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Motorcycle Exhausts 101

Apr 20, 2013

Why would a rider buy an exhaust? Is it something riders need? Something we want? Are they easy to install?

All of those questions have a variety of answers, and we'll try to address them in general terms in this Moto 101 tutorial. To keep it simple, we're going to focus exclusively on four-stroke engines. Unless you have a fairly old street machine or a dirt bike, odds are very good you have a four-stroke engine. Exhausts for two-stroke engines are quite different.

What is the purpose of the exhaust system, anyway?

Motorcycles are fitted with exhausts for a few reasons:

  • They route combustion gases away from rider and passenger. You and your passenger don’t want to suck down a carbon monoxide cocktail or burn your boots on a hot pipe, right? Exhaust pipes move exhaust to a more convenient place!
  • They muffle noise. Putting a muffler on an engine is a wonderful way to make it reasonably quiet. If you’ve ever been to the track, dragstrip, or your local muffler shop, you have some idea of how face-meltingly loud a vehicle is with no exhaust system.
  • They help the engine perform better. Believe it or not, bolting a simple tube on a motor can actually help it run lots better. There’s an amazing body of research dedicated just to exhaust in fields like thermodynamics and laminar flow, but suffice it to say that the right exhaust can help you go faster.

So why would I want to replace my factory system?

Your bike probably came with an exhaust system already on it. So why would someone remove a perfectly good stock exhaust system? There’s a bunch of good reasons to replace an exhaust:

  • Goin’ fast! Remember, the OE manufacturer had to meet a budget. That means that they might have been unable to produce the best motorcycle exhaust pipes possible for your bike at the price you purchased it for. They may have used a simpler or easier-to-manufacture design rather than the best design. They are constricted by emissions regulations and by having a broad base of customers who may all be doing different things with their bikes, leading to an exhaust that may be a compromise.
  • Goin’ on a diet! Replacing, for example, a big, steel, dual exhaust system with an aftermarket titanium unit with a single short muffler can yield 15 pounds or more of shed weight. Depending on your bike, that could be five percent of the total machine! When it comes to handling and performance, dropping weight in the right places can be as good (if not better than) increasing horsepower.
  • Goin’ to save some money! Dollar for dollar, it’s hard to replace a component that has as big an effect on the look, sound, and performance of your bike. Very few aftermarket parts do so much for such a low cost. An exhaust system isn’t necessarily cheap, but it is an excellent value. Aftermarket offerings are generally much more affordable than an OEM one, if your original one is worn out or broken.
  • Goin’ deaf! Depending on what you ride and what type of exhaust you install, you can really change how your bike sounds. A small-displacement metric cruiser typically experiences an auditory change that could be characterized as going from a sewing machine to a fire-breathing monster. Mildly buzzy sportbikes turn into an unstoppable cacophony when “uncorked.” While there are levels of sound available from quite tame to vision-blurring, most motorcyclists will tell you that the “bark” an aftermarket exhaust adds is pleasurable.

How do aftermarket performance exhausts improve my bike's performance?

There are several ways to make the engine do more work — enlarging it, making it rev faster, compressing the air intake, etc. For a more thorough explanation of these options, see Fuel Management 101.

The relevant approach when you're talking about an exhaust is to improve your engine's efficiency. Your engine can be thought of as a pump, moving a precisely metered mixture of air and fuel into itself, and moving burned air and fuel (exhaust) out. Improving that pumping efficiency is the most affordable way to pick up a few extra ponies. An exhaust that flows better will reduce the work the engine must perform when it expels exhaust gases, especially when coupled with improvements to the flow of air and fuel on the intake side.

What type of exhaust should I get?

Excellent question! What type of riding do you do? A heavy fella who carries loads of camping gear on his Harley bagger is going to want a vastly different exhaust than someone tearing up the track on a lightweight CBR.

Motorcycle exhaust systems typically come in one of two types:

  • Mufflers only (also called slip-on or bolt-on systems). This type of exhaust retains the factory headpipe(s) and replaces just the muffler.
  • Full system. This type of exhaust replaces everything from the head(s) to the back of the bike.

Most slip-on systems are typically (not always, but usually) designed to be the only thing that gets changed on your bike. The performance boost is not as great as a full system, but the initial cash outlay is reduced and installation is typically quite simple.

Most full systems need fuel management changes to work correctly. This will typically entail a jet kit for a carbureted bike, or a fuel controller for a fuel-injected motorcycle. This type of exhaust system is typically more complex to install than the mufflers-only system, but the reward is a greater power gain, and often a more aesthetically pleasant exhaust system.

To correctly select an exhaust pipe, we think several things are important. First, figure out your final goal with the bike. If you just want a little more rumble and a bit more pep, an inexpensive slip-on may be the way to go. If, however, you are planning on future headwork, camshaft changes, or different carbs, a full system will allow you to take complete advantage of all the aftermarket parts and work. Each modification, ideally, will be made with the other modifications in mind. A properly thought-out engine modification plan can make great gains with fairly modest expense. Similarly, ill-chosen parts can often cost lots of money and work poorly together. If you need help determining what parts play well together, get in touch with us!

If you are on a budget, but have lofty goals, some exhaust manufacturers offer systems that are “modular.” That is, you can buy a slip-on muffler now and replace the factory headers later, when budget and other modifications dictate, while retaining your slip-on muffler. Not all manufacturers do this, so if you think you might hot-rod your bike down the road, this is a great way to get your feet wet without breaking the bank.

What style should I buy?

There are many, many exhausts to choose from for Harleys and all kinds of street bikes. Here are some factors you might want to think about.

  • Configuration: Often, a factory four-into-two exhaust (four headers for a four-cylinder engine splitting to two mufflers) will be made aftermarket as a four-into-one system, because the exhaust can be moved almost as quickly through the less restrictive system and the excess weight of the second muffler is eliminated. Some engine types work better with one configuration over another. V-twins, for instance, typically give great torque gains when two-into-one exhausts are used. Some research or a call to our Gear Geeks is encouraged to help you get an exhaust that will meet your needs.
  • Material: Titanium and carbon fiber are becoming popular due to their low weight. However, their cost can be prohibitive. Aluminum offers some benefits of weight loss and lower cost. Stainless steel is ever-popular due to cost and durability. Chromed steel is often prized for its beauty and longevity. How fastidiously you maintain your bike’s appearance, and how you want it to look are going to play into your selection of material, as well.
  • Outlet type: Maybe a big, carbon fiber can is your look. Maybe you like old-timey cocktail shakers or cigar mufflers for your Brit bike. Maybe goose-cut turn-outs or fishtails are your style. Some aftermarket exhausts, especially for cruisers, can be fitted with a variety of easily swappable tips to give a different look. It's all about what you think looks good!

Should I run drag pipes or an open exhaust?

Maybe. If you’re competing at the drag strip or track, probably. If you’re anywhere else... probably not! Realistically, open or unmuffled pipes make horsepower high in the rev range, where the engine is expelling the highest amount of exhaust. However, because they do not promote good velocity in lower portions of the rev range, they tend to yield worse performance in typical street use. Tuning the bike is very difficult with drag pipes, and almost all open exhausts leave weird “flat spots” in the torque curve where power is delivered erratically, or can even decrease!

If you’re honest with yourself, you’ll probably find that wide-open throttle most of the time is not really how you ride on the street.

There are a great many federal, state, and local laws in the United States, and typically they mention unmuffled pipes as being problematic. Again, every place is different, but by and large, having at least a muffler is usually the minimum exhaust requirement you’ll need to meet for most vehicle inspections.

The fact is that the exhaust manufacturers spend quite a lot of time and money developing an exhaust product that will lend you the most horsepower and torque for your particular motorcycle without being overly obnoxious. Loads of dyno-tuning and testing back up their products. It is pretty rare to achieve a better all-around exhaust for street use than the ones the big players in the exhaust game have developed.

Will my bike pass inspection with an aftermarket exhaust?

Because states have different requirements (and some don't even require periodic inspections), your best bet is to get an answer from your local police department, inspection station, state department of motor vehicles or motorcycle shop.

How loud will it be?

Well, that depends on a few things. Nearly every aftermarket exhaust is louder than the factory exhaust. If you purchased an aftermarket Harley exhaust, it will likely be fairly loud. Some of the performance gain depends on not needing to silence the bike. If the exhaust is too loud, some manufacturers offer quieter baffles that you may install to bring the volume of the bike closer to stock.

If you purchased an exhaust for a sport bike, there is often a removable baffle in the tip of the muffler (commonly referred to as a dB killer) that can be removed or installed for similar manipulation of sound levels. Performance is usually unaffected by the presence or lack of these baffles.

I heard I need to change my fuel delivery setup, too — true?

The answer depends on a lot of things. What kind of bike you have, what kind of exhaust you have, and how you want your bike to behave! As a general rule, most slip-on or “muffler-only” exhaust systems do not require alterations in the fuel delivery, but even a bone-stock factory motorcycle will benefit from fuel management changes.

If you are purchasing a full exhaust system, most manufacturers strongly recommend fuel system changes. Effectively, the exhaust helps the engine breathe better, but without adding more fuel to compensate for the greater ease with which the engine gets air, the ratio of fuel to air becomes imbalanced. That is known as a lean condition, which can make the temperature of the engine catastrophically high. Couple this situation with the fact that recent EPA regulations have been causing manufacturers to set bikes up quite lean from the factory, and a plan for poor performance and longevity is afoot!

Typically, “richening” the air-fuel mix (more fuel per amount of air) is done by purchasing a fuel controller for a fuel-injected motorcycle, or jet kits for carbureted bikes. A dyno run and tuning session is often in order, so if you are not an experienced tuner, consider these costs in your performance budget.

Before you purchase your exhaust, you might want to consider a few things:

  • Engine modifications work with each other. The sum of carefully selected parts usually leads to greater performance gains than any part offers individually.
  • Modifying your engine is very personal choice. Changing your bike to perform differently than the factory intended is possible and often desirable, but the changes must be made thoughtfully.
  • If you do not want to change your fuel delivery method, you may be best served by a simple slip-on system made by a manufacturer that does not require a fuel system change.

How do I keep my pipes from turning different colors?

The honest answer here is that you don’t. All pipes discolor to some degree, due to heating and cooling, the presence of environmental contaminants like tar, oil, and road debris, and the material the exhaust was created from. Stainless pipes, for example, turn gold. There is nothing wrong with this. It is a property of the alloy. Cruiser pipes often “blue,” especially near the heads. “Bluing” is a result of how the bike is running, not a defective finish. Camshaft choice, engine timing, carburetion, and other factors all affect exhaust temperature, the biggest determinant in keeping exhaust the color it was intended to be.

These are some of the best ways we at ZLA know of to keep exhaust looking brand-spankin’ new:

Make sure your bike is in a good state of tune. Properly gapped spark plugs and proper jetting go a long way to keeping pipes looking great.

Buy double wall pipes. Effectively a pipe within a pipe, the outer visible pipe never achieves the high temperatures the inner pipe does, thus leaving it stain-free.

Buy pipes with extensive heat shielding. Some brands of exhaust come with heat shields so voluminous, they appear to be the exhaust pipe. However, since there is a layer of cool ambient air between pipe and shield, the shields never attain the temperatures the inner pipe is subject to.

A word about wrapping your pipes with fiberglass tape: Originally seen on the dragstrips to keep exhaust temperatures high to improve exhaust velocities out of the exhaust, pipe wrapping has become popular on the street, due to its unique look. It can and does tend to hold moisture near the metal, thus speeding the oxidation process. Most manufacturers consider wrapping pipes to be “abuse” and typically do not warrant problems stemming from wrapped pipes. Do it at your own risk!

How hard is it to install a motorcycle exhaust?

Well, it depends. What do you ride? How good a “wrench” are you? What kind of system are you installing?

The answers vary greatly. We’ve put bolt-on exhausts on a bike and gone riding on it in under 20 minutes, whereas some bikes require considerable time simply to remove the body panels necessary to gain access to the exhaust. Consider your mechanical aptitude and understanding of the exhaust system and the related systems requiring disassembly to access the exhaust. Take stock of your tools on hand, and your ability to follow directions.

If any of these seem like obstacles, don’t be afraid to call us and ask for a recommendation on how to attack your specific bike and pipe combo. Sometimes, an install from a competent shop for a fair price can give you peace of mind, while relieving you of any possible problems from an exhaust job that was tougher to tackle than originally estimated.

What about staying legal? What about C.A.R.B.?

Staying legal can be tricky. Ultimately, it’s up to each rider to follow the laws of the countries, states, and municipalities we live in, ride through, and visit. C.A.R.B. is the California Air Resources Board, and while not all riders fall under its jurisdiction, the laws it promulgates frequently have far-reaching effects for nearly all North American riders. 

What do I need to know before I order?

A few things:

  • Year, make, model, and engine displacement are imperative. Guessing leads to the wrong parts being sent out.
  • An exhaust system usually includes just the pipes. Re-using gaskets works sometimes, but not every time. Fresh gaskets are always a good idea.
  • Hardware is another component that often suffers, as are oxygen sensors. Typically made of mild steel, hardware can rust, bind up, or break due to the heat cycling it receives over the course of its life, and oxygen sensors are often damaged to the point of unusability upon removal. Be prepared to find or purchase parts that may break while you are working, or purchase them beforehand. A word of caution: Oxygen sensors can be removed with normal wrenches, but specially designed sockets and wrenches exist and they typically extract sensors unbroken with a much higher degree of reliability than standard hand tools. Consider one!
  • Once an exhaust shows signs of being mounted, it is not typically returnable. We do not send out exhausts that have previously been installed. Consequently, we cannot take a return of one in such a state.
  • You need to know how an exhaust will work with your bike. An exhaust system is an aftermarket modification. They are usually made for stock motorcycles, so if your rig deviates from stock, the onus of responsibility for determining if two components will “play nicely” together must rest with you. If you have a question or concern, call us. If we don’t know, we’ll find out.
  • In a similar vein, some exhausts require modifications to your motorcycle. You may need to relocate components such as oxygen sensors, and possibly remove componentry like high-mount centerstands. Sometimes, directions can be less than explanatory, and light fabrication work can be required.
  • Finally, parts often must be reused. Brackets, side stand bumpers, flanges, and clamps are often pieces that need to be pressed into service once again. If yours are missing or damaged, you may need to obtain them from your dealer prior to your exhaust installation.

Hopefully, you're now less baffled about aftermarket exhausts. (Ha! Get it? Baffled? I’ll be here all week.) If you enjoy a little bit of wrench twisting, some bark with your bite, and a little bit of “get-up-and go,” a new exhaust might be the hot ticket for falling in love with your bike all over again! If you’ve got any concerns, questions, or just thoughts for us, we love calls, and we’d love to talk about hot-rodding your bike.