A timing belt, or cam belt (informal usage), is a part of an internal combustion engine that controls the timing of the engine's valves. Some engines, like the flat-4 Volkswagen air cooled engine, and the straight-6 Toyota F engine use timing gears. They have replaced the older style timing chains that were in common usage until the 1970's and 1980's (although in the last decade there has been some reemergence of chain use). This term is sometimes used for the more general case of any flat belt with integral teeth, although such usage is a misnomer since there is no timing or synchronization involved.
Typically rubber with high-tensile fibres (e.g. fiberglass or Twaron / Kevlar) running the length as tension members.
Rubber degrades with higher temperatures, and with contact with motor oil. Thus the life expectancy is lowered in hot or leaky engines. Newer or more expensive belts are made of temperature resistant materials such as "highly-saturated nitrile" (HSN). The life of the reinforcing cords is also greatly affected by water and antifreeze. This means that special precautions must be taken for off road applications to allow water to drain away or be sealed from contact with the belt.
Older timing belts have trapezoid shaped teeth leading to high rates of tooth wear. Newer manufacturing techniques allow for curved teeth that are quieter and last longer.
Aftermarket timing parts may be used to alter
engine performance. OEM timing belts "will stretch at high rpm,
retarding the cam and therefore the ignition. Stronger, aftermarket belts, will
not stretch and the timing is preserved. In terms of engine design,
"shortening the width reduces weight and
friction.