The battle is on! 2-stroke vs 4-stroke dirt bikes
There are lots of differences between 2-stroke and 4-stroke dirt bikes, and each one has their pros and cons for riders. We take a look at all of the differences between the two, to help you decide which one to get.
In this post you'll learn all about the two different engine types, with a look at exactly how they work. You'll learn the major differences in the engines including weight, power, fuel, and running costs. And we'll look at how each engine will affect the dirt bike, including it's power delivery, and handling.
2-stroke vs 4-stroke dirt bikes: The engines
Until recently, 2-stroke engines were the go to choice for all dirt bike manufacturers. They have super high revving capabilities, they're very light, and they're cheap to manufacture. This, along with the fact that they are also very easy to repair, even in your garden shed, made them the top choice of riders for decades.
2-stroke engines get their name because they complete one power cycle, with two movements of the piston, or two strokes. This happens in a single revolution of the engine's crankshaft, and moves the piston up once, and back down once.
How do 2-strokes work?
There are two types of 2-stroke engines available: traditional carburettor type engines, or fuel injected engines.
With a traditional carburettor 2-stroke, fuel and air are combined in the carburettor to create a highly combustible mixture. As the piston moves up, this mixture is drawn into the crankcase, and as the piston moves down the mixture is compressed and forced through the transfer ports into the cylinder.
As the piston moves up, it compresses this mixture by forcing it into the combustion chamber. The spark plug fires and ignites the mixture, which forces the piston back down. As the piston moves down, burnt fuel exits through the exhaust port, the fresh mixture is compressed in the crankcase, and the process starts again.
Fuel injected 2-strokes use a pump that injects 2-stroke oil directly into the throttle body. This passes into the crankcase to lubricate the engines's internals, along with sufficient air.
At the same time, fuel is injected into the cylinder via two injectors that are placed in the transfer ports. The fuel mixes with the air and fires in the traditional way. This technology was only introduced to the market in 2018, so it's still pretty new, but it cuts down on emissions and lowers fuel consumption.
When new emission rules were introduced in 2006, 4-strokes suddenly became very popular. With some of the big manufacturers including Honda, and Suzuki stopping production of adult 2-stroke dirt bikes, 4-stroke technology improved rapidly.
Although modern 4-stroke engines are producing more power than ever before, size for size they still can't match the power of a 2-stroke engine. At the same cylinder capacity, 4-strokes will produce about half the power of 2-strokes. Because of this, 4-stroke 450 cc bikes will race against 250 cc 2-stroke bikes in the same race class, this is mirrored in other race classes.
How do 4-strokes work
4-stroke engines are a little more complicated than 2-stroke engines. Where a 2-stroke will complete a power stroke in two movements of the piston (up, down), 4-stroke engines require four up and down movements, or four strokes.
As with 2-stroke engines, there are both carburettor and fuel injected 4-strokes.
1. Intake stroke: on the intake stroke the piston starts at the top, also known as top dead centre. As the piston moves down, the intake valve opens to let fuel and air from the carburettor pass into the cylinder. This stroke finishes at the bottom, or bottom dead centre, and the intake valve closes.
2. Compression stroke: this stroke begins at bottom dead centre, and as the piston moves up it compresses the fuel air mixture that has just been drawn into the cylinder. The stroke ends at top dead centre, both intake and exhaust valves are still closed.
3. Combustion stroke: the combustion stroke, or power stroke, starts with the piston still at top dead centre. The spark plug fires and ignites the fuel air mixture, forcing the piston back down to bottom dead centre. This is the stroke the provides the energy to keep the crankshaft rotating.
4. Exhaust stroke: the piston starts at bottom dead centre, and as it begins to travel back up, the exhaust valve is opened. This forces the burnt fuel gases out through the valve, and into the exhaust port. The piston returns to top dead centre to start back at the intake stroke.
Fuel injected 4-strokes use an injector that squirts fuel directly into the throttle body, where it is mixed with air. The process is exactly the same as above, but a computer controls the amount of fuel and air entering through the intake valve.
This computer communicates with the throttle and the injector, and it also reads engine RPM, temperature, crankshaft position, air temperature and oxygen content in the exhaust. This is essential to feed the exact amount of fuel and air into the engine.
Fuel injected 4-stroke dirt bikes have been about for for a long time, so it's likely that if you choose a 4-stroke, it will have this system. This cuts down on emissions and fuel consumption, and also increases engine performance.
Engine pros and cons
- Less moving parts = cheaper to buy.
- Cheap to rebuild.
- Easy to maintain and repair.
- High revving means more engine wear.
- Premixing of fuel is required (not on fuel injection).
- High emissions (much lower, and below regulations on newer models).
- Lower revving = less maintenance.
- Smoother power delivery.
- More efficient.
- Less emissions.
- Better low down torque.
- More moving parts = heavier.
- Twice the size is needed to mach 2-stroke power.
- More expensive to buy and rebuild.
- More complicated to work on.
2-stroke vs 4-stroke dirt bikes: The bikes
So now you know how the two engines differ, what difference does it make to the dirt bike?
2-stroke dirt bikes
The first thing you'll notice when you look at a 2-stroke dirt bike is the big exhaust pipe. These pipes look great, but they are there to serve a purpose, not just to look cool.
The big front pipe is known as the expansion chamber, and this is a special tuned exhaust pipe. This strange shape is designed to reflect the pressure waves that leave the exhaust port, back to the port, at a particular time in the cycle.
These tuned exhausts are essential for 2-stroke performance, but you won't find an expansion chamber quite like this on 4-stroke machines.
The second thing that stands out is the size of a 2-stroke engine. They are very small, and as previously mentioned, this makes them very light.
I mentioned earlier that you need to premix the fuel, this means mixing it with special 2-stroke oil before adding it to the dirt bike's fuel tank. This is essential to lubricate the engines internals, piston, cylinder, etc, and without it the engine will overheat and seize solid.
If you're riding a fuel injected 2-stroke dirt bike, the bike will have a separate 2-stroke oil tank, so fuel doesn't need pre mixing. This is also true for most 2-stroke road bikes, so you won't need to carry oil with you to mix at the filling station.
Starting a 2-stroke is very easy, and a couple of kicks will usually be enough. Riding will feel strange to complete beginners, as the power delivery is very sharp and aggressive. This is because they have a very steep power curve, meaning there will be a sudden sharp increase in power at a certain RPM; this is called the powerband.
There's little engine breaking compared to a 4-stroke, and they will require more gear shifting due to being powerbandy, with less low down torque.
4-stroke dirt bikes
4-strokes lack the huge expansion pipe that is essential for 2-stroke performance. But as technology improves, we are seeing it on some of the newer 4-stroke models. Bikes like the husqvarna FC are great examples, but the addition of an expansion chamber is only a small improvement compared to a 2-stroke engine.
Because the 4-stroke engine has valves, and cams to operate the valves, the top half of the engine is much bigger. This, with the addition of a sump full of engine oil, and a separate lubrication system, makes them heavier than a 2-stroke. A 4-stroke will usually weigh 10-20% more than 2-stroke.
Fuelling the 4-stroke dirt bike is much easier than a regular 2-stroke, because you will not have to pre mix the fuel. As I just mentioned, the sump holds the oil needed for lubrication, so you don't need to add any extra.
Starting a 4-stroke can be more tricky than a 2-stroke, especially when the engine is warm. For this reason many manufacturers include a hot start lever, which lets in more air to make the mixture lean for easier starting. This is also useful for starting a flooded engine.
Riding the 4-stroke will be a totally different experience to the 2-stroke, as the power is delivered at a lower RPM. This makes the 4-stroke quicker on up hill sections, and pulling away from tight corners. There is also more engine braking than you experience on a 2-stroke, this can help during racing and tight cornering.
2-stroke vs 4-stroke dirt bikes: Which to buy
It's really hard to say which one you'll like best until you ride them both. But in general, beginners will usually do better with 4-strokes due to their smoother power delivery. This won't be true for all beginners though, especially very young children who need a lighter dirt bike.
You've read the comparison, and you now have a good understanding of the differences, the next part is up to you. Good luck with your choice!
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