4-stroke outboard motors are an excellent choice for your boat. They’re powerful, efficient, and compared to their 2-stroke counterparts, run more smoothly and are less harmful to the environment. But, how can you tell if an outboard motor is a 4-stroke?
Here’s how to tell if an outboard motor is a 4-stroke:
- A 4-stroke has an engine oil cap, reservoir, and dipstick.
- 4-stroke motors have a lower rumble and make less noise.
- 4-stroke motors are heavier than 2-stroke motors.
- 4-stroke motors produce less smoke than 2-stroke motors.
2 and 4-stroke motors may have the same job to perform, but the way they accomplish this varies significantly. Although 2 and 4-stroke engines have key differences internally, this article focuses more on distinguishing each motor without taking them apart.
1. A 4-Stroke Has an Engine Oil Cap, Reservoir, and Dipstick
A 4-stroke engine will have a separate oil and fuel tank, as opposed to a 2-stroke engine that only has a single tank to house the oil and fuel mixture, which will help you identify which. But, to better differentiate the two motors, it helps to know how each of them works.
How 2 Stroke Motors Produce Power
As the name suggests, a 2-stroke engine produces power with two motions; the ignition and power stroke. These two motions include an upward and downward movement to complete one complete revolution from the engine, which means every cycle produces power:
- The ignition/compression stroke: The ignition stroke is when the piston moves up to allow air and fuel to enter through an inlet port to be compressed and ignited by a spark plug.
- The exhaust/power stroke: After ignition, the pressure in the chamber builds up and pushes the piston back down, allowing it to get rid of unwanted exhaust gas through an outlet port.
A 2-stroke motor requires an oil and fuel mixture to provide lubrication to perform its power cycle. Not having a separate lubrication system means that 2-stroke engines only have one tank to house both the lubricant and fuel (source).
How 4 Stroke Motors Produce Power
4-stroke motors, on the other hand, require twice as many motions to complete a power cycle, which means it requires two complete revolutions (or cycles) from the engine to produce power.
Below is what happens with each stroke:
- The intake stroke: The intake valve opens to allow the air fuel to enter with a downward stroke of the piston.
- The compression stroke: An upward stroke follows the first step, which compresses the air-fuel mixture.
- The power stroke: Once fuel compression occurs, it’s ignited to produce power and pushes the piston back down.
- The exhaust stroke: The final upward stroke in this cycle pushes burned gasses out through the exhaust valve. When the piston reaches the topmost position, the exhaust valve closes, and the intake valve reopens to repeat the cycle.
Similar to 2-stroke motors, 4-stroke motors also require lubrication. However, 4-stroke engines don’t burn oil but recycle it instead.
In fact, one of the easiest ways to tell a 4-stroke engine apart from a 2-stroke is if it has a separate oil reservoir. And since 4-stroke motors don’t burn oil, the oil they use must be checked and replaced periodically, which means 4-stroke engines will also have an oil cap and a dipstick.
2. 4-Stroke Motors Have a Lower Rumble and Make Less Noise
Another way to identify a 4-stroke outboard motor is by listening to the sound it makes. 4-stroke engines run quieter and have a low-pitched rumble compared to a 2-stroke motor which is louder and has a higher pitch (source).
The reason 2-stroke engines are louder is that combustion occurs with every cycle of the motor, unlike a 4-stroke engine where it happens after every two cycles or revolutions of the engine.
To put things into perspective, a 2-stroke outboard motor revving at 2000 rpm will have fired 2000 times, while a 4-stroke outboard motor revving at the same speed would have only fired 1000 times.
You can watch this video to hear the difference between the sound of a 2 and 4-stroke engine:
3. 4-Stroke Motors Are Heavier Than 2-Stroke Motors
4-stroke outboard motors can weigh up to 50% more than 2-stroke outboard motors. The difference in weight is due to their overall construction and the number of parts they use.
To be more specific, 4-stroke outboard motors have a more complicated design since they have a valve train which will require a bigger cylinder head. These 4-stroke engines also have a more complex lubrication system and, as stated earlier, have a separate reservoir for oil.
2-stroke engines only have intake and outlet ports instead of valves, which translates to fewer moving parts. These engines use only one tank for the oil and fuel mixture, making their overall design a lot simpler and lighter.
Source: Boats.net: 2 Stroke Vs. 4 Stroke Outboards: Pros and Cons
4. 4-Stroke Motors Produce Less Smoke Than 2-Stroke Motors
If you get the chance to fire the motor up and see that it starts up quite quickly and produces less smoke, chances are it’s a 4-stroke motor.
4-stroke outboards have cleaner emissions. 4-stroke motors can be up to 90% less harmful to the environment than 2-stroke motors.
Since 2-stroke motors run on blended oil and fuel, the combustion of oil causes them to emit more smoke.
The absence of valves also means that some air-fuel mixture can leak through the exhaust port. This leak, combined with oil, increases the smoke 2-stroke engines produce (source).
Key Takeaways
The differences between 4 and 2-stroke outboard motors may not be evident at a glance. However, closer inspection and even using them briefly is enough to reveal their unique characteristics.
Once you spend a bit more time using one, or better yet, examine its internals more closely, you’ll also notice the following traits of a 4-stroke motor:
- It requires a heavier flywheel
- Better lubrication generally allows for longer lifespan
- It’s mechanically efficient due to more moving parts
- It’s more fuel-efficient
- It runs at cooler temperatures
- It can be either air or water-cooled (2-stroke engines are always air-cooled
