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.

If your track rules specify the use of a muffler, but no decibel limit is listed, then get one of the short, straight-through types that offer virtually no restriction. All you need to do is put these on the ends of the collectors.

No story on mufflers would be complete without discussing some of the ways to get the exhaust to the muffler. We are not speaking about headers or cast manifolds. We'll hold that topic for another time. What we are going to be examining is how the exhaust is fed to the muffler(s).

I called a friend, Greg Fuesler, who has exhausted this subject with Dynatech (pun intended). My questions concerned the use of x pipe systems,Y pipes, and H pipes, among other things. Fuesler's thoughts are incorporated into the rest of this article.

It seems there is no single exhaust system that is correct for all applications. When class rules restrict the use of any type of side-to-side connection, then a straight-through muffler attached to each collector is thought to be best. There is some extra power available when you connect the two sides of a V-8 exhaust. In some cases, you may want to bring both collectors into one muffler. This applies to engines at the 525-550hp range or less. Also, you want to stay with 3-inch diameter pipes and the matching muffler. These sizes maintain the speed of the exhaust flow.

Higher horsepower engines running on tracks longer than a 11/42 mile can take advantage of 311/42-inch diameter pipes.

Where the Y pipe arrangement is not practical, such as in crossover headers, there are other solutions that can work just as well. The crossover headers have the left-bank cross behind the distributor to join in a Y with the right bank. If stock firewalls are required, these can't be used because the crossover pipes must go through the firewall.

The x pipe system or H pipe arrangement allows the exhaust system to possibly be placed more efficiently in the chassis, usually under the car. Both of these systems function virtually the same. The nature of the H pipes is not to change the flow between the two sides, but to allow the pressure waves to equalize. The size of the H crossover pipe is not too critical; however, it would be preferable to have it the same diameter as the collector.

For the H pipe to be most effective, it should be attached to the collector close to the point where the individual pipes come together.

With the x pipe system, the two sides are brought together and then apart in an x shape. There can be some easy crossing of the exhaust pulses from one to another, which happens if an exhaust pulse reaches the x and finds a lower pressure on the other side. Still, its primary function is to equalize the pressure waves between both sides of the engine's exhaust.

The mufflers can be mounted anywhere in the system, but are probably more effective near the end of any exhaust pipe. Exhaust pipe length can be adjusted for the desired power range. While it is more effective to change header primary pipes to change the power range, it can be far easier to make the changes with the exhaust pipe length. Longer tubes provide more torque, which may work well on bullring tracks.

Another trick you might use for a track that requires a low decibel rating is to have the exhaust terminate far under the car. This will break up some of the noise events.

Noise is a fact of life in a race car. Controlling it is up to the racer, just like chassis setup. The more you know, the better you can race.