A Four-stroke cycle engine, is one of two main types of engines. The other type being the two-stroke cycle. Besides motorcycles, four-stroke cycle engines are commonly used in cars, trucks, and many other things mainly because they can be muffled without slowing the engine down as much. The name four-stroke refers to it taking four piston strokes in a four-stroke engine to complete the four main engine processes(intake, compression, power, exhaust). The four-stroke is known for making a loud revving noise in the engine. For years, four-strokes were nearly absent in AMA motocross and Supercross events. Recently, however, the AMA has allowed for larger engine sizes when four-strokes are used(250cc instead of 125cc and 450cc instead of 250cc), this is most likely because four-strokes are more enviromentally friendly.
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