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This article is about the auto part. For the rock band, see The Glasspack.

A glasspack is a type of automobile muffler in which the exhaust gas passes straight through the center of the muffler. The basic design consists of one smaller tube centered inside of a larger outer tube that is enlarged or swollen in the middle. The gap between the swollen part of the larger outer tube and the smaller diameter center tube is packed full of fiberglass, hence the name "glasspack".

All mufflers impede the flow of exhaust gas to some extent. The more that the muffler / exhaust system impedes the total flow of the engine, the more reversion and "back pressure" is generated. This not only lowers the potential for power, but also fuel efficiency. Muffler designs involve trade-offs between noise, back pressure, weight and cost. Replacing the factory muffler with one that creates less back pressure is a modification that is often made by hobbyists interested in increasing the power of their vehicles. Glasspacks are an old, simple, and relatively inexpensive design. They are perceived as being effective at reducing back pressure, but not very effective at muffling noise. Having said that, a longer glasspack will reduce noise to a greater extent than a shorter one.

There are several other factors involved in terms of the overall total flow capacity & muffling capacity / sound of a glasspack. This has to do with how sound is directed into the fiberglass damping material. Higher flow mufflers simply have a multitude of holes drilled into a smooth bore center core. While this increases total flow capacity, the ability to muffle the sound passing through the core is reduced. The smoother bore of such a design does little to create turbulence and / or direct the sound waves into the fiberglass damping material, resulting in less muffling capacity. Walker / Thrush / Dynomax & Magnaflow brand glasspacks make use of this type of high flow core design.

Most other glasspack mufflers utilize perforated louvers punched into the center core. This can adversely affect total flow capacity. How much it lowers total flow capacity will vary depending on how far the louvers protrude into the airstream and which direction the louvers face. The turbulence created by the perforated louvers therefore achieves greater muffling capacity at the expense of total volume of air flow. The more turbulence created, the greater the muffling & less total air flow / power production capacity. Depending on the directionality of the louvers, once can choose between slightly higher flow capacity or slightly greater muffling. This lower flow but slightly quieter design approach is used in most all glasspack mufflers, including Cherry Bomb brand glasspack mufflers.

Some modern muffler designs are similar in principle to the glasspack, but use more sophisticated sound-absorbing materials such as stainless steel mesh, and more advanced acoustical engineering, reducing noise while retaining the power-preserving advantages of a straight-through exhaust flow.

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