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Should you buy a Two Stroke OR Four Stroke Outboard Engine?

Mercury Outboard Motors

Should you Buy a Two Stroke OR Four Stroke Outboard Engine?

Outboard engines have been around since 1896 and have done a good job of getting smaller craft from point A to point B with simple efficiency. As time has gone by the simplicity of these motors has diminished as the technology advanced. Along with greater power, speed and efficiency came more sophisticated motors with more complex components and systems. Today’s outboard is a high tech, computer-driven powerhouse with vastly improved fuel efficiency, faster speeds, better response, more power, improved reliability and is better for the environment.

The argument to go with a two stroke or  four stroke outboard is a very subjective one. All the major manufacturers claim their motors are superior and each brand boasts unique technology that puts its motors above the competition. If you are in the market for a new engine how do you know which one is right for you? With advances in technology the offerings are greater than they have ever been before. From small 2.5hp kickers to whopping 557hp monsters, the choices seem endless. Should you go with a two-stroke model or a four-stroke? What horsepower will allow for the best performance of your vessel? Which brand will be the best fit? Should you go with a single engine, duals or even triples? With all the choices and possibilities, selecting a motor can be a daunting task. This series of articles will attempt to help with your selection by taking some of the mystery out of these modern marvels, explaining the technology behind them and the advantages and drawbacks they have to offer.

Outboards are typically categorized as either two-stroke or four-stroke. This terminology refers to the number of strokes the piston makes to complete a single power cycle. Two-stroke and four-stroke motors are based on the same principles of a combustion engine but are engineered very differently. Each has its advantages and disadvantages. Two-strokes are said to be noisy, low performers at or near idle speeds and not very fuel efficient with high emissions. They are, however, said to be very responsive, fast, powerful, light and simple to maintain. Four-strokes, it’s said, are heavier than their brethren, less responsive off the line and more expensive to maintain. They are also said to be quieter than two-strokes and more fuel and emissions effective. All of these statements may be true to some extent, but engines of both types on the market today are blurring the ‘traditional’ advantages and disadvantages of each.

The first outboards commercially produced back in the early 1900’s were actually four-stroke models, as existing motors were modified for the application. But as time moved on, the two-stroke version we know today became the mainstream outboard flooding the market and became the outboard motor of choice. The nature of its mechanics allows for a lot of power in a small compact package; it’s because of this that its popularity grew. Back in the day, four-strokes could just not output the same power in a comparable physical size of the two-strokes. Seemingly at the time, two-strokes were the perfect solution. Recently technology has arrived enabling the more intricate and complicated four-strokes to be engineered effectively for the marine environment, producing the results required by the demanding boat owner. There has been a more recent trend to four-stroke motors due to cleaner emissions requirements and a demand for more fuel efficiency with soaring fuel costs. As greater advancements and improvements occur in both, the differences and disadvantages of the two types is becoming more and more blurred, making selection of one over the other ever more difficult.

Mercury Outboard Motors
Mercury Outboard Motors

To understand the benefits and pitfalls of each, one must first understand the basic principles of how they work.

Two stroke motors function by completing a power cycle (intake, compression, power and exhaust) with one turn of the crankshaft and a resulting two strokes of the piston. There are intake and exhaust ports on either side of the cylinder and they open and close by the motion of the piston. In a full power cycle several events occur in each stroke; as a result the motor tends to be more rapidly responsive. Because the ports on either side of the cylinder do not have valves there is less mechanical hardware with a two-stroke motor and as a result there are less moving parts (no valves, valve springs or cams). This results in a lighter weight than a comparable four-stroke motor.  Fuel enters the cylinder as the piston moves up and combusts when ignited by the spark plug. The resulting controlled explosion causes expansion, pushing the cylinder back down. The gases escape through the exhaust port as the cylinder moves up again.

This is where the lower efficiency of older two-strokes comes into play. At this point of the power cycle, the intake (where the gas enters the chamber) is open, as is the exhaust port (where the exhaust is sent out of the motor). This allows for unburned fuel to remain in the chamber and also to escape out of the exhaust without being combusted, reducing fuel efficiency and increasing hydrocarbon emissions. This process is known as scavenging. Newer two- strokes are starting to address this issue, but we will get into specifics in a later portion of this series. These engines were lubricated by mixing oil with the gas, which also resulted in higher emissions and a smoky motor at low rpms. New direct-injected motors resolve a lot of these issues, and improving technology is keeping the two-stroke a viable option in today’s world.

Four-stroke motors by their mechanical nature have some advantages and disadvantages. The combustion process is very similar to the combustion process of the two-strokes. Unlike two-stroke outboards, however, the new four-stroke motors have to make two full revolutions to complete one power stroke. It is for this reason that they have tended not to be quite as responsive to the throttle and in acceleration as the one revolution per power stroke two-stroke engines. The combustion process is very similar to that of the two-strokes. However, four-stroke outboards have valves for each cylinder that are held closed by springs and are opened at the appropriate time by a camshaft driven by a crankshaft. Because they have this system, the valves remain shut when needed so uncombusted fuel cannot escape through the exhaust chamber and exhaust gases do not remain in the chamber. This typically translates into a cleaner, more fuel-efficient motor. This additional hardware adds weight and physical size to the motor, which can end up being a disadvantage.  These motors do, however, enjoy a reputation for being extremely reliable, easy to start and they operate smoothly and quietly at idle or low speed. Their emissions meet ever-increasing environmental policies and are being welcomed exclusively in certain bodies of water due to their environmental friendliness. All this comes at a price, however, as these motors tend to be more expensive than their simpler, lighter competitors.

With new technology has come a blurring of the lines between new two-stroke motors and four-strokes. Arguments for one or the other can be countered in many cases with new technology that is being applied. In the following articles we will delve into what new technology is being used by each of the major outboard manufacturers to improve their range of motors and will try to enlighten the new buyers as to which motor would be the best option for their application. We will talk with experts in the field and will explain the sophisticated technology that is producing advanced outboards that would be a perfect fit for you.  So hang on, technology is going to ensure you have the ride of your life.

Continue reading with our How to Buy an Outboard Engine Series

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One comment

  1. After reading this article, it seems like advantages in technology have made the decision between two and four stroke very subjective. I like how the article mentions though that the bottom line is the same; outboard motors today bring more power, speed, and reliability. I think a tip that could be helpful for choosing which outboard motor is right for you would be to speak with people that utilize a four or two stroke and ask what they like or dislike about their motor size.

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