To understand the complexities of an automotive muffler, particularly one intended for racing or high-performance engines, it's necessary to include some basic informational components. Expansion pressures tailing the end of combustion are emitted at energy levels much higher than atmospheric pressure, often resulting in objectionable sounds. These can be pleasant or not. For racing engines, it's becoming increasingly not, especially in areas with growing environmental concerns.
Sound And FlowHere's an analogy to consider: Let's say you are standing at the edge of a creek or any flowing stream of water. The moving liquid has weight (or mass) and velocity, therefore it displays kinetic energy (K.E.). How much K.E. it contains depends on the quantity of both these elements: mass and velocity. In the exhaust flow of a running engine, our example is now exhaust gas, and the amount of K.E. it displays is still a function of its mass and velocity. Simple enough.
Now, suppose you throw a rock into the water. Ripples are created, which are carried along with the movement of the water. These disturbances are like sound waves in the flow of exhaust gases.
Continuing our example, let's assume a piece of screen wire is placed across the stream just below the rock's splash. As the waves pass through the screen mesh, some energy is dissipated, and the waves that emerge on the other side are of a different size and shape-typically reduced. There is an energy exchange, or transformation, as the waves interact with the screen wire, resulting in a conversion of energy from one form into another.
That, basically, is what happens when high pressure, disturbed, heated exhaust gases leave an engine's combustion space and enter a "muffled" exhaust system.
Technically, sound may be defined as an alteration of pressure or particle displacement propagated in an elastic medium, producing variations in pressure. In a practical sense, an engine's exhaust sound can be either pleasing, of no consequence, or damaging. Its energy can be diminished or absorbed when passing through a medium or by hitting against a surface. Both can be and are used as automotive mufflers, racing or otherwise.
Sound In A Vacuum? If you have an atmosphere inside a container (in this case, a muffler), then a medium exists through which sound waves can be propagated. As atmospheric pressure is decreased, the density of this medium is reduced, which diminishes its ability to move sound waves. While a pure vacuum is unlikely to be found in a muffler, the ability to establish and maintain low-pressure areas within the muffler effectively increases its ability to deaden sound. It is possible to design interior portions of a muffler that accomplish this objective.
Sound Frequency And AmplitudeStrictly speaking, we may define acoustical frequency as the number of sound waves (per second) passing any point in a sound field (any medium through which sound is moving). Units of sound frequency are typically measured in cycles per second. Since sound is an auditory sensation, the conditions that describe its presence relate specifically to what we can hear.
For example, the strength of a sound may be termed as its amplitude. High-strength sounds have high amplitudes. Combinations of high frequency and high amplitude are common in auto-motive mufflers, particularly on racing engines. Sound also travels faster in a heated medium than cooler ones. As a result, mufflers on racing engines are exposed to high-frequency, high-amplitude sound waves in hot exhaust gases, all of which increase the complexity of muffler design.
There is also the issue of frequency change that can be addressed in a muffler. Since certain sound frequencies are more pleasant (or less damaging to the ear) than others, it is possible to design mufflers that cancel (or alter) specific sound frequencies while allowing others to remain in the mix of exhaust sounds.
It may surprise many racers out there to learn that it's possible to design mufflers to reduce specific sound frequencies-while still providing low backpressure conditions. Networks of baffles, instead of a combination of baffles and packing material, frequently characterize this type of muffler. For purposes of this discussion, baffles may include dimpled or perforated tubing, as well as indentations resembling louvers that either absorb or reflect pressure waves.
A device called a Helmholtz resonator is often used to change the tone (frequency and amplitude) of exhaust pulses. By definition, this type of resonator is characterized by an enclosure that communicates with outside pressure conditions by way of a small opening, usually a tube (see Helmholtz illustration). Based on the volume of the resonator and the communicating tube, Helmholtz resonators will resonate at a single frequency, thereby affecting exhaust tone at that frequency.
Exhaust Backpressure Historically, the muffling or attenuation of engine exhaust sounds has been accomplished by adding a system that also hinders the free flow of exhaust gasses, increasing backpressure. Low backpressure is important because we're trying to rid an engine of combustion byproducts-particles, or residuals, that don't tend to burn a second time.
An inefficient exhaust system (muffled or not) can cause these residuals to remain in the combustion space, not only preventing the entrance of fresh air/fuel mixtures on the next cycle but also reducing combustion heat via fuel-charge dilution. While there may be instances when a measure of back-pressure is desirable, particularly during low rpm and to prevent "overscavenging" of cylinders, this is not the case in a majority of situations. Backpressure also causes natural EGR (exhaust gas recirculation). This reduces combustion temperatures and oxides of nitrogen, both of which are counterproductive to optimum power. In a racing engine, avoidance of combustion contamination is one of the rules to producing power. So we don't want racing mufflers that promote or cause backpressure.
Backpressure is also affected by piston displacement, power output, and rpm. All things being constant, backpressure (and exhaust sound) increases with rising rpm. It is important to keep all these elements in mind when selecting a muffler system.
Open Pipes vs. Mufflers It is the net of pressure excursions (cycle) traversing an engine exhaust system that affect its output of sound and power. The notion that an open exhaust always makes the most power is not necessarily true. Examples abound, but here's one that gets to the core of the issue:
Consider a single-cylinder engine running at a constant rpm. Its exhaust system is a simple length of open pipe. Visually, there appear to be no restrictions to flow. However, by lengthening or shortening this pipe, we discover a corresponding change in rpm. If no physical impediment (e.g., a muffler) has been added to the exhaust's path, how can the engine's rpm change by merely altering the pipe length? Simple-the longer pipe through which the exhaust gases had to travel increased backpressure.
So what if we did install an energy-altering device (possibly a muffler) that aided net flow while treating sound pressures as a separate issue. Now we're getting into the realm of exhaust tuning by lowering the effective backpressure created by an open exhaust system. By comparison, it can be said that a muffler with this design objective makes horsepower over an open exhaust.
In instances involving exhaust tuning of this type, a reduction in pressure at the system's exit amounts to reduced backpressure. If a muffler is designed to reduce its exit pressure below that of its inlet pressure, the device becomes a sort of one-way valve, which benefits combustion efficiency through reduced contamination in the combustion space. However, such a muffler must also prevent atmospheric pressure from causing flow backwards during times of low muffler pressure-like during valve overlap-if it is to qualify as a "one-way" valve.
Mufflers, Backpressure, and Valve OverlapIf backpressure is reduced by installing more efficient mufflers, then the opportunity exists to revisit the issue of valve overlap and how decreasing it may increase power. The same holds true in reverse if you're installing mufflers for the first time or even putting them on and off with regularity. Some valve-timing changes may be in order.
If the term scavenge is appropriate (scavenge is the effect of low-pressure conditions in the exhaust path pulling the burnt contents of the cylinder into the exhaust system as the exhaust valve opens), its efficiency is said to improve with increases in overlap. During this period in valve events, atmospheric pressure can "see" all the way from where air enters to where it leaves the engine. This can wreak havoc on carburetor calibration and net volumetric efficiency (or torque). The addition of a muffler can not only affect carburetor fuel-metering signals, but its effects on combustion efficiency also cause changes in initial ignition spark timing. Typically, as backpressure is increased, fuel-metering signals weaken, combustion efficiency falls off, and more spark timing is required to ignite and sustain the burn. In addition, it may be necessary to increase valve overlap and/or retard the cam for power at higher rpm where contamination becomes worse (less time to evacuate cylinders of residue).
The corollary to these conditions is what happens when backpressure is reduced. In this instance, combustion efficiency stands to improve. But what can happen is cylinders may become "overscavenged," netting a reduction in volumetric efficiency because fresh air/fuel charges are passed through the combustion space during overlap. This possible loss of power can be addressed by shortening the overlap period, particularly closing the intake valve.
In addition, due to the potential for improved combustion efficiency (a reduction in air/fuel charge contamination), it's often possible to reduce ignition spark timing. Consider this: By delaying the point of ignition, less negative torque is produced at the crank, culminating in a higher net torque (increased IMEP) and more low-rpm performance-particularly for accelerating out of corners and when passing. o it's not just about mufflers and their sound-attenuating properties. Companion parts and tuning considerations are key to making the most of a muffled situation.
If you give proper attention to the engine components that are affected by the installation of a muffler, it's possible to minimize power loss because of increasing backpressure. Overall, a race engine should be an integrated package of parts that work toward optimizing volumetric efficiency in the typical rpm range that is seen in a race. Properly developed torque ranges contribute more to on-track success far more than top-end power numbers do. There's no reason why the installation of a muffler needs to detract from that concept. It's just good, sound advice.
Saturday-Night Criteria for Selecting Racing MufflersTouchy subject here, largely because of the many variables involved. But here are some guidelines:
*If you're faced with muffling a race engine, find out what sound limits are imposed. There's no reason to reduce sound below what's necessary, particularly since you have some choices among mufflers to select.
*Let's suppose you're racing in an area (or class) that requires the use of a spec muffler. If this is the case, the competitive playing field gets leveled out. In this scenario, it's critical that you optimize all other aspects of the engine as outlined in this article.
*Once a muffler system is installed, keep an eye on engine coolant temperature. While backpressure can contribute to contaminated combustion and reduced flame temperature, it also tends to make the engine retain heat in the engine block and heads.
*Unless the muffler you've installed allows a particularly low backpressure, don't be surprised if on-track fuel economy declines-especially if you find additional jetting is required to restore performance.
*Mufflers that do allow low backpressure virtually require you to rethink valve timing, particularly valve overlap periods. Shorter is the norm in many cases.
*Wherever possible, use a crossover pipe between collectors. This provides sound-pulse damping and a measure of noise reduction. You may find that a less restrictive muffler in conjunction with a cross-over pipe will help you achieve required sound levels and maintain open-pipe performance.
*Overall, a muffler is an additional tuning volume placed in the exhaust system. If you can, experiment with its placement along the tailpipe. You likely will find a location that's superior to others along the length. One way to determine this location is to paint the muffler, run the car awhile, then find an area where the least amount of paint has been burned from the case. This is a signal for a location that minimizes backpressure of the muffler you're using.
*In the vast majority of cases, a length of tailpipe extending past the end of a muffler will aid in its performance, both in sound attenuation and its effects on power. Typically, both are good.
ManufacturersBorla Performance IndustriesDept. CT095901 Edison DrOxnard, CA 93033805/986-8600
FlowmasterDept CT092975 Dutton AveUnit 3Santa Rosa, CA 95407707/544-4761
Specialty Products DesignDept. CT0911252 Sunco DrRancho Cordova, CA 95742916/635-8108