Several years ago, Mercury Marine literally bet the farm on a totally new concept in outboard engines: supercharging. Not that supercharging gasoline engines is new–it's been around since the 1930s, but no one had yet been willing to try it on a production outboard. In Merc's case, it appears that the risk has been well justified.

Why supercharge an outboard in the first place? The gear required adds weight and complexity. But the pathway that led to this is interesting.

The high-horsepower segment (e.g. 150 hp and up) of the 4-stroke outboard market is very competitive and primarily dominated by large-displacement V-block engines (exception: Suzuki's 4- cylinder inline 150 and 175). The engineers at Mercury Marine elected to draw on the company's history with "6-storey" inline two-stroke outboards in the 90 - 135 hp range. This time, though, they designed a small-displacement inline six and supercharged it to get roughly the same amount of horsepower other manufacturers get from their bigger Vblocks. Mercury calls this new line Verado, and it makes a slim package, compared with the Vblock designs. Although the configuration isn't much lighter than the Vs, it leaves more room on the transom to hang one or two more engines for those who like to go very fast in large boats.

Verados have some other advantages, too. Whisper-quiet is one of them. Recently I had the pleasure of taking a ride aboard a 33-foot Yellowfin with four 275-hp Verados on the transom. That's a total of 1,100 horses just eight feet abaft where we stood, turning 5200 revs and blasting us along at 72 mph–yet we were able to converse easily without raising our voices. If anything,wind and water noise were much more noticeable than the sound of the engines. At idle speed, the tachometer was the only way to tell which, if any, of these engines was running.

HOW IT WORKS

The supercharger is housed in a relatively small box on the front of the powerhead. An impeller, driven by a belt off the engine's crankshaft compresses intake air, stuffing the combustion chamber with far more volume than atmospheric pressure can do alone. This compression process also heats the air, reducing its density. To increase the density, the air has to be cooled. Downstream of the compressor the air flows through an intercooler, which is cooled by air or water, making intake air as dense as possible. This compressed, cooled air is force-fed into the engine, giving it a much higher concentration of oxygen than it would get through normal aspiration. More oxygen allows more fuel to be burned efficiently per cylinder, thus more horsepower per cubic inch of displacement. An electronic throttle sends commands to a computer-controlled sequential multi-port electronic fuel injection (EFI) to make this happen. The net effect is that the supercharged engine burns about the same amount of fuel per horsepower as does a larger displacement naturally aspirated engine.

But, because the Verado doesn't need this boost except in the higher rpm range, a bypass tube that is regulated by an electronically controlled bypass valve recirculates a portion of the boosted air back to the supercharger. When the boost is needed most (e.g. hole shot, high rpm, etc.), the bypass valve closes, and the engine gets the higher concentration of oxygen and fuel it needs.

By the way, turbochargers serve essentially the same function as superchargers. The main difference is that turbos are powered by exhaust gases coming out of the engine and superchargers are driven mechanically via gears, a chain, or as in the case of Verado, a serpentine belt.

THE END RESULT

The Verado, with just 159 c.i.d, produces up to 275 horsepower, while a 4-stroke 250-hp V-6 requires 205 to 221 c.i.d. It also delivers a hole shot like a rocket, and can easily supply enough extra power for finger-tip power steering that controls up to four or five engines at a time. Mercury Marine has priced the Verados to be competitive with other 4-strokes in its horsepower class.

After several years of working successfully in the field with these supercharged 4- strokes, Merc has extended the Verado line downward from 200 to 135 -175 horsepower, using a 4-cylinder version of Verad's design. All current Verado models carry a 3-Star Ultra-Low rating from the California Air Resources Board for long-term emissions compliance. They'll also run just fine on 87 octane gasoline, although Mercury recommends 92 octane for maximum performance. Given today's skyrocketing fuel prices, my guess is that most of us would willingly give up a couple of mph at top end to significantly reduce the pain at the pump.

Contact: (920) 929-5040 www.mercurymarine.com.