's header pipe was routed behind the transmission and down the bike's right side before joining with the front pipe. Additionally, just below the seat, the rear pipe Y-ed off and made a sharp radius to the bike's left side, traveling over the primary case, eventually mating with the left-side muffler under the saddle bag.

Rinehart True Duals system

The Rinehart "True Duals" system uses a separate header pipe and muffler for each cylinder, providing a balanced flow for both cylinders. Notice that the rear header does not connect to the front-cylinder header.

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For 2009, the rear pipe is radiused forward and away from the seat, bending in front of the transmission on the bike's right side, then following along the bottom of the frame rail into a merge collector. The front pipe also has a new design for 2009. It still runs under the engine's gear cover but now goes under the rear pipe and eventually into the merge collector. The collector has two pipes exiting from it. One is a crossover pipe that travels under the frame from the bike's right side toward the left. It then makes a 90-degree turn toward the rear and connects to the left-side muffler. The second pipe exiting the collector runs rearward and connects to the right-side muffler. Check the photos for more details.

S-bend pipe

Ideally, pipe length should be the same for both cylinders. Some companies use a large S-bend design coming from the rear cylinder to maintain similar pipe lengths.

D. William Denish

The new 2009 Touring exhaust system not only reduces heat to the rider and passenger but also should improve performance because the collector should even out exhaust flow through each muffler while providing better scavenging. With pre-2009 Touring exhaust systems, roughly 3/4 of the exhaust was directed through the right-side muffler.

Vance & Hines Pro Pipe

The Vance & Hines Pro Pipe is a proven 2-into-1 design that makes excellent power and is available with replaceable baffles so exhaust flow can be tuned to the engine combination. Two-into-one designs generally increase torque below the engine's peak torque rpm for improved low- and midrange power.

D. William Denish

Conclusions

Although most riders buy an exhaust system based on sound and attention-grabbing looks, remember that for optimum performance, pipe diameter, length and design are critical. Consider the exhaust system an integral engine component that should be tuned to the engine displacement, cam and induction system. Header diameter normally is the most important factor for exhaust system design because it sets the torque curve. Larger diameter improves top-end power at the expense of low-end torque. Changing pipe length moves the torque curve either up or down the rpm band. A shorter length generally improves top-end horsepower while a longer pipe increases low-end torque. Straight pipes typically improve power above 4,000 rpm but reduce throttle response in the lower rpm ranges. Finally, if a key component or specification such as displacement, cam, induction tract or combustion chamber is changed, the engine may require a different pipe design and should be retuned for best performance.

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