September 27, 1996
Mufflers come in two basic flavors. Packed and unpacked. The stock BMW mufflers are of the unpacked variety, as are most all OEM units for cars and bikes. Unpacked mufflers have no renuable packing inside. The noise is reduced by winding the spent gas back and forth between a set of internal baffles. The baffles consist of a series of plates and tubes that force the gas to go a longer distance inside the muffler. As the gas winds its way through the muffler it looses the noise. The bad news it requires a pressure drop to push the gas back and forth through the passages. This pressure drop causes power loss.
Some unpacked mufflers use a Hemholtz chamber to reduce noise. Sound comes in one side of the chamber and sets up a resonant frequency. This cancels out the original noise. This method only works for a specific frequency, however it causes less pressure (power) loss. In a car this is called the resonator.
Packed mufflers consist of a perforated tube with a packing material on the outside. An outer shell covers the whole assembly. The tube it pretty much a straight through affair. Noise comes in the perforations and gets damped by the packing material (usually fiberglass). This type of muffler muffles poorly, but has a low pressure drop. That is why many of the after-market exhaust systems use this method.
The packing material slowly breaks apart, eventually blowing out in little pieces with the exhaust. As this happens the exhaust gets louder and louder. When the muffler looses all or most of the packing it must be repacked. The stock mufflers do not need repacking since they do not have packing.
Exhaust is corrosive stuff. It is especially corrosive when it is combined with moisture like when the bike is first started. An unpacked muffler can fail when the internal baffles are corroded, or a hole develops in the outer housing.
I hope this explains how a muffler works.
Ride Safe,
Joe Dille
(joedill@frmail.frco.com)