Two stroke and four stroke dirt bikes have been around for a long time. Each type has it’s advantages and disadvantages. Furthermore, over the years each has had its shining moments as the preferred type of dirt bike for certain applications. The reason for these shining moments is primarily because of engineering improvements. Of course, the bike of choice among the populace can be influenced by marketing, word of mouth and rider preference. Whatever the reasons, there are some general facts that make the two stroke and four stroke dirt bike engines very different, power being one of these.
With regard to power, we have to realize that many variables can affect power of an engine whether an engine is a two stroke or four stroke. We just need to realize what we are comparing. To be fair, always compare apples to apples. A fair comparison, as an example, would be to compare different models of 450 four stroke motocross bikes to each other. An unfair comparison would be a 125cc two stroke with a 450cc four stroke. Now I do not mean to say these cannot be compared. They can indeed, but simply from the perspective of what each one is, generally. It is unfair to say which one is better because their target riders and purposes may not be the same.
Here though, we simply want to get at the power difference between a two stroke and a four stroke. So, is it true that a two stroke has twice the power of a four stroke? The answer is yes, give or take some. This why we have seen the AMA motocross series evolve as it has. Not long ago, two strokes ruled the motocross arena. There were the 250 cc classes and the 125 cc classes (and others also, but let us reflect only on the two). Then, in the late 90’s I believe, four strokes began to show up on the scene. I remember when Doug Henry was ripping it up on the early model YZ400 four stroke, which evolved to a 426 then finally a 450. Everyone else was riding a 250 two stroke. Soon thereafter, Honda produced the amazing 2002 CRF 450. Now, four strokes rule the motocross tracks.
Back in the 1990’s, a typical 250 two stroke’s horsepower peaked in the mid 40’s. To achieve the same horsepower with a four stroke required an increase in displacement by nearly two times. I’ll keep this basic because there can be exceptions. A two stroke engine fires on compression every other stroke. In other words, the engine fires every single rotation of the crankshaft. How the internal porting routes the fuel air mixture is what makes this possible. The porting is always open and flow control is accomplished by the presence of a reed valve in the intake. The position, size and angle of ports are used to tune the timing of air-fuel mixture arriving and exiting the combustion chamber.
A four stroke engine fires once out of every four strokes (every two full crankshaft rotations). Unlike the two stroke, a four stroke engine uses mechanical valves to control the flow of fuel-air mixture. These mechanical valves are controlled by a camshaft which is timed to the crankshaft via a timing chain. The crankshaft rotates twice for every rotation of the camshaft. If you would like to learn more details about how a four stroke dirt bike engine functions, please click HERE.
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Generally, we can say that a two stroke produces a power stroke twice as often as that of a four stroke. If each engine type was assumed to be 100% efficient, then we could conclude that twice the engine displacement would be required by a four stroke to produce the same power output as a two stroke. Of course, no engine is 100% efficient and there is more to engine performance than just peak horsepower.