Boat Outboards 2013

By Michael Vatalaro
Published: April/May 2013

The past decade has seen an amazing array of changes and improvements in outboard engines. If you haven't shopped for one lately, you're in for a nice surprise.

Teaching An Old Camshaft 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 down 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.

Photo of the Honda 250 Outboard Engine
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.

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 mid-range punch and throttle response into an engine."

Computer rendering of the new Yamaha F70 engine
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.

Mercury decided to try to get more horsepower out of less displacement, using a supercharger to force more air into their Verado outboard in 2004: "A four-stroke takes twice as long to perform the same engine operation as a two-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 supercharger also poses no heat issues. So we designed the engine with oversize cranks and bearings to be supercharged."

The other side of the performance coin is weight. Due to their complexity, and the need for parts like an oil sump and pump, four-strokes must shed weight in other areas to have the same horsepower-to-weight ratio as a two-stroke. One strategy is to squeeze more power out of a smaller block, like Mercury's Verado. Another is to put an existing design on a diet, rigorously examining each part for excess weight. Yamaha has pursued a combination of these strategies recently, and the results are striking.

A few years ago, Yamaha's flagship 350-hp outboard and the 300-hp version of it were based on eight-cylinder blocks. But at the end of 2009, they introduced a 4.2-liter V6 outboard that put out 300 horsepower, and weighed around 250 pounds less than its V8 predecessor. Besides extracting nearly the same amount of power from fewer pistons, thanks in part to larger-bore, longer-stroke, and well-thought-out air-intake design, they also shed weight throughout the V6, from the cowling to the cylinder sleeves. They've even reduced the weight of the alternator by using smaller, more powerful, rare-earth magnets (more on this later). Their new four-cylinder F200, released last fall, continued the trend, shedding nearly 120 pounds compared with their earlier V6-powered 200-hp models. In fact it's just 14 pounds heavier than their 200-hp, DI two-stroke model.

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