Glass packs are very restrictive, use mufflers for better flow. The only things glass packs do well is make noise and fit in tight places. Most folks look in a glass pack and see right through it so they think it flows like a straight pipe, not so. Air flow is strongest around the perimeter of the pipe and all those perforations slow the flow of air dramatically. You don't want the air slowed, you want the sound waves dampened.
Occasionally different magazines do flow tests of mufflers, find one of these issues and you'll see. If you have a flow bench you can test them with that too. Almost any muffler outflows a glass pack.
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