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Two-stroke and Four-stroke Outboard Motor: What are the Differences?

Blog | 07 December 2018

Whether a boating enthusiast is replacing an old outboard engine or he’s buying an entirely new craft, there are some elementary engine differences he should know about right off the bat. The differences between two-stroke and four-stroke outboards, and there are major differences, are one example of fundamental boating knowledge. So, if there are still doubts about those alternating engine architectures, here’s where we clear up the matter. 

The Telling Differences: Two-Stroke Outboard Motors

Folk tend to make sweeping generalizations when they talk about the two engine families. Let’s try and avoid this unfortunate habit. First off, two-stroke outboards are economically designed. When the engine stroke commences, they carry out the intake and compression stroke in one action. Likewise, the expansion and exhaust stroke take place as a second action. That compact cycling effect delivers power fast, but there are costs to be exacted when engine pistons multitask. For older two-stroke motors, expect noise and fuel inefficient operation. So far, then, things don’t look so good for two-stroke outboards, but we’re not finished with them just yet. 

Powerplant Efficiency: Four-Stroke Outboards

Refusing to multitask, the pistons in a four-stroke outboard motor separate the actions described in the previous paragraph of text. There’s a separate stroke for the intake action, for the compression stage, expansion, and the exhaust reaction. That’s all very good, so why are two-stroke outboards still around? Well, four-strokes are large. They also require more maintenance, probably because there are more moving parts under their beefy engine cowls. Still, even with the size issue causing problems for a boat’s transom, they’re more fuel efficient and up to 50% quieter than their smaller, noisier counterparts. 

Weighing the Performance Tradeoffs

Up to now, this post has been all about major differences, about performance gaps that seemingly make a clearcut buying choice easy. Unless a buyer’s boat is small and its hull is fragile, then a four-stroke outboard is the superior choice. Not so fast, though, technology has muddied the waters a little. Because of newer, more innovative designs, the latest two-stroke motors run quieter and they’re more fuel efficient, to boot. They’re also lighter and often more responsive than larger four-stroke motors, and they don’t require complex valving configurations or intricately designed camshafts. At the end of the day, just remember the key points mentioned above, then let the various manufacturers show off their wares. 

Using the above buyers-be-aware approach, boaters know that two-stroke outboards generally make more noise, but they’re usually responsive, so they take the time to seek out the latest generational models. Meanwhile, four-stroke outboard motors are quieter, they do discharge fewer emissions, and they’re extremely fuel efficient, but there’s still that chunky disposition to consider.

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