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Volvo Penta D4 and D6 Marine Diesels

Volvo's D4 and D6 diesels were designed from a blank sheet of paper. In other words, they are not merely marinized truck engines. The D4 is a 3.7-liter, four-cylinder, in-line motor rated 210 horsepower. While its brother in the blood, the six-cylinder D6, displaces 5.5 liters and is rated 310 hp. Both of them are intercooled and turbocharged.

Heart and soul of the D4 and D6 is common rail fuel injection. The benefits of the technology is the essence of simplicity. A single fuel line supplies all of the injectors. Fuel in the rail remains under constant pressure, as opposed to conventional systems where fuel must be pressurized with each pulse of the injector before it can be injected into the combustion chamber. So with common rail's pre-pressurized fuel, throttle response is a fraction of a second quicker. The payoff is measurably crisper throttle response.

D4 and D6 power is managed via electronics, allowing individual cylinders to more quickly respond to a change in throttle position. Once again, the payoff is razor sharp throttle response. In mapping the power band, Volvo engineers paid particular attention to low rpm torque in order to speed a boat's acceleration, even when loaded to the gunwales. Legendary diesel fuel economy becomes even more miserly and cruising range is extended. Volvo engineers went one step further. By their design, diesel fuel pulses into the combustion chamber as often as three times per power stroke. The net effect is a softer, more progressive combustion, a characteristic that lowers the noise level. Translation: Diesel knock is non-existent.

No big surprise with this micrometer like fuel metering, emissions are low and smoke is virtually non-existent at startup and running. These engines met the requirements for both the US and Europe in 2006. The foundation for the dynamic duo of new diesel engines is a particularly robust block featuring integrated cylinder liners. A ladder, strategically located between the block and crankcase stiffens the engine so that vibration are substantially reduced.

D4 and D6 diesels are available in both inboard and sterndrive packages. The sterndrive is also new. Predictably, it's a DuoProp, Volvo's proprietary technology that spins twin, counter-rotating propellers. For those few who may be unfamiliar with the technology, Duo Prop's twin propellers better grip the water, allowing a greater conversion of engine horsepower into thrust. Greater thrust pays big dividends in stronger acceleration, improved fuel economy, and more precise handling, both at speed and when docking. Reverse maneuvering is particularly agile. The DuoProps comes in two different configurations, the DP and DPR. The DP version is intended for boats with top speeds up to 45 mph, while the high performance DPR is intended for high-speed boats.

Both of the new Duo Prop drives were engineered to handle the greater torque of the new diesel engines. Gear sets, bearings and shafts are robust. Key features include integral exhaust and cooling water passages that obviate the need for boring inlet holes through the transom. The over-sized passages flow twice the cooling capacity of previous models, insuring cool running. Also, exhaust back pressure is reduced by 30 percent, significantly boosting engine's ability to generate its maximum hp. The steering system features a powerful servo that includes a feedback valve that lends the helmsman the feel of control resistance and response. In other words, the driver feels more at one with the boat.

Nine paired propeller sets are available. The front propeller is three bladed, the rear four. Each set of props is designed for a particular speed range (from 25 to 45 knots). Each pair was painstaking tailored so that front and rear wheels share work equally, lending the engine maximum efficiency. With the matched sets, the days of calculating propeller pitch versus wide-open throttle rpm are history. Propeller sets are chosen on the basis of the boat's top speed. One curious note is the speed bump found on the lower unit near the propeller hub. Derived from supersonic spy aircraft, the bump eliminates cavitation of the propeller blades so every last drop of horsepower converts to forward motion.

Propeller casting are machined from nickel, aluminum and bronze, an alloy commonly found on inboard installations. Volvo, though, is the first marine engine company to use the alloy on a stern drive wheel. The alloy allows high precision casting and machining at the same time it stands up to brutal engine torque. Obviously corrosion resistance is also very good.

Finally, the reliability of the new engine and drives, Benny Hedlund, before he retired was Volvo Penta's Technical Project Manager who oversaw the development of the DuoProp. He says his engineers logged nearly 15,000 hours in various boats, in various surface conditions, just to make sure they had things right. A goodly number of the endurance tests ran for 300 hour endurance, long enough to have run the boat all the way from Sweden to New York City. Given extensive testing and Volvo's sterling reputation, it seems likely the new power packages will do rather nicely.

Specifications: Nomenclature D4-210 Nomenclature D6-310
Displacement 3.7L 5.5L
Configuration Inline-4 Inline-6
Horsepower 210 @ 3500 rpm 310 @ 3500 rpm
Weight 1012 pounds 1232 pounds
Alternator Output 115 amps 115 amps