A Glasspack® is a type of muffler made popular in the US in the 1950s. Consisting of a straight through design, the Glasspack® muffler uses a fiberglass mat packed inside of a steel tube with a perforated steel tube center. The engine exhaust and sound pass through the perforated steel tube of the Glasspack® muffler and the sound is absorbed by the fiberglass packing. While many companies utilized this design, a few companies such as Cherrybomb, Thrush and Smitty's became cultural icons and actually became synonymous with the Glasspack® moniker.
Marketed as a performance exhaust component, the Glasspack® muffler was said to reduce back pressure and create a free-flowing exhaust system. As the fiberglass packing began to burn out of the muffler, the volume of the exhaust began to increase. With the increased volume also came a low rumble. This rumble became the signature sound of the 1960s muscle car era in the US. Automobile manufacturers were offering Glasspack® mufflers as options for performance vehicles, but they had to be installed by the purchaser.
In search of the performance sound that came from a worn-out pair of Glasspack® mufflers, some car owners began to pour gasoline inside of the mufflers and burn the fiberglass mat out. This method of "seasoning" the fiberglass was frowned upon, and after several people were injured by exploding gasoline, word got around the hot rod scene that the practice should be abandoned. Publications such as Hot Rod magazine were instrumental in convincing people to halt the dangerous practice.
The use of fiberglass-packed mufflers for performance use has given way to sound-canceling mufflers. These sound-canceling mufflers use no packing to quiet a vehicle's exhaust. Small steel barriers in varied shapes are positioned and welded inside of the muffler body and bounce and deflect the exhaust into itself. The colliding of the exhaust into itself cancels out different frequencies in the exhaust and creates a definitive exhaust note.
The resurgence of the muscle car craze has led to the re-emergence of companies like Cherrybomb. The release of original style Glasspack® mufflers from Cherrybomb have been featured on special edition vehicles from the Daimler-Chrysler Motors company. These muffler manufacturers have also released exhaust systems for the retro-styled vehicles from all of the big three US auto manufacturers. Now a consumer can purchase a Mustang, Challenger or Camaro that not only looks like their 1960s era predecessors, but that sounds like it came straight from the muscle car era.