Two-stroke engines are the simplest of internal combustion designs, simply possessing a combustion stroke and an exhaust stroke. The four-stroke engine has four-strokes and uses valves, in conjunction with pistons, during their four-stroke cycle. Up until the late 1990s, the four-stroke engine exclusively powered trail bikes, while two-stroke engines and their zippy powerbands were the engine of choice for motocross riders. Cannondale, a bicycle company, entered the market in the late 1990s and started pushing the four-stroke engine for motocross bikes and started the debate over the pros and cons of each engine style.
Two-stroke engines emit a slight black smoke from the tailpipe because the engines burn oil, much like a diesel engine. Four-stroke engines burn cleaner and emit fewer toxins into the air. Dirt bikes with two-strokes are known for their high-pitched buzzing sound, which is loud if you are in the general vicinity but the noise does not project far out. Four-stroke engines generate a louder rumble that is not as piercing at a close distance, but does maintain its pitch over a longer distance.
Four-stroke engines have a smoother powerband, allowing novice riders to lay on the throttle without hitting a sharp bite of power. Two-stroke engines, however, have abrupt powerbands and can often buck an inexperienced rider who is not familiar with the torque curve of the bike. A two-stroke engine with the same displacement as a four-stroke engine will generate more power. However, American Motorcyclist Association regulations allow four-stroke engines of larger capacity to compete in the same class against two-strokes that are capped at a lower engine size.
Two-stroke engines require a specific mix of gas and oil in the fuel tank, while four-stroke engines use gasoline exclusively in the fuel tank, with oil being placed in a separate reservoir. Two-stroke engines typically require more frequent top-end rebuilds, which can add to operating costs. This is partially offset by the two-stroke engine's requiring less frequent oil filter changes than four-stroke engines.
Four-stroke engines are inherently more complex due to their higher number of moving parts. Modifying one of these engines is more expensive and more laborious. The exhaust pipe is one of the most commonly modified parts of a dirt bike due to the increased horsepower that the modification produces. This modification can cost two to three times as much on a four-stroke due to the extra complexity of the engine's exhaust components.
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