It's a debate as old as time: 2 stroke dirt bikes versus 4 strokes. (Ok, maybe not as old as time. But it's still a huge debate that's been going on forever.) All over the world, dirt bike riders argue about whether 2 strokes versus 4 strokes are the better engine for dirt bikes.
A stroke is a motion of a piston, meaning a two-stroke dirt bike has 2 different motions of the piston, while a four-stroke has 4. 2 Strokes are generally more unstable and accelerate faster, while a 4 stroke is more consistent and has a higher top speed.
But this debate goes so much deeper than the simple question, "which is better?". What should really be considered in the 2 stroke versus 4 stroke dirt bike debate is what is good and bad about each of them as well as how both of them work in different situations, settings, and riding styles.
What's the difference?
Before we get going, I'm going to touch over a few examples of two-stroke engines, and four-stroke engines. Once we know the contrasts between the two, it will be easier to understand the different kinds of pros and cons that exist between the different engine builds.
What even is a "stroke"?
Prior to my life of dirt biking, stroke meant one of two things. Either something horrible was happening to your health OR you had just had the best idea in the history of best ideas. However, when it comes to dirt bikes, we're talking about something entirely different, that has nothing to do with hospital visits or sudden flashes of inspiration.
So, when it comes to your dirt bike, what exactly is a stroke?
A stroke is the movement of a piston in the dirt bike's engine. To put it simply, a stroke is a step in the process to get the dirt bike's engine going. Following the trail of common sense, two-stroke engines use two "movements", or strokes, to complete a full "engine cycle", while four-stroke engines use four strokes of the piston to accomplish the same thing.
Easy enough? Good. Now let's get to the good stuff: what's the difference between two-stroke dirt bikes and four stroke dirt bikes? Keep on reading to find out!
Two Stroke Dirt Bike
Two stroke engines are the much less complicated option out of the two engines. They are designed to complete their engine cycle in two piston movements, instead of four, like the four-stroke.
The two-stroke engine begins its cycle with a power stroke. The ignited air, fuel, and oil mixture force the piston down until the mixture reaches the exhaust port, an opening on the side of the cylinder. The piston travels downward, and it pressurizes the air, fuel and oil mixture that was previously drawn into an attachment on the side called a “crankcase”. The mixture was pulled into the crankcase during the most recent compression stroke, the one prior to the current power stroke that hasn’t completed yet.
An exposed intake transfer port lets the next air and fuel mixture into the cylinder, right as the crankshaft begins its next rotation, pushing the piston back up. This action blocks off the exhaust and intake ports, enabling the piston to compress the fuel and air mixture. The upward action of the piston pulls in the next fuel and air mixture from the carburetors and keeps it underneath the piston. The currently compressed air and fuel “charge” above the piston is ignited by a spark plug, and the whole thing repeats itself, over and over again.
This operation produces less waste than a four-stroke engine of similar or equal power output. They have a more efficient build, enabling them to be assembled and maintained using thirty to fifty percent fewer moving parts than a four stroke. Two strokes are the easiest of the two types of engines to clean. They do, however, have elevated fuel consumption, and because of the way the stroke of the engine works with the openings in the chamber, it produces more emissions.
Due to the intake and exhaust ports being open at the same time, with each piston rotation a portion of the air and fuel mixture escapes out the exhaust port without being used to power the engine. That being said, direct injection and catalytic converters are the exceptions to those rules and will reduce the number of unburned hydrocarbons in your emissions.
Due to the way they’re assembled, they require a mixture of fuel and oil. Two-stroke engines generally require more maintenance, but the parts are cheaper. Bikes with two-stroke engines are generally lighter and faster, with more of an initial kick to the “get up and go”. While they require half the strokes to accomplish the same purpose as a four-stroke engine, they do make twice as much noise. Two-stroke engines will give you more torque at a higher RPM.
Two-stroke engines basically combine a handful of steps and complete them in just two movements. So efficient!
Dirt bikes with a two-stroke engine are generally lighter, produce a higher pitched noise, and are cheaper than dirt bikes with 4 stroke engines.
Here's a handy-dandy video that shows you exactly how two-stroke engines work, in addition to the parts they have:
Four Stroke Dirt Bike
Four stroke dirt bike engines are a little more complex than their two-stroke counterparts. "Why?" you ask? Like the name suggests, a four-stroke engine takes four strokes, or movements, to complete an engine cycle. Doing this requires more parts, resulting in a more complicated, heavier engine.
Four stroke dirt bike engines have four steps: intake, compression, combustion, and exhaust. Instead of firing every single revolution of the crankshaft like the two-stroke engine does, four-stroke engines fire every two revolutions of the crankshaft.
A four-stroke engine takes four piston movements to achieve a singular engine cycle. On the first, or “intake” stroke, the piston lowers, pulling in a mixture of air and fuel. Next, it raises for the “compression” stroke, which ignites the air and fuel mixture. This ignition forces the piston down for the “power” stroke, followed immediately by the “exhaust” stroke, pushing the exhaust out of the engine. These cycles repeat over and over again, the whole time the bike is running.
Four stroke engines “fire” every two revolutions of the crankshaft, which delivers a steadier and more easily managed output of power. Typically a better bike for beginners, a four-stroke engine is great for both trail riding and racing. When trail riding on a four-stroke, you’ll rarely ride above second gear, because they can rev higher than a two-stroke can.
This means less work for you and your engine. There are more parts to a four-stroke engine, which enables you to spend less time braking and shifting your bike when you have to slow down.
Maintenance is needed much less frequently since the work the engine is doing has been spread out through more parts. Unfortunately, this also means that when maintenance IS needed, it is generally more expensive, since more parts need to be worked on and replaced. These additional parts also make bikes with four-stroke engines heavier than their two-stroke counterparts.
Four stroke engines are a very clean burning engine from the viewpoint of emissions testing. This is because they have a higher fuel efficiency than the two-stroke engine, and you don’t have to mix oil with your fuel in order to make the engine run.
Four stroke engines are heavier, which results in a heavier bike when similar cubic centimeters are concerned, sometimes weighing 50% more than comparable two-stroke engines.
They are also much harder to clean, due to the higher number of parts they’re built of. Four stroke engines will give you more torque at a lower RPM.
Four stroke engines make a lot of sense (creating a lot of pros that we'll talk about later), but they aren't as efficient as a two-stroke (which leads to some cons that you'll learn about in a moment).
Dirt bikes with four-stroke engines are heavier than dirt bikes with two-stroke engines, make a deeper noise, and are cleaner burning when it comes to fuel consumption.
To see an animated four-stroke engine in action and learn a little bit more about their parts, check out this fantastic video:
How are they Similar?
Before getting into the similarities, here's a quick review: Two-stroke dirt bike engines have two movements of the piston that take place in order to keep the engine going. Four stroke dirt bike engines require four movements of the piston to accomplish the same thing.
In describing the differences, there are already some similarities that we can see:
- Both two-stroke engines and four-stroke engines have pistons that keep them running.
- Both engines complete four steps: intake, compression, combustion, and exhaust. (But again, what makes them different is that two strokes complete the four steps in two movements.)
- Their processes are designed to keep the dirt bikes running and moving. Simple.
Regardless of the engine built, both two strokes and the four strokes are great engines that make great dirt bikes. However, two strokes work better on some terrains and are more efficient on different types of dirt biking than four-stroke engines are, and vice versa.
Yet another similarity between two-stroke engines and four-stroke engines on dirt bikes is that they both have pros and cons. Keep reading to find out all of the good, the bad, and the ugly things about two-stroke engines and four-stroke engines. (Ok, not the ugly. But we're gonna talk about the good and the bad.)
Two Stroke Dirt Bike
Two stroke dirt bike engines have a lot of really awesome things going for them. But they also fall short in a few areas.
As you read the pros and cons for two-stroke dirt bike engines, keep in mind the kind of dirt biker that you are. Where you ride and how you ride your dirt bike is honestly what should determine whether a two-stroke dirt bike engine is a good choice for you.
Because two stroke dirt bike engines are simpler, condensing four steps into two movements, they are smaller and lighter than four-stroke engines. Having a lightweight dirt bike can be a good thing for a number of reasons.
For example, they have faster acceleration and are capable of decelerating more quickly, which enables you to make split-second decisions that your bike can actually react to. Why? The reason is simple: weight slows you down.
Think about it with a childhood memory: running with your backpack on. If all you had in your backpack was a folder and maybe a notebook, running was no issue at all. You could get going without much effort, and slowing down was only as hard as slowing down is when you're a kid. BUT if you had two or three workbooks, your lunchbox, and a water bottle in your backpack plus the folder and the notebook, getting started running was a little bit more challenging because you had extra weight to move. Slowing down was harder because the extra weight was pushing you forward.
Get the picture? It's the same with engine weight. If your engine is lightweight, you can get started and slow down with ease.
Having a lightweight dirt bike is also good for smaller people like myself (or regular-sized children). I've said this in previous posts, but it's really important that the size and skill of the person riding the dirt bike are balanced by the size and power of the bike. If this is weight ratio is not balanced, riding a dirt bike is going to be pretty darn challenging. Lightweight dirt bike engines can also good for trail riding at times. If there's an obstacle that you don't feel comfortable jumping or riding over, you can easily pick up your dirt bike and lift it over.
Now, I know it's not that much of a weight difference for the overall dirt bike if you have a two-stroke instead of a four-stroke, but every pound counts in dirt biking. (At least, it does in my opinion.)
If you've read anything else I've written, you know that I am big on cost-effectiveness. So, of course, I had to include this in the pros of two-stroke dirt bike engines!
Brand new two-stroke dirt bikes cheaper upfront than four strokes are. If you buy a used two-stroke, though, you're gonna save even more money. Who doesn't love that?
Not only is the initial cost of a two-stroke dirt bike cheaper, but upkeep is also generally cheaper on these engines. Because these engines are simple machines that only need two piston movements to complete their process, the necessary maintenance is not only cheaper but easier to complete.
Two stroke engines are typically cheaper also because they have fewer parts than four-stroke engines. Less material means less money. This comes in handy when you have to rebuild your two-stroke engine.
They're easy to clean.
Two stroke engines are assembled using fewer parts than the four-stroke, which is definitely the more complicated counterpart. In this case, the fewer parts and lack of complicated operation make cleaning the engine a piece of cake.
Because the two-stroke dirt bike engine operates on just two strokes to complete an engine cycle, it produces more power per stroke. While this does provide for things like quicker initial acceleration, it does have some downsides. For example, it can't "hover" in a gear as well as a four stroke dirt bike can. This means that you'll have to be shifting more often, as the threshold for variability in different gears is comparatively smaller with the two-stroke engine when compared to a dirt bike with a four-stroke engine.
While having a more simple engine is an absolute dream when it comes to doing your own repairs, there are some definite downsides to the two-stroke engine.
Is it easier and cheaper to maintain and repair? Absolutely! Unfortunately, since there are fewer parts, each of the parts does more work, and are subject to more wear on average. This means that, while maintenance and repair are easier and cheaper, you're also going to have to perform more maintenance in general.
As long as you're prepared to be heavily involved in the maintenance, upkeep, and rebuild of your engine, this could still be a good choice!
They're not so great for the environment.
Because a two-stroke engine works at twice the speed of a four-stroke engine, they have higher fuel consumption. Two stroke dirt bikes also tend to "smoke" during startup. Burnt oil gets released into the air as well with the exhaust. These things put out a lot of emissions into the air, which hurts the environment.
I know some dirt bikers laugh at this environmentalist issue, but it's important to note it. I grew up in a home with a plant-loving mom and a dirt bike-loving dad, so I learned to find a balance for a lot of these issues. If keeping the air clean, and the planet happy is important to you, knowing that two strokes are not super environmentally-friendly is important knowledge.
Four Stroke Dirt Bike
I said this before with the two-stroke engines, and I'll say it again because it's important! Before getting into the pros and cons of four-stroke dirt bike engines, I would like to remind you that what engine you have on your dirt bike should match what kind of dirt biker you are.
Keep an open mind as you read on, and don't forget to be honest with yourself about what kind of dirt biker you are. Depending on how and where you ride your dirt bike will be the deciding factor for whether a 4 stroke engine is right for you.
Due to the way a four-stroke engine is built, a dirt bike with a four-stroke engine has something called a wider "powerband". Powerband is the range of speeds an engine can run most efficiently. Because a four-stroke engine has a wider range of speeds at which it can run effectively, braking, shifting, and clutching, you don't have to shift as often, leaving you more room to focus on other things, like not dying.
Easier to ride
Dirt bikes with two-stroke engines are more "jumpy", and while that does give them their vaunted faster acceleration, it does make them a little harder to handle, especially if you're smaller, less experienced, or don't like feeling like you're riding your grandparent's old lawnmower.
More Managable Power Output
As we consider the old axiom, "with great power comes great responsibility", we can apply a similar outlook to dirt bike engines. "With more manageable power output, comes more manageable responsibility". As a two-stroke engine will give you twice the number of firing strokes a four-stroke engine will, a four-stroke engine will give you a more steady and reliable output of power, which makes your bike easier to "read" while riding it and will consistently give you a smoother feel.
As the name entails, a four-stroke engine uses four strokes to complete a single engine cycle, while a two-stroke uses (you guessed it), two. This means that the four-stroke engine uses more parts than the two-stroke engine does, making it the heavier and more complicated of the two engines. This is important to note because of weight, when dirt biking, is very important.
If you generally consider yourself to be "just" a trail rider, or your dirt bike for fun, you probably don't have to worry as much about excess weight, or the effect a couple of extra ounces or pounds can have on your top speed. That being said, if you are a racer, top speed is going to be important to you, and you may be more concerned and conscientious about the base weight of your bike.
Maintenance is more expensive.
Maintenance for a dirt bike with a four-stroke engine is definitely something you need to consider before you purchase one.
Four stroke engines are heavier because they have more parts, sometimes making them fifty percent heavier than their two-stroke counterparts. Having more parts means more things to replace, and a higher chance that something will break, simply by virtue of there being more things TO break.
Now, because the work the engine does is spread out through more parts than the work of a two-stroke engine, the four-stroke engine will need repairs, maintenance, and rebuilding less often than a two-stroke engine will. Unfortunately, repairs on a four-stroke generally are more expensive, since there are more components used to keep the engine running.