Straight-through, or flow-through, mufflers have perforated baffles and packing material that provides a conservative level of sound absorption while still keeping sound output moderately loud. Glasspack mufflers perform similar to resonators due to their design and lack of sound-absorbing packing material, so they tend to be louder and higher pitched. For compression-ignition (i.e., diesel) engines, the most commonly used catalytic converter is the diesel oxidation catalyst (DOC). This catalyst converts particulate matter (PM), hydrocarbons, and carbon monoxide to carbon dioxide and water. These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping reduce visible particulates. These catalysts are ineffective for NOx, so NOx emissions from diesel engines are controlled by exhaust gas recirculation (EGR).
To maintain catalyst efficiency, the air–fuel ratio must stay close to stoichiometric and not remain rich or lean for too long. All of Milltek's performance exhaust systems are manufactured from T304L stainless-steel