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Why Gasoline Inboards May Rise Again

Volvo Penta V8-300C

Dynamic Duo

Boaters finally get the latest gasoline-engine technology in two new engines.

The fact that I grew up in Detroit and San Diego explains my two passions: engines and boats. These two passions have produced a lot of enjoyment but also a certain frustration. As an engine guy I found it galling that boats had hand-me-down gasoline engines when there was so much new technology on the road. I was cursing the ignition points in my boat engine long after electronic ignition had eliminated them in cars, and my balky marine carburetor gave me fits that my car’s electronic fuel injection never did.

One excuse for this disparity has been that the marine environment is more demanding than the automotive one, both in terms of the corrosive atmosphere and the consequences of a breakdown.

Another excuse is what engineers call duty cycle, a fancy word that describes the job an engine has to do. Your car has a multigear transmission to help it accelerate that your boat doesn’t. Moreover, a car’s engine is under much less load at cruising speed than a boat’s. Thus high-revving V-6s and V-4s do fine in an automobile but not well in a boat. That’s because torque, not horsepower, puts a boat on plane, and torque is a function of displacement. In other words, big engines make more torque.

But variable valve timing (VVT) has altered that equation. In a conventional gasoline engine, intake and exhaust valves always open at the same time relative to the position of the crankshaft; with VVT, these openings vary according to rpm. Today most new cars have VVT, and their small engines accelerate like big ones on the low end and rev up ike small ones on top. VVT engines also have better fuel efficiency and produce fewer emissions.

VVT has been in cars for over a decade, and now it’s available for boats, thanks to Volvo Penta. Finally, boats can enjoy the same high-tech engine features as cars. The engine is General Motors’ Generation V 5.3L V-8, which the company has marinized and now sells for stern-drive applications as either the V8-300-C or the V8-350-C.

In terms of technology, GM’s engine can hold its own with any in the world. This 5.3-liter (323 cubic-inch) V-8 is all new; it shares virtually no components with any previous GM motor. Both its cylinder block and heads are aluminum, which alone makes it unique in the marine industry. This feature saves roughly 120 pounds and also means that freshwater cooling is standard. Although it has only two valves per cylinder and pushrod activation, it does come with common-rail direct injection (DI), a technology that appeared in cars only about five years ago. Instead of injecting fuel into the intake manifold, upstream of the valve, this system sprays it directly into the combustion chamber, very close to the spark plug. DI not only allows for more complete combustion but also improves efficiency by allowing the use of a leaner mixture. It also cools the combustion chamber, which permits a much higher compression ratio (11:1) without the need for premium fuel.

While DI improves combustion, VVT flattens out the torque band, meaning there’s more torque sooner, a key factor in planing. Peak torque for both engines (382 pound-feet) comes at 4100 rpm, but at 1200 rpm the engines are already making 300 pound-feet. Maximum horsepower output for both the 300-C and 350-C is at 5600 rpm.

GM includes a number of other engineering features that make this engine noteworthy, including a variable-displacement oil pump and pistons that are cooled by jets of lube oil sprayed on their undersides; together they should ensure longevity even under the increased loads common to marine applications. The throttle is electronic, eliminating balky mechanical linkage, and the spark plugs are tipped with iridium for extended life. These two engines come mated ony to DuoProp stern drives—for now.

Of course in order to comply with current emissions regulations, these engines are fitted with catalysts and wideband oxygen sensors, the latter being an industry first, says Volvo. They also come with Volvo’s rpm-based speed control, which automatically maintains boat speed regardless of changes in load.

The bottom line is impressive. I was able to test boats powered by both new engines as well as similar boats powered by the engines they replaced and had no trouble telling which was which. The difference was dramatic, especially at the point where VVT kicks in—it really throws you back in your seat. Acceleration is significantly quicker—the boats with the new engines were, I’d estimate, 20 percent quicker to plane and even more so to top speed. Time did not allow fuel-consumption measurements but Volvo claims an improvement of 10 percent, which would be remarkable given the improvement in performance.

It’s unclear when these engines will be available as straight inboards. All I can say is that this duo bristles with the latest engine technology, and for once boaters are going to get it.

This article originally appeared in the March 2016 issue of Power & Motoryacht magazine.