Fast & Quiet(er): Race Muffler Design and Function

Racing and mufflers are two words that don't seem to fit together. However, sometimes they are forced together in a shotgun wedding of sorts. While some racers love to hear engine noise, there are neighbors near racetracks who have other opinions, and these neighbors are sometimes armed with lawyers. Depending on the situation, mufflers may be the solution to this conflict. It might be that the cure is the use of mufflers that take some of the high-pitched sound from the exhaust note. Other times, a quieter muffler is required. In this case, there is usually a maximum decibel requirement.

There is another reason for reducing the noise in your race car. If your car is quieter than another, then you can hear the other cars around you better. More than a few drivers do this.

There is a problem to all this that should concern the racer. When noise is reduced, it often means that power is also reduced. This may not always be the case, but it does take some investigation to find the answers, and the answer will possibly be different for each application.

One thing we need to know is how a muffler works. A muffler does not work just by restricting flow. In the basic sense, for the muffler to reduce noise, it must separate that noise in a way likened to leveling the mountain top to fill the valley. By separating the peaks and valleys produced in the noise and then reassembling them at the end of the muffler, the noise is somewhat neutralized.

This noise separation occurs by several means. Each of these means has parts of the noise traveling paths of different lengths through the muffler. Thus, a particular noise event (e.g., one cylinder's exhaust valve opening) is broken up so that parts of it reach the muffler outlet at different times. This tends to blend the noise events. While this won't make a totally quiet muffler, the peaks and valleys will be evened out to a level at which the muffler is designed to work. Therefore, our ears perceive the noise to be a lower volume.

However, it is easy and perhaps inexpensive to greatly reduce the noise by slowing the flow through chambered mufflers, as found in many stock automobiles. These are very quiet and very restrictive. Many straight-through mufflers (glasspacks, and so on) have internal louvers that disrupt flow, so while appearing straight-through, the small louvers actually present a restriction to flow. These types are often found on street machines.

Grain-auger mufflers also split the flow of exhaust. I remember seeing wheat or other grain being fed into silos with this type of auger, which resembles a screw. These augers make a real racket when they are running inside sheetmetal tubing. They have been adapted for use in racing mufflers. The spiral is continuous in a grain auger. Some of the spiral flow mufflers use several segments of this spiral to split the noise events further.

When put inside a pipe forming the outer shell, the spirals in a tube become mufflers. Some of the flow goes straight through the center pipe, while the greater portion spirals around through the auger. This splits the noise events with little restriction.