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1.
Mufflers that are packed with fiberglass to muffle the sound. They come in different lengths. The shorter the glasspacks, the louder the car.
When I get my Corvette I am going to put glasspacks on it.
by OneBadAsp October 24, 2006
 
2.
A noisy, inexpensive muffler design made with fiberglass surroundings and stainless steel. Also referred to as cherry bombs due to a manufacture that produces glass pack mufflers.
When I get my Jeep Wrangler, I am going to put a glass pack muffler on it
by goods_25 September 27, 2009
 
3.
A type of exhaust. Features a straight through exhaust tube to provide as much exhaust flow as possible (with holes or baffles in it to direct the exhaust into the packing) that is surrounded by a steel wool or fiberglass packing (sometimes both, varies by manufacturer) to make it relatively quiet considering its a straight through exhaust. Also look up fart can performance exhaust. Sometimes called cherrybombs because it was a major manufacturer of the exhaust.
"The fart can is the same thing as a glasspack"

"Both the glasspack and fart can exhaust work on the principle of absorption"
"The glasspack enables exhaust to flow better than most chambered exhausts"
by TheFlacker99 December 13, 2012