Low-emissions DFI (direct fuel injection) two-stroke outboard motors have been available since 1996, so it would seem like this technology is old-news. Unless you’re a boating Rip Van Winkle, like my neighbor—who has decided that this is the year he’s going to replace his chalky old runabout and its trusty V-4 90 hp motor. Perfectly satisfied with this package, he’s been motoring about in a cloud of two-stroke fumes, oblivious to a decade of advances in outboard technology.

There are plenty of old outboards prowling around out there, and a lot of boat owners like my neighbor. If you've been out of the outboard loop, it may be time to get up to speed with a quick tutorial on the modern two-stroke outboard.


etec fuel injector two stroke

Modern DFI two-strokes utilize injectors like this one from the Evinrude ETEC line, to gain a huge efficiency boost.


Two-stroke outboard motors with DFI (direct fuel injection) were developed in response to an EPA mandate requiring a 75 percent reduction in exhaust emissions between 1998 and 2006. Since the days of Ole Evinrude, almost all outboard motors used a traditional two-stroke powerhead, which was lighter and less expensive to manufacture than a four-stroke engine. Unlike the four-stroke engine, the two-stroke does not have moving valves to seal the combustion chamber and control the flow of intake and exhaust. In a two-stroke engine the rising and falling piston covers and uncovers ports in each cylinder through which a fuel/air mixture flows into, and exhaust flows out of, the combustion chamber.

A traditional two-stroke produces very high exhaust emissions because the fresh charge of fuel and air, streaming from the crankcase through the transfer ports and into the combustion chamber, arrives while the exhaust port is still open. Some of the unburned fuel mixture escapes with the exhaust, creating the cloud of hydrocarbon emissions we all used to take for granted. In fact, the unburned fuel often escaped by design, to ensure that all of the spent exhaust was “scavenged” from the combustion chamber. This was especially evident at low speeds when up to 40 percent of the fuel entering a traditional two-stroke motor escaped unburned. At its most efficient speed, generally 4000 to 4500 rpm, a traditional two-stroke may still expel eight percent of its fuel with the exhaust. To get a visual, check out this video of