A longer glasspack will reduce noise to a greater extent than a shorter one, as sound is 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.
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 acoustic engineering, reducing noise while retaining the power-preserving advantages of a straight-through exhaust flow.
Glasspacks and other modifications that increase the noise level of exhaust systems are illegal in the United Kingdom.