A close look at how MAN added more muscle to its 401 diesel reveals that bulking up an engine involves more than just a bunch of add-ons.

THE ENGINES in a diesel series are typically built on the same short block, but they have multiple horsepower ratings and vary, sometimes dramatically, in price. The price difference often seems greater than the cost of the added parts, such as turbochargers and aftercoolers. You may be left wondering if you're being charged more than you should be for chat extra horsepower. Wouldn't it be cheaper to simply buy the base engine and have the additional parts installed?

The truth is chat increasing horsepower involves a lot more than simply adding power-boosting components. A look at how MAN increased the output of its D2848 LE 401 engine from 680 hp co the 800 hp of the D2848 LE 403 is a perfect example of this. (Note: MAN uses metric horsepower ratings; by U.S. standards these engines are 671 hp and 789 hp respectively).

The first seep was to increase turbocharger efficiency and fuel-injection flow rates, thus increasing the thermal load in the cylinders. But, over time, this would cause the pistons, and possibly the valves, to burn out. So the pistons had to be redesigned (without increasing diameter) to handle the higher heat.

Instead of using heat-absorbing inserts or ceramic coatings, MAN chose to redesign the rod to allow more cooling oil to be directed to the piston. The small end of the piston rod was reshaped from a rectangle to a trapezoid, thereby increasing the open volume under the piston. Then the cooling oil spray pattern was changed co a targeted stream to get more oil where it was needed. You can't see this change, bur it took time and money to make.

Increased cooling-water capacity was also required. A new, higher volume raw-water pump was added, as was a larger air cooler. The number of tubes in the heat exchanger was also increased.

It was necessary for the turbos to provide high performance - even at low rpm. Low-inertia turbochargers were added co deliver the needed increase in boost pressure to burn the added fuel required for extra horsepower. However, such a turbocharger would provide too much boost at high-horsepower rpm. So MAN developed a waste-gate turbocharger system that dumps a portion of the exhaust before it gets to the turbo. This keeps the turbo rpm in its designed operating range while providing extra performance at lower engine speeds to reduce black smoke. MAN then added a special surge-control system to the turbochargers. This system adjusts turbocharger output to smooth abrupt throttle changes and keep the air and fuel ratios in balance.


To further reduce black smoke, the 403's new governor mimics electronically controlled engines by shaping fuel output to engine rpm and load using a curved cam in place of a simple low-idle/high-idle adjustment. The air-cooling system also helps keep smoke levels low. When the engine is cold, intake air is heated by the engine cooling system until normal operating temperatures are achieved.

Many of these smoke-reducing changes could be accomplished with electronically controlled fuel-injection systems, but most engine manufacturers tell me that electronic controls are more costly than mechanical upgrades.

Emissions requirements and the quest for better performance have both helped produce enormous improvements in diesel engines in the past decade; so has customer demand for cleaner, smoother running engines. Today's engines are free of almost all of the smoke and vibration that used to plague the marine diesels, and are in significantly better value. In short, they deliver a lot more bang for the buck.

It should now be obvious that you have to look a lot deeper than merely calculating the cost of added external components to discover the reason for higher prices for higher horsepower and improved performance. The next time you're looking at the prices of engines with various horsepower ratings, I hope you're aware that the differences are a lot more than "skin deep". 

This review/article originally appeared in Motorboating & Yachting Magazine, September 1996 and is written by Jim Daly. For more great yacht reviews, visit their website and subscribe at: https://www.magazinesdirect.com/az-magazines/34207321/motorboat-and-yachting-subscription.thtml?j=QMY