Ben:

One of the real early muffler systems to put on your own engine was a strap-on stack. It had small slits near the outer end to smooth the exhaust flow. They didn't work well. Open enough to let the engine run they didn't quiet anything, closed far enough to quiet the exhaust the engine didn't have any power. These look similar.

Squeeze them right down,
. Engine makes not a sound.
Open for a run that's tops,
. Neighbors call the cops.

Bill.