Loud motorcycle noise can prove exhausting

BY Jim KerrAUTO TECH. Nov 03 04:00 am

This summer I visited one of the world’s largest collections of unmuffled motorcycles — the Sturgis, S.D., motorcycle rally.

Most of the motorcycles were Harley-Davidson models, but there were many other makes too. To be fair to Harley, their bikes are the most popular at the rally and originally come with a fairly quiet exhaust system.

Many owners modify them. Some claim that “loud pipes save lives”. True, you do notice them more, but there are just as many on the street with quiet exhaust systems that seem to survive. There is one thing about loud exhaust on any vehicle that many fail to consider — they are tiring. Whether it be on a motorcycle, automobile or even an airplane, loud noise over an extended period of time wears on the senses, making drivers and passengers less alert.

Automobile exhaust systems were originally designed to carry noxious fumes away from passengers. The same still holds true today. Exhaust systems exit behind the passenger compartment so that fumes and poisonous gases are not drawn into it. Inspecting exhaust systems for leaks should be done yearly, preferably before winter when cars are idling more and windows remain closed. Carbon monoxide in the exhaust is a colourless, odourless gas that can kill. It sneaks up on you by making you drowsy and unaware of what is happening. The next step is sleep and unconsciousness.

Many provinces have mandatory vehicle inspections. Some are done every year. Others require it only when a vehicle is imported from out of province. A body integrity check is part of the inspection. They look for panels rusted through where exhaust fumes could leak into the interior.

Any open holes or rusted panels that would allow carbon monoxide into the interior will fail the inspected vehicle until it is repaired.

Catalytic converters, part of the exhaust system, have reduced a lot of the carbon monoxide gases by converting them into carbon dioxide (CO2). The converter uses precious metals such as platinum and rhodium to create a chemical reaction in the exhaust gases.

Hydrocarbons (unburned fuel and oil) are broken down and combined with oxygen to form CO2 and H2O. Oxides of nitrogen are broken down to form nitrogen and oxygen.

The original catalytic converters used to be fairly restrictive to exhaust flow and hindered engine performance. Today’s converters use thousands of small passages, as many as 900 per square inch, to allow gases to flow freely. There is now as much restriction caused by the bends in the exhaust pipes as there is in the catalytic converter.

The converter also acts as a muffler by breaking up the pressure waves created by the engine’s cylinders firing. The Harley motorcycle exhaust sounded so noisy because there were only two cylinders and the exhaust pulses are very noticeable. Smooth out the pulses, by either adding more cylinders to create more pulses, design an exhaust system with tubing lengths that will create even pulses, or breaking them up by routing them through a series of passages or through the rotating impeller of a turbocharger, and you’ll have a quieter sound.

Mufflers are the most frequently replaced part of the exhaust system. Water rusts the muffler from the inside out. One day it may look good — the next day it has a big rust hole in it. Tapping on the muffler will usually detect weak spots where it’s about to fail. Short trips don’t allow it to get hot enough to evaporate the water, so city-driven vehicles usually need new mufflers sooner than highway-driven vehicles.

The muffler quiets the sound by routing exhaust pulses through several channels and breaking them up. Some manufacturers such as Nissan have used a spring-loaded flap inside the muffler of some models that will open up the passages to increase exhaust flow during full-throttle acceleration. Porsche offers an optional exhaust system that uses an electric control to open the muffler for higher performance, and others have used vacuum controls to reroute the exhaust for higher flow.

Aftermarket high-performance mufflers have fewer baffles inside to increase exhaust flow, but they also increase interior and exterior noise levels.

Noise is tiring, so a quiet exhaust is more pleasant for everyone. The best designs are capable of offering low restriction to exhaust flow and a quiet sound. Chevrolet’s Corvette is a great example of how modern design accomplishes this.

james.kerr@sasktel.net