strokes. Making all those valves open and close at precisely the right
time requires a whole complement of extra parts — camshafts, lifters,
rockers arms, and so on. The result was also around 50 to 100 more
pounds in parts in a midsize four-stroke compared with the simpler
That’s where things stood for a long time. Four-strokes were inherently cleaner and heavier, and two-strokes were lighter but dirtier.
But Honda, Mercury, and Yamaha have all spent the past few years
developing new four-strokes that perform and weigh comparably to
two-strokes. And Evinrude, Mercury, and Yamaha have all developed
two-strokes that are as clean running as a four-stroke. Each has pulled
off a neat engineering trick.
Teaching an Old caMShaf T new TrickS To understand how a camshaft dictates the power of an engine, it’s important to know what it looks like. Picture a stack of nickels on your desk, maybe 18 inches high. Now carefully push just a finger’s width of the middle of the stack off-center slightly and repeat up and own the stack. Now you have a stack of offset circles, which looks a lot like a camshaft, except that on a camshaft, they are oval in cross- section, not perfectly round. As the camshaft rotates, these ovals push on arms that pop the valves open and closed. The shape of the oval, or lobe, and its orientation on the shaft relative to the other lobes determine when the valve opens to allow air in (or exhaust out), how high it opens, and for how long. The camshaft’s offsets dictate how the engine will perform, and the shape of the traditional camshaft can’t be changed. “In four-stroke engines, the amount of horsepower generated is driven by the camshaft,” says Dennis Ashley, assistant national sales manager for Honda Marine. “A fixed-camshaft can be designed for low-end power or high-end power, but not both.” Variable-valve timing is a way around that. In effect, it allows the camshaft to shift into a higher gear, which lets the engine work up to its full potential. Honda uses technology developed on the automotive side called VTEC, for variable valve timing and lift electronic control system.
It’s complicated, but the system effectively adjusts the shape of the
camshaft, improving the power and performance of the outboard. It’s
like swapping out camshafts while the engine is running, allowing
Honda’s outboards to generate more power at high rpm, while still
having low-end efficiency.
Yamaha has taken a different tack, but still tweaks the camshaft to
improve the performance of their four-stroke models, like the F200
that was unveiled at the end of 2012. They use variable-camshaft
timing, where the camshaft is rotated to advance the lift of the
intake valves. “It’s like taking a big breath before trying to lift a heavy
weight,” says David Meeler, Yamaha’s product marketing information
manager. “It gives the engine a deep breath of fresh air, a great way to
put midrange punch and throttle response into an engine.”
Mercury decided to try to get more horsepower out of less dis-
placement, using a supercharger to force more air into their Verado
outboard in 2004: “A two-stroke takes twice as long to perform the
same engine operation as a four-stroke,” says Steve Miller, senior
category manager for Mercury Outboards. “With the technology
at the time, we wanted the performance of a two-stroke with the
four-stroke benefits. When we considered the supercharger versus
a turbo as a means of boosting performance, we found the turbo
needs exhaust to spin up, so it doesn’t respond as quickly. The
Mercury’s Optimax engines are beloved by bass fishermen
for their quick throttle response and fuel-thrifty ways.
honda developed the aMP+ system for the U.S. coast
guard, which bumps the idle up 100 rpm when needed to
provide more juice to the batteries.
a computer rendering of the new Yamaha f70, showing
the single-camshaft design, where the camshaft opens
and closes both the intake and exhaust valves.