By Jim Kerr
With summer weather, household windows are open to let the fresh air in. Unfortunately for many, it also lets in the noise of traffic, especially those loud raspy aftermarket mufflers found on tuner cars and sports motorcycles. Do these mufflers really offer any advantages, or are they there to make a statement? (One that annoys many!)
What is the real purpose of a muffler? Car manufacturers spend a considerable amount of time developing exhaust systems that maximize fuel economy and power while maintaining a quiet noise level. Exhaust gases exit the engine in pulses as each cylinder fires. These loud percussive waves travel through the exhaust and if unchecked, are transmitted out the tailpipe where our ears pick them up. The muffler is designed to even out the flow or pulses of these exhaust gases so they don’t impact so heavily on our senses.
Some sound frequencies are much more pleasant than others. A low rumble may be noticeable but usually doesn’t bother us. A high-pitched wine or snarl is much more bothersome because it interferes with our normal communication frequencies. An exhaust doesn’t have to be loud or bothersome to increase performance. Just ask many of the local stock car club drivers, who have been regulated into using mufflers on their race cars because of their proximity to housing developments. Not only do the mufflers make the cars quieter, it makes them more powerful too!
To accomplish this, the muffler must allow for the expanding hot gases to change their path so as to break up the sound pulses. If you look at factory mufflers, they are huge! The larger the volume of the muffler, the more performance is available from the vehicle because there is room for the gases to expand, which reduces backpressure in the exhaust system. This is something that many aftermarket mufflers don’t accomplish, because they are designed to fit in compact areas as an add-on device.
To redirect the pulses, there are many concepts in muffler design. Some use the maze method, which causes the gases to pass through many chambers in the muffler before exiting. Others use simple deflectors, which produce less back pressure but don’t break up the pulses as well. Flow-through designs may have straight passages with many perforations in them that allow the exhaust pulses to expand into a filter or sound trap. The cheapest of these use fiberglass wrapped around the perforated core, while better ones use steel mesh. This type of muffler may at first appear to be better at letting exhaust flow through, but other designs often can produce more power with less noise. The noisiest types of mufflers often have several plate baffles at the exit and the number of plates can be increased or decreased by the owner in a few minutes. Sometimes called “jam cans,” these mufflers can be very loud and are as much about making a statement about the vehicle’s owner as they are about controlling exhaust noise.
Today’s vehicles also have a catalytic converter in front of the muffler. The catalytic converter has numerous passages that let exhaust gases through while creating a chemical reaction to convert them into something less harmful to the environment. There are many who think converters rob power from the engine, and the first units decades ago did, but the ones used today allow almost as much flow as an open pipe would. The converter also helps break up the sound pulses, so the muffler has less work to do.
There are aftermarket “cat back” systems (they bolt on behind the catalytic converter) that are designed to improve the performance of your vehicle. These systems feature mandrel bends — smooth radius bends — in the tubing and freer flowing mufflers. If your factory system was restrictive, the cat back systems can help power but typically only at higher engine speeds or loads where there is more exhaust flow. A louder system may be the downside of any aftermarket system.
Do mufflers have to be noisy to perform? No. Can aftermarket systems be quiet? Yes, if selected properly. I like performance as much as any driver, but vehicles with noisy exhaust systems tarnish the image of all enthusiasts and lead to increasingly restrictive noise bylaws. Let’s be good citizens and keep noise levels low.