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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.
Glasspacks are an old, simple, and relatively inexpensive muffler design that are effective at reducing back pressure, but not very effective at muffling noise. A longer glasspack will reduce noise to a greater extent than a shorter one. Sound is also dissipated into the fiberglass damping material. Other glasspack mufflers have perforated louvers punched into the center core which can reduce total flow capacity. 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, one can choose between slightly higher flow capacity or slightly greater muffling. This lower flow but slightly quieter design approach is commonly used in 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.