High performance internal combustion engines of the type used on racing cars have been the subject of considerable empirical design work and some theoretical studies for both commercial and racing applications. The exhaust systems for these engines, however, are often treated secondarily by racing teams and car manufacturers in the effort to increase or maintain engine performance. Exhaust systems are conventionally regarded as decreasing engine horsepower, rather than being a possible source for increasing horsepower.
High performance engines are generally designed to provide peak power at higher engine speeds, and free flowing exhaust systems, and particularly mufflers, for such engines are highly advantageous. While a slight back pressure from the muffler system may aid engine acceleration at low engine speeds, at a high RPM, back pressure is highly undesirable.
Exhaust system induced back pressure tends to impair breathing of the motor, thereby limiting top end speed. Thus, for high speed performance minimizing the back pressure of the exhaust system is a primary consideration in exhaust system design.
Race cars, for example, normally run straight pipes, eliminating any type of muffler. This unattenuated or unsuppressed engine noise, however, is unacceptable and intolerable for non-race applications. In fact, even race tracks are now under pressure to reduce the noise levels during racing, especially at those tracks situated near urban areas.
The use of mufflers on conventional non-racing cars, of course, has been mandated by various laws in order to meet sound attenuation standards on public roadways. Original equipment muffler manufacturers for non-racing cars are only marginally concerned with the horsepower drop which occurs as a result of the muffler's sound attenuation. Performance-minded owners, therefore, tend to look to after-market muffler manufacturers for higher performance mufflers for their cars, while still keeping these cars "street legal," i.e., meeting the legal sound attenuation requirements.
For many years, therefore, there have been after-market muffler assemblies available which produce a throaty sports car exhaust sound, which sound is still within legal noise limits and which sound is accompanied by at least somewhat enhanced engine performance. One such after-market muffler has been produced in many similar versions, which versions are generally known as "glasspack" mufflers. These mufflers employ an elongated tubular casing having a layer of fiberglass material around the inner periphery of the casing, which fiberglass is retained in place in the casing by a perforated tubular shell mounted inside the casing. Various gas-directing partition or baffle structures have been used inside the fiberglass retaining shell to assist in dispersing gases and sound for attenuation, but in mufflers which generate the least back pressure, the gas dispersing baffling is minimal.
Glasspack mufflers initially have the desired sports car sound, but with time, the high gas temperatures and exhaust gas velocity break-down and erode away the fiberglass. This problem is exacerbated by cars which have catalytic converters because the exhaust gases reaching the muffler are much hotter. Fiberglass can withstand 800.degree. F., but catalytic converters can raise the exhaust gas temperatures from 800.degree. F. to about 1200.degree. F., which greatly accelerates fiberglass breakdown.
Thermal erosion of fiberglass has been addressed by substituting a ceramic fiber blanket as a sound attenuation means in mufflers. While this approach has been suitable to address the thermal breakdown problems caused by the heat of the exhaust gases as they pass through the muffler, high velocity of the exhaust gases still erode ceramic blankets.
The use of partitions in glasspack mufflers to attenuate sound has been accompanied by three undesirable side effects. First, the partitions have tended to increase back pressure by choking flow through the muffler. Second, the partitions have often increased exhaust gas velocity proximate the fiberglass, thus increasing the rate of fiberglass erosion and breakdown. Thus, to the extent that glasspack mufflers are essentially straight-through mufflers (do not include sound attenuating dispersion partitions) sound attenuation is reduced. If they include sound attenuating, gas-dispersing, partition structures, back pressure and fiberglass erosion have been undesirably high. Third, glasspack mufflers also have a tendency to rap (make a cracking sound) during acceleration and deceleration. This is also known as "school busing" and is caused by sound waves that are not allowed to expand.
As a result of these problems, glasspack mufflers are considerably less popular in the muffler after market than was the case 20 or 30 years ago.