Learn about Exhaust Muffler Packing Material

The phrase 'blown out muffler' accurately describes the type of mechanical failure that occurs with muffler packing. Exhaust muffler packing material have a limit to the amount of heat they can withstand before becoming brittle. Fiberglass, one of the more common fiber materials used to make exhaust muffler packing, begins to lose much of its strength once it is cycled past 620 degree Celsius. Up to this temperature, the glass fibers are soft and flexible. Once pushed past this temperature limit, fiberglass becomes weak and brittle, even 'crunchy' to the touch. Exhaust pulses eventually pulverize the glass fiber into particles small enough to go though the core where they are blown out the muffler.

Heat and vibration work together to destroy the glass fiber in muffler packing. A modern four-stroke engine is capable of hurling you from corner to corner and generates heat in the process. Even in the hand of a novice rider, the gas temperature in the muffler exceeds 530 degree Celsius. Opening the throttle with high load and relatively low ground speed produces more impressive heat. Twist the throttle long enough on a hot track-day and temperatures can approach 1000 degree Celsius.
Exhaust muffler packing materials most commonly take the form of mats, rovings and yarns. Each has its own benefits and drawbacks.
  • Mats – which can be made of glass, ceramic, silica or basalt fibers – come in a range of thicknesses and densities. Mats can withstand 620 degree Celsius to 1100 degree Celsius (silica-based), but mechanical strength is not their forte. Exhaust gas turbulence can quickly pull the short fibers into the exhaust stream. As with any engine, a layer of stainless steel wool around the core will greatly increase the life on a mat.
  • Roving-style packing is identified by its angel hair-like appearance. Roving has been used in automotive mufflers since the '80s and more recently in motorcycle applications. Supplied in bags that get wrapped around the core, roving are mechanically strong, due to their larger fiber diameter, and capable of withstanding 620 degree Celsius to 760 degrees. More durable than mats, their primary limitation is that they tend to get pushed around and migrate to one end of the muffler canister. In some cases, 30 to 40 percent more roving is required than yarn to compensate for settling. OEM replacement roving-filled pillows can be expensive.
  • Yarn packing, which has loop-looking construction, is more durable than mats and weighs less than roving. Its continuous fibers resist blowout better than the short fibers in mats. Additionally, the lops act like tiny springs to create a fluffy, high-memory filling. Standard yarn materials can take up to 720 degree Celsius and are well suited for two-stroke engines. Hotter four-strokes benefit from higher performance materials, some of which remain strong up to 950 degree Celsius. These materials are available in 250 gram to 500 gram bags. More convenient yarn-filled pillows are also available. In a hot four-stroke, a layer of stainless steel wool around the core wool can increase the durability of yarn packing.
The next time you rebuild your muffler, note whether the packing is wearing evenly from beginning to end or most of the damage is confined to one area. Localized damage or areas where nothing is left may need a higher density of packing or a layer of stainless steel wool and wire cloth. Places where the core changes diameter or near any obstruction to the gas flow, like quiet inserts, should be beefed up with more durable materials. More on how to repack your exhaust muffler can be found at Learn how to repack your exhaust
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