As published in British V8 Newsletter, Volume XV Issue 2, September 2007
by: Larry Shimp
The focus of this article is on the silencing efficacy of an exhaust system, and how to get the minimum sound with the minimum flow restriction. This may not be of interest to everyone, but for those who want to drive for long distances, especially with air conditioned cars and closed windows, this may be more applicable.
Source of noise:
The combustion process and exhaust cycle of the engine impart vibrations to the air stream that make themselves known as noise. Noise tends to increase in volume as an engine gets bigger and as compression increases, and to increase in frequency (and volume) as engine RPM is increased.
Muffler principles:
There are four main performance muffling principles: expansion, “turbo”, noise cancellation, and noise absorption. Old OEM mufflers worked primarily by restriction, but these will not be considered for performance use.
Dale Knapke’s Supertrapp mufflers utilize the expansion principle.
ExpansionExpansion is one of the oldest designs and works by letting the hot exhaust gases expand and cool so both their velocity and volume are reduced. These mufflers are simply long, big diameter tubes with no internal baffles, usually with an increasing diameter from the inlet to the outlet. As the exhaust gases slow down, the amplitude of the noise they carry is reduced. Expansion is relatively efficient in that the exhaust flow is not really hindered. Unfortunately, it is only practical for smaller, low power, low speed engines. It worked on Model T Fords and similar cars, but modern V8s put out too much exhaust volume for any reasonably sized expansion muffler to take care of.
However, hybrid expansion mufflers are still around. The megaphone mufflers seen on some motorcycles work by expansion in the increasing diameter exhaust tube, and by sound absorption in the fiberglass lining. This is also the fundamental principle of SuperTrapp mufflers (without the plates installed). At this year’s British V8 meet, Dale Knapke utilized SuperTrapp mufflers on his Ford SVO Turbo Triumph, and had good sound attenuation.
Leonard Marshall‘s Summit Turbo mufflers utilize the same noise canceling principles as OEM mufflers.
Chambered mufflers look externally similar, but utilize the “noise canceling” principle.
Turbo”Turbo mufflers” are a variation on OEM style mufflers, but are less restrictive, so they are in the performance category. They were originally developed for the early GM turbo cars (especially the Corvair) because the turbocharged engines didn’t like back pressure. Besides, the turbochargers already provided some muffling effect through expansion and cooling, so less sound attenuation was required from the mufflers. Turbo mufflers consist primarily of a pair of parallel, perforated dead-end tubes, sometimes with a baffle in between. Old style OEM mufflers are similar but usually have smaller tubes and less perforations, and sometimes extra baffles. The exhaust escapes from the holes in the inlet tube, and exits through the holes in the outlet tube. Noise cancellation is by restriction, which slows down the exhaust gasses, and by some noise cancellation as sound waves get reflected among the many inside surfaces. There is some case resonance, but the sound is otherwise similar to noise absorption mufflers.
ChambersNoise canceling is the principle used in chambered mufflers. A well known manufacturer of chambered mufflers is FlowMaster. (Although they externally resemble “turbo” mufflers, their internal construction is quite a bit diferent.) Chambered mufflers have a series of compartments that are designed to cause the sound waves to be reflected back on each other. Sound waves have peaks and valleys. If two waves are 180 degrees out of phase so that the peak of one wave corresponds with the valley of another wave, the peaks and valleys will cancel and the result is silence. Noise canceling ear phones work by electronically generating signals exactly 180 degrees out of phase to detected noise signals. The ultimate car interior cancellation system is therefore an electronic noise canceling circuit that sends counteracting sound waves through the car’s speaker system. This would let the driver determine the amount of interior noise at the turn of a dial. Prototype systems actually do exist, and I understand that some car companies were considering such a system for their production cars.
Mufflers are not (at least not yet) electronic devices and so they must generate the canceling waves mechanically. At certain frequencies, some sound waves will cancel in this type of muffler. The range of frequency cancellation is often increased by having sloped baffles that will work on a variety of frequencies at once. Perfect cancellation is not needed in order to be effective, even partial overlap of the sound waves will result in a reduction in noise. Because the mufflers are limited in size, they mainly cancel higher frequency sound waves (the higher the frequency, the shorter the wavelength). This gives chambered mufflers their characteristic deep (low frequency) sound.
The main appeal of chambered mufflers is their sound characteristics. Low frequency sound is much less irritating than high frequency sound and that is why these mufflers are often described as “mellow”. Another characteristic of chambered mufflers is interior resonance. The sound within the muffler tends to vibrate the muffler case, turning it into what is, in effect, a speaker. This amplifies the sound inside the car and can be either an advantage or a disadvantage, depending on personal preference.
The main disadvantage of chambered mufflers is that they tend to restrict flow. This is a consequence of their sound wave reflection operating principle because as sound waves are reflected, so are actual exhaust gasses. However, they tend to be less restrictive than most turbo mufflers.
There are three ways to minimize this restrictive effect on performance. The first is to use as large a muffler as possible. But space in an MGB or similar car is very limited so this is not practical. The other is to place the muffler as far back from the engine as possible. In this position the exhaust gases have cooled as much as they can, and cool gases take up less volume; in effect giving the same outcome as a larger muffler. Many modern cars have their main muffler at the very rear of the exhaust system for this reason. However, space for an MG exhaust is very limited behind the rear axle, so this is not practical (but it may be practical for some cars like Triumphs). The third solution is to use an X pipe or an H pipe and a dual exhaust system. The connection between the pipes (before the mufflers) permits each exhaust pulse to use both mufflers, which gives the same effect as using a much larger muffler. Because the firing pulses in a V8 engine overlap to some extent (unless the engine has a single plane crankshaft), not every pulse has the whole exhaust system to itself, but the principle is still effective.
Car Chemistry brand mufflers are a variation on the out of phase noise canceling principle. They divide up the exhaust stream into two parts, and slow down one stream relative to the other. The velocity difference causes some out of phase variation in the sound waves between the two exhaust streams which results in some noise attenuation. For the maximum effect, the gas stream has to have a high inlet velocity, and so these mufflers are best installed as close to the engine as possible.
The Car Chemistry mufflers are not a good choice as a primary muffler because their overall sound attenuation is not as great as a conventional muffler. But they are useful as a supplement. These are available as a complete muffler, or as an insert that fits into the exhaust tubing. The inserts are the ultimate answer to ground clearance issues, but there is some restriction from the inserts. It is therefore best to oversize the exhaust tubing if using the inserts.
Al Wulf’s glass pack mufflers utilize the absorption principle.
AbsorptionAbsorption is the principle by which a glass-pack muffler works. There is a perforated tube within the muffler, and the sound radiates out through the perforations where it is absorbed by the fiber filler. Essentially, the sound is dissipated trying to vibrate the filler material and, in addition, the filler material does not reflect the sound waves back to the exhaust stream. Unlike chambered mufflers, these mufflers absorb sound at all frequencies and the result is the normal exhaust sound, only quieter. They will not produce a nice rumble like a chambered muffler without a lot of higher frequency noise accompanying it. The goal of an absorption muffler is as quiet a sound as possible; otherwise the higher frequency components of the sound can become annoying. One advantage is that, because most of the sound is absorbed before it gets to the outer case, the case resonates very little, and so interior resonance is generally less compared to a chambered muffler.
Absorption mufflers can be more efficient than chambered mufflers, but that depends on the design and installation. Many of these mufflers have a louvered internal tube. When the flow goes against the louvers, the sound is efficiently absorbed, but the louvers create turbulence that restricts flow. Turning the mufflers around greatly reduces turbulence, but it also greatly increases noise. Some mufflers, like the Magnaflow, have perforated tubes with plain holes and no louvers. These can be installed in either direction, and have the best compromise between flow restriction and sound absorption.
Of interest is that absorption mufflers both get louder and lose flow capability if the packing starts to come loose. Loose packing creates larger internal voids and this causes greater turbulence in the exhaust gases as they penetrate farther into the muffler outside of the main flow tube.
Additional IssuesMuffler configuration:
For the least noise it is always best to have two mufflers in series. The second muffler will absorb sound missed by the first muffler, and there will be some wave type noise canceling in the tubing between the two mufflers. Ideally, the second muffler should do most of the silencing but even a small resonator at the end of the exhaust system will make a big difference.
Exhaust outlet:
The exhaust outlet should extend beyond the separate bumper found on MGBs and most vintage sports cars. This is because the curved inner surface of the bumper will reflect the exhaust noise back towards the car contributing to “droning” during cruising. Another alternative is to used turned down exhaust tips to keep the noise away from the bumper/reflector.
Exhaust pipe size:
As a rule of thumb, a 2 inch dual exhaust is fine for a mild 215 engine, a 2 1/4 inch diameter is good for up to about 300 cubic inches, while a 2 ½ inch exhaust is good for a highly tuned 350. A 3 inch exhaust is better for larger performance engines. In theory, using too large of an exhaust can decrease low end torque because the increased gas velocity of a smaller pipe helps scavenging. However, I doubt this is too strong of an effect, and probably header diameter and length is more important for scavenging. But with an old British car, ground clearance is limited and that is a good reason for not using too large of a pipe.
In any case, the largest piping is needed before the X or H pipe. As was explained earlier, the X or H pipe allows the exhaust pulses to use both exhaust pipes, while the pulses are restricted to only one pipe before the cross over connection. Also, the exhaust gases have cooled slightly by the time they reach the cross over and will take up slightly less volume, but the cross over connection is the main effect. Logically, then, a way to minimize ground clearance issues and retain many of the advantages of a large diameter exhaust is to make a custom crossover piece with larger diameter inlet than outlet pipes.
To summarize, there are ways to make a quiet exhaust system without sacrificing power.
Disclaimer: This page was researched and written by Larry Shimp. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.
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