Custom Motorcycle Exhaust - Technical Analysis

How To Select A New Set Of Pipes

Of all the possible upgrade options available to bike builders, no other modification will have as great a combined impact in altering the overall visual, power, and auditory personality of your bike as changing the exhaust. The trick is how to wade through all the exhaust b.s., understand the terminology and options, figure out what's best for you, and then choose a set of pipes with some assurance that they will fulfill the urge. Wouldn't it be nice if there were a simple, logical process for a bike owner to select new pipes? This is not an exhaustive technical article covering every aspect and component of how the exhaust system works or the finer points of tweaking an exhaust for any specific setup. This piece is one attempt at a process that includes the things a bike owner should consider and should know when choosing an exhaust system.

Selection Process TopicsSo what is this mysterious selection process, and what are the topics to which a bike owner should give at least a cursory review? Not surprisingly, the topics aren't magic, and they apply to virtually any project. The most important point is to carefully consider how the topic relates to your individual situation. That awareness will help you prioritize and balance the four basic selection elements of looks, performance, sound, and cost.

Here are the topics to bear in mind when selecting an exhaust system:* Your overall bike plan: Which performance and appearance changes do you want to see on your bike?

  • Exhaust 101: Know how the exhaust system works to judge upgrades.

  • Your budget: How much can you spend?

  • Final answer: Last-minute questions to consider before making the big decision.

Your Overall Bike PlanObviously, exhaust pipes don't operate alone. They are one part of the integrated complete powerplant for your bike and one design element for the overall look of your bike. Most owners have some strategy for their bikes. Therefore, when considering any exhaust upgrade, remember to include ideas for both performance and appearance objectives. It's also helpful to list all the parts you'll need and when you expect to install them. Matt Held, of Holley/Hooker Headers, described this planning process as "future-proofing" your bike. The basic idea is that with a plan, you can select pipes that are flexible, tunable, and tolerant of change so that they will continue to work with future components.

Performance PlanTo create a performance plan, it's best to use the industry's general "stages" descriptions as a guide. There is no industry-standard definition for each stage, but there are widely accepted specifications that will help you predict what components you'll need to achieve a desired horsepower gain within a given budget. The stages refer to gradually increasing horsepower performance from a stock engine.

Stages I, II, and III build on each previous level and are the most streetable combinations. Stage IV, on the other hand, is basically a "start over" approach because most of the early-stage stuff either can't be tuned up to extreme performance or won't survive at that level.

Stage I focuses on better engine breathing by getting more air and fuel in and putting less restriction on exhaust going out. The modifications require no internal engine changes, no special tools, and no training. In other words, almost any bike owner with a set of wrenches could upgrade his or her bike to Stage I. The components include low-restriction air filters, carburetors, ignition systems, and, of course, exhaust systems. Expect to gain from five to 10 hp over stock and spend from $500 to $1,500.

Stage II readily builds off the first level and continues to improve engine breathing. These changes require a couple of internal bolt-on components, which in turn require special tools and knowledge of engine mechanics. However, a knowledgeable owner with experience, time, and patience can perform these modifications, perhaps using a bike shop for advice and for any necessary milling or porting. The internal components include camshafts and ported stock or aftermarket heads. Stronger, adjustable pushrods and roller rockers could also be added. Expect to gain from 15 to 30 hp over stock and spend from $1,500 to $2,500.

Stage III requires the improved respiration of the earlier stages, because now you'll increase engine displacement and compression. These alterations go beyond simple bolt-on components and should only be performed by experienced mechanics with knowledge of high-performance engines, as well as the essential shop tools. Stage III parts include stroker kits (cylinders, crank, and pistons), new cases, and heads. Turbo- and superchargers could also be added. Expect to gain from 20 to 70 hp over stock and spend $2,500-plus.

Stage IV uses essentially the same parts as the previous stages-only the parts are bigger, meaner, stronger, and more expensive. This is serious racing, Pro-Stock territory with anything-goes, scratch-built engines. If you're in this category, then you either know a lot more than this article can tell you or you have a really good mechanic-or both. Horsepower is only limited by money (starting at $6,000).

Performance features to look for are step pipe design, ceramic or heat-management coatings and wraps, replaceable baffles, replaceable fiberglass cores, diffuse discs, reversion or torque cones, and tunable end caps. Many of these will also change the exhaust sound.

Appearance PlanIn addition to the performance plan, create an appearance plan that considers theme, color, chrome, and other styling accessories. Decide if you want your pipes to blend in with or stand out from the overall design. Remember, large mufflers or long pipes are positive enhancements when they hide unattractive (i.e., stock) rear wheels, rotors, and swingarms. However, if you have a gorgeous rear wheel, you may want slim mufflers or short pipes. Features to look for are replaceable heat shields, replaceable end caps, and slip-on mufflers.

Exhaust 101Next, you'll benefit from a little knowledge about how the exhaust works. Armed with that knowledge, you'll understand what your mechanic is saying, and you'll see through the marketing fog of the manufacturers' literature. Some questions to ask are: Which parts are included in the exhaust system? What does the exhaust system do? What are the performance variables? Which variables are controllable, and to what degree? How does exhaust interact with other engine parts? What really matters, and what doesn't? How do different exhaust options compare? Which options emphasize or diminish looks, performance, and sound?

Reference the schematic from a Harley-Davidson service manual (shown in Fig. 1) to visualize a typical exhaust system. This drawing illustrates a stock rear cylinder pipe. The front cylinder pipe would be very similar and would use the same basic parts. Starting at the neck where the pipe bolts into the exhaust port, the pieces include the exhaust-port gasket, C-clip retaining ring, mounting flange, header pipe, muffler clamp, and slip-on muffler. Also shown are a mounting bracket and a heat shield with worm-screw clamps. Not shown are such items as torque and anti-reversion cones, removable and replaceable baffles, and replaceable end caps.

Exhaust Operation and InteractionAs we see in the diagram shown below, the exhaust system is really only a length of pipe to conduct hot exhaust gases away from the exhaust port. The muffler's job is to quiet the loud combustion noise.

A tuned exhaust system can increase engine power over stock by getting more of the fresh air/fuel mixture into the cylinder. Since the potential power in an engine is determined by the amount of fuel available for combustion, then adding more fuel will increase potential power. The tuned exhaust adds fuel by doing the following: being more efficient in getting the spent gas out of the cylinder, thus creating more room for a fresh charge; and creating a low-pressure area at the exhaust port to help suck in more fresh charge during cam overlap.

PulsesTo begin our understanding of how the exhaust functions, visualize two pulses traveling down the pipe. Each exhaust stroke produces both a physical pulse of hot, spent gas and a sonic pulse of sound energy waves.

The gas pulse travels down the pipe at approximately 300 feet per second. It goes primarily in one direction-out-until it is dispersed into the atmosphere. Its important attributes are velocity and temperature. It has mass and inertia. The flow of the gas pulse is affected by changes in pipe diameter, length, shape, and thermal properties.

The sonic pulse travels at the speed of sound (between 1,300 and 1,700 feet per second in the hot gas) and easily outraces the gas pulse. The sonic pulse is readily reflected up and down the exhaust system. The reflections are called reversion waves. With velocity constant, its important attributes are amplitude and frequency. It has no mass. The sonic pulse is reflected by a cross-section change in the pipe, such as collectors, mufflers, or the end of the pipe.

As these pulses travel in the exhaust system, each one produces a zone of high pressure before it and a zone of low pressure behind it. Exploiting and controlling these pressure zones is a key component to exhaust turning.

ScavengingScavenging is when low pressure in the pipe at the exhaust port helps extract gas from the cylinder. The low-pressure zones in both the gas pulse and the sonic pulse can be used to scavenge, called inertial scavenging and wave scavenging, respectively. Inertial scavenging is strongest when gas-flow velocity is greater. Wave scavenging is most effective when the reflected sonic pulse arrives at the exhaust port while the exhaust valve is open. Sonic waves that arrive when the exhaust valve is closed have no direct impact but do help to prolong the low-pressure zone of the gas pulse for the next exhaust stroke.

Cam or valve overlap is that brief period of time at the end of the exhaust stroke and at the beginning of the intake stroke when the intake and exhaust valves are both open. Scavenging doubles up during this time because the low pressure in the exhaust pipe not only sucks the spent gas out of the cylinder-it also helps suck in more fresh charge through the intake.

The amount of time the valves overlap is a function of engine rpm. Therefore, timing the arrival of reversion waves also becomes a function of engine rpm. Thus, exhaust performance peaks at a specific rpm band. Designers use several techniques to spread performance over a wider rpm range but never maintain maximum (or peak) power from low to high.

Backpressure, Velocity, and VolumeBackpressure is one of the more debated and misunderstood topics concerning exhaust systems. Some say you should eliminate backpressure for a free-flowing system and maximum power, while others say you should keep some backpressure to prevent over-scavenging (drawing fresh air/fuel into the exhaust) and loss of power. Let's define backpressure and see how it impacts the performance of the exhaust.

Backpressure is the resistance in the pipe to the flow of gas. As mentioned earlier, the engine must work to push exhaust gas down the exhaust system. That work is spent overcoming this resistance. Obviously, the greater the backpressure, the more work is required to oppose it, and so the less power is available to move your bike. Conversely, the lower the backpressure, the less work is required to oppose it; therefore, the more power is available to move your bike.

If that was all there was to it, then the free-flowing experts would win, and we'd all have big-diameter, smooth, straight drag pipes.

Ahh, but there's more. There are two reasons to reduce backpressure: to recover the power lost overcoming resistance, and to increase flow velocity. Lowering the backpressure by smoothing the inside of the pipe, by eliminating restrictions, and by removing obstacles not only lowers the resistance to the flow but will also increase flow velocity. Or, if you really want to be precise, lowering the backpressure decreases the rate at which the flow velocity decreases. The gas always slows down because the pipe isn't frictionless; with lower resistance, it just doesn't slow as fast or as much. For our layman's discussion, we'll leave it simple. But then, what has flow velocity got to do with exhaust performance? Everything. And you can't consider flow velocity without also considering flow volume.

A higher flow velocity will produce quicker throttle response and more torque in the low-to-middle-rpm range. Greater velocity increases the high- and low-pressure zones surrounding the gas pulses and thereby improves scavenging. A higher flow volume will produce maximum power in the middle-to-high rpm range. High volume is required at the high end because much more gas must move through the engine. Intuitively, then, we tend to agree with our free-flowing experts again and see that the best exhaust system will minimize backpressure to maximize flow velocity and flow volume. Unfortunately, this is where the compromises and the misunderstandings begin, because the laws of physics say that the same pipe can't do both.

In addition to backpressure, pipe diameter is a major factor in determining both flow velocity and flow volume. Think about blowing through a straw. With a small straw, it's hard to blow, but your breath comes out really fast. With a large straw, it's easier to blow through, but your breath comes out more slowly.

So here's the dilemma. Choose a large-diameter pipe with minimum backpressure and large volume to get tremendous high-rpm power, but the slower flow velocity will rob the low end of torque. The solution to these flow dynamics is to pick a design compromise that offers satisfactory throttle response, torque, and power over the entire rpm range.

As for the debate about the need for backpressure-it's not whether some is needed or not; it's that in the overall design compromise some backpressure is always left. That's misinterpreted as a need for backpressure.

Header-Pipe ConstructionSmoother is better. Gas moves like a fluid and will flow better if it's treated to gentle, well-rounded turns and smooth surfaces. Fortunately, most of today's motorcycle manufacturers, unlike auto exhaust makers, have not resorted to the use of low-cost, thin-wall, compression-bent pipe in their stock exhaust. While compression bends are easy to mass-produce, they can collapse or crimp the pipe in the bend, often cutting the exhaust flow in half. The better exhaust systems use mandrel bends, which are made by a pipe-bending machine that uses a noncrushable insert or stiffener (the mandrel) that goes into the pipe while bending to prevent the pipe from becoming crushed.

Pipe diameter impacts both backpressure and flow velocity. It also impacts gas temperature. Exhaust gas is hot, and we'd like to keep it hot. Cold air is denser, and therefore heavier. The heavier the air, the more work the engine does to push it out the pipe. A large exhaust pipe will slow the exhaust flow, giving the gas time to cool. Coating the pipe with an insulating material, such as header wrap or a ceramic thermal coat, also helps keep the gas hot. An added bonus in moving hot gas away as quickly as possible is that it helps cool the cylinder.

A stepped header pipe offers a slight compromise on the small-versus-large-diameter pipe dilemma. The first few inches of the tube are small diameter, which is then mated to a slightly larger-diameter tube. This creates a Venturi effect that maintains high flow velocity at the exhaust port, then allows the higher gas volume to expand farther into the pipe.

Does the length of the exhaust pipe play any part in tuning an exhaust system? Yes, it does. The end of the pipe is a major reflector of the sonic pulse. As the pulse moves down the exhaust and surges into the ambient atmosphere, a reversion wave is created that travels back up the pipe. The length of the pipe is calculated so that the reversion wave arrives back at the exhaust valve before the valve closes, thus allowing the reversion-wave low-pressure zone to aid in scavenging spent gas from the cylinder.

The MufflerThe muffler is a basic component of the exhaust system. Its sole purpose is to silence or muffle engine noise. However, in general, mufflers achieve sound dissipation or absorption at the expense of performance. To make a typical stock engine quiet requires a muffler that has several internal structures that slow and cool the hot exhaust gas and absorb sonic energy. These create restrictions that generate backpressure. To work, mufflers use two similar designs with either a smooth or baffled inner core, along with several techniques for absorption and reflection.

Most high-performance mufflers use a smooth, straight-through inner core. Inside the body of the muffler is a straight pipe perforated with many small holes. Wrapped around this inner pipe is a heat-resistant padding such as stainless-steel mesh, ceramic fibers, or fiberglass wadding. As the exhaust gas passes through this pipe, sound energy enters the holes and is absorbed by the padding. The flow of the gas is unrestricted.

Baffled-inner-core mufflers can have a wrapped, straight pipe inside the body, but instead of small holes the pipe will have louvers that stick down into the exhaust flow. These louvers direct sound energy and exhaust gas into the padding. They also create turbulence to diffuse the sound.

Baffling can also be achieved with inner chambers, which redirect the exhaust flow and reflect the sound energy inside the muffler body. The spacing of the chamber walls is designed to cause similar frequency waves to collide, thus canceling one other. These mufflers may also use absorption materials.

On some exhaust systems, the inner cores, whether smooth or baffled, are removable and the padding replaceable. In addition, some manufacturers offer variable-flow devices, such as adjustable disks, which allow the owner to modify the exhaust characters for sound and performance.

Other stuffTorque cones, power cones, monster cones, and anti-reversion cones all refer to similarly designed diameter-reduction devices that usually are placed near the exhaust flange to restore low-rpm throttle response, power, and torque on straight drag pipes. Torque cones create a Venturi effect that increases flow velocity as the gas passes through the narrow part of the cone. Anti-reversion cones are designed to diminish and disrupt the negative reversion waves. Step-pipe headers can also act as anti-reversion devices.

CollectorsA collector is unique to two-into-one-design exhausts and refers to the joint where the two header pipes converge. The header pipes must come together in a smooth taper to maintain high gas velocity, while the single exit pipe must be large enough to accommodate the combined gas volume.

A collector generates power in several ways: First, by simulating the end of the exhaust pipe, it can create reversion waves. The timing of those waves depends on how far down the header pipes merger. Second, by interleaving the exhaust pulses, the pressure waves are shared between the two header pipes. This tends to enhance and multiply the effect of each. Next, by creating a Venturi effect as the gas expands into the larger collector, the lower pressure acts to pull gas from the header pipe. Finally, a collector can act as an anti-reversion device to stop unwanted reversion waves from the end of the pipe.

Collectors usually spread performance improvements over a wider rpm range and often create more top-end power. The crossover pipes found on many stock exhausts improve performance much the same as a collector.

Rules of ThumbNo exhaust system delivers maximum peak performance in all applications across the full rpm band. All systems compromise performance in one area to deliver better performance elsewhere.

Since this is a layman's guide, we'll skip the formulas and list a few general rules here:

  • Minimize backpressure and maximize flow velocity, then work backward from there.

  • Small-diameter pipes increase low-rpm throttle response and torque but choke at high rpm. Conversely, large-diameter pipes increase high-range maximum power but gasp at low rpm.

  • Short pipes make more horsepower. Long pipes (of the same diameter) make more torque.

  • Collectors, i.e., two-into-one exhausts, are good for widening power across the rpm range. If not collectors, then crossover tubes.

  • Loud doesn't make power. And mufflers are good-for your neighbors.

Exhaust Upgrade OptionsExhaust systems can be organized into four purchasing options: stock, slip-on, full bolt-on, and custom. Stock, of course, is whatever comes with the bike from the original manufacturer and represents the base or no upgrade. Slip-on refers to exhaust components that replace only part of the stock system and most often are slip-on mufflers. Full bolt-on systems are, as the name implies, a complete replacement for the stock system and require no fabrication. Custom, on the other hand, also replaces the entire stock set but is completely fabricated from scratch or pre-formed components. Each of these options has varying strengths and weaknesses with regard to the four basic selection elements of looks, performance, sound, and cost.

For example, when comparing looks custom is the best because, as custom pipes, they can be built precisely to satisfy the exact appearance an owner desires. When comparing performance and sound, then full bolt-on is best for the average bike owner because those sets are designed by the aftermarket manufacturers to increase power and enhance appearance for a specific motorcycle model. It could be argued that custom should also be best for these elements; however, many custom sets are unmuffled straight pipes designed primarily for looks. Stock is the obvious winner on cost because those pipes are already paid for. A full bolt-on replacement set is the best compromise across all the elements.

Your BudgetThe fact is that only you can decide how much to spend on your bike's exhaust. But now that you have knowledge and a plan, you should have a good idea of either how far your money will go or how much performance you can buy. Remember the compromise thing.

Final AnswerA stock exhaust is the original manufacturer's compromise among looks, performance, sound, and cost. The goal of the OEM engineer is to design an exhaust that appeals to the broadest range of buyers, delivers an acceptable amount of power, keeps the engine quiet and in compliance with governmental regulations, is cheap to produce, and lasts throughout the warranty period.