Today, 4-stroke engines are much more popular than 2-stroke engines. This is because, as we will see, they are more easier to control and are more environmentally-friendly. However, there’s an argument that is as old as time about these two categories of dirt bikes. It’s of course about which is the better option for off-road racing.
When it comes to finding a dirt bike for motocross or supercross racing, you want to consider the weight, handling, engine type, among others. All of these considerably determine how fast the bike will be.
These criteria, therefore, bring 2-stroke dirt bikes out of the oblivion and back into the debate: which is faster? 2-stroke or 4-stroke dirt bike?
Here in this article, we will dissect these two bike types, see which is faster and what actually makes it faster. But first, let’s delve into the smaller concepts and explain how each engine works.
Bike engine strokes: what are they?
A reciprocating engine (the type of engine in your bike) is called a stroke when the piston moves from one endpoint of the stroke to the other. These endpoints are referred to as top and bottom dead centers.
The four-stroke engine
As you can now imagine, the four-stroke engine got its name from the fact that the 4th stroke is a working stroke. Ignition only takes place after every other 360° rotation of the crankshaft. This is what it looks like in detail:
- First stroke: In the first stroke, the piston starts at the top dead center. As the piston moves downwards, a fresh gas mixture enters the cylinder via the inlet valve. When the piston reaches the bottom dead center, the inlet valve is closed.
- Second stroke: The second stroke is compressed and ignited. The piston moves again from bottom to top dead center and compresses the gas mixture. Once at the top, the compressed gas is ignited by the spark plug.
- Third stroke: The third cycle is the work cycle. The gas mixture burns at 2200-2500°C. The piston is pushed towards the bottom dead center. Once it has arrived there, the outlet valve opens.
- Fourth stroke: In the fourth and last stroke, the piston moves again from bottom to top dead center. It presses the exhaust gases out of the cylinder via the exhaust valve. Once the piston has reached the top, the outlet valve closes and the process starts all over again.
The operation of a 4-stroke engine
Unlike a 2-stroke engine, in this one, the gasoline and oil do not come into contact. This type of propellant gets involved with a greater number of parts, such as the cylinder, connecting rod, spark plug, two valves, crankshaft, and other complex components that ensure its correct operation.
One the main advantages of four-stroke engines is the considerable saving in oil, which does not burn during explosion. This also results in the reduction of odor and polluting emissions to the environment
Its mechanics include a more complex structure, to ensure that the different tasks do not overlap and achieve more efficient use of fuel.
Note: As you can see, with four cycles to complete, it will take a little longer for an average 4-stroke engine to complete a single speed revolution. Meanwhile, that’s not the single factor that can help us conclude that four-stroke engines tend to be slower than two-stroke engines. We will look at how the two-stroke engine completes its cycles and highlight other factors which affect speed.
The two-stroke engine
The two-stroke engine bears its name from the fact that every second stroke of the engine is a work stroke. So, that’s a cycle in which mechanical energy is generated. The cycles run as follows:
- First stroke: In the first cycle, the piston in the cylinder moves from bottom to top dead center. The fuel-air mixture in the cylinder is strongly compressed. The pressure and temperature in the cylinder rise sharply. Because the piston moves upwards, the space in the crankcase increases.
This creates a vacuum, which draws new gas out of the carburetor. Just before the piston reaches the top dead center, this compressed gas is ignited by the spark plug. It then creates very high pressure in the cylinder.
- Second stroke: During the second stroke, the hot gas expands and pushes the piston back towards the bottom dead center. Mechanical energy is created. The volume in the crankcase is reduced and the fresh gas there is compressed.
Shortly before the bottom dead center, the outlet opening opens, allowing the exhaust gas to escape. Then the overflow or inlet channels open. The fresh gas enters the cylinder through this. Now the first bar begins again.
The operation of a 2-stroke engine
The composition and operation of 2-stroke engines are much simpler than in the case of 4-stroke engines. This type of engine requires fuel to pass through all parts of the cycle and mix with oil in order to lubricate. During the process, the oil burns, which influences the strong odor that is emitted and the increased contamination.
Operationally, 2-stroke engines operate at a much higher speed than their 4 stroke counterparts. The only steps a 2-stroke cycle performs are intake-compression and combustion-exhaust. This simplicity of operation causes the engine to rotate much faster than a 4-stroke. And, in addition, it provides much more performance in acceleration.
The higher engine speed that 2-stroke engines must experience causes greater wear. Although, on the other hand, they are cheaper to manufacture and their maintenance is less than in the other case.
Two-stroke vs. Four-stroke Engines: Verdict on Overall Speed and Performance
As already mentioned, due to its design, the two-stroke engine is more powerful than the four-stroke engine with the same displacement. This is because of the fact that this engine does work during every second stroke and not just every fourth stroke. With the same cubic capacity and an identical number of cylinders, it does work twice as often.
Since the power of 125cc bikes is limited to 15PS, most engines come close to this limit. If you compare a four-stroke engine with a two-stroke engine with the same number of horsepower, the differences in the power are, however, not significant.
Two-stroke engines don’t require as much power to run as four-stroke engines. This also means they accelerate better than four-stroke bikes.
Speeding is better achieved on a two-stroke bike than on the other one in off-roading. With two-stroke bikes, you don’t need to rev up the engine to get the optimal performance from the engine.
Also, while four-stroke dirt bikes produce more torque for lesser horsepower, two-stroke bikes produce more horsepower with less torque.
On average, a 2-stroke 125cc dirt bike has a speed of 100 mph, while a 450cc 4-stroke dirt bike can only make 87mph on average, according to the Dirt Bike Planet.
Other Factors Which Affect the Speed
The way a bike is handled can affect how fast it moves. The two types of engines have different handling engineering. These are known as front-wheel handling and rear-wheel handling. So, which is best to have and which ensures more speed.
- Front-wheel handling: The rider of a four-stroke engine bike can handle and control the bike from the front wheel. This type of handling allows the rider to make use of the front tire for basically everything – speed and controls.
While the back wheel of a four-stroke bike provides the drive power, the front wheel is used for controlling and maneuvering the bike. This overload of functions, however, makes the front wheel wear out fast. While this lets you have better control over change of direction, it doesn’t help with speed. You’ll see why!
- Rear-wheel handling: The rider of a two-stroke engine bike can handle and control the bike from the rear wheel. You will guide the bike through railings, sleepers, and missiles by making use of the rear wheel in changing direction. Direction-changing starts from the front wheel and is then completed by the back wheel.
The major advantage with rear-wheel handling as regards to speed is that you can get faster RPMs. The use of the back wheel means the front wheel can be raised up (as you saw in Motocross). So the rear one in which the engine controls can find better traction and get optimal performance.
The ability to raise the front wheel in two-stroke bikes so the rear can work also helps the rider off-road through hilly and rugged terrains. This of course ensures a faster speed than when you’re on a four-stroke bike where the power is concentrated on the front wheel.
There’s a reason small cars are considered fast cars while the bigger vehicles are considerably slower. On average, a modern 4-stroke dirt bike tends to be heavier than a 2-stroke bike because the former has more parts.
In fact, four-stroke bikes are on average, about 50% heavier than two-stroke ones, according to the Dirt Bike Planet. So, it’s simple and plain; the chance of a heavier dirt bike being faster than a more lightweight two-stroke bike is slim and almost impossible.
Which is Faster 2 Stroke or 4 Stroke Dirt Bike (Comparison Table)
To know which is faster, below is a table showing how both types compare to each other in practical use:
|Criterion||2-stroke dirt bike||4-stroke bike|
|Weight||Lighter than 4-stroke bikes||Heavier than 2-stroke bikes|
|Horsepower per torque||More horsepower with less torque||Less horsepower with more torque|
|Speed (miles per hour)||100 mph||87 mph|
Two-stroke engine dirt bikes seem to be faster among the two types of bikes. However, controlling a four-stroke dirt bike, due to the front wheel handling, is faster and effective than on a two-stroke bike. Four-stroke bikes are more acceptable today because of their low emissions, low noise, among other reasons.
However, we can’t deny the fact that if you need a faster off-roading bike, two-stroke dirt bikes are your best bet. They are light in construction, have just two cycles to complete a revolution, and produce more horsepower with lower torque.