You could have them on backwards , It will make a big difference ..
Not always true, it depends on the glasspack design. Glasspacks either have holes or they have louvers on the inside pipe. Some designs have directional louvers which dictate exhaust flow directions and some don't.
There are several other factors involved in terms of the overall total flow capacity & muffling capacity / sound of a glasspack. This has to do with how sound is directed into the fiberglass damping material. Higher flow mufflers simply have a multitude of holes drilled into a smooth bore center core. While this increases total flow capacity, the ability to muffle the sound passing through the core is reduced. The smoother bore of such a design does little to create turbulence and / or direct the sound waves into the fiberglass damping material, resulting in less muffling capacity. Walker / Thrush / Dynomax & Magnaflow brand glasspacks make use of this type of high flow core design.
Most other glasspack mufflers utilize perforated louvers punched into the center core. This can adversely affect total flow capacity. How much it lowers total flow capacity will vary depending on how far the louvers protrude into the airstream and which direction the louvers face. 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, once can choose between slightly higher flow capacity or slightly greater muffling. This lower flow but slightly quieter design approach is used in most all glasspack mufflers, including Cherry Bomb brand glasspack mufflers.http://en.wikipedia.org/wiki/Glasspack
