New EPA Tier 3 Diesel Engine

Cummins replaces their venerable 5.9 liter diesel with a cleaner, quieter, more powerful 6.7 liter engine.

Daniello cummins article

Daniello cummins article

Cummins, Inc. is unveiling its new 6.7-liter common rail diesel at the Miami International Boat Show. The engine has been in Dodge Ram pickups since 2007. Why so long to bring it to the marine market? Cummins didn't simply marinize the truck engine -- new EPA emissions standards for recreational boats were met without aid from catalytic converters used in highway applications. While they were at it, Cummins engineers increased power without increasing engine size, weight or fuel consumption.

The new inline six-cylinder QSB diesel will be available beginning this summer in 380-, 425- and 480-horsepower offerings, mirroring the current horsepower ratings of the 5.9 liter engine it replaces. Measuring just 1.6 inches taller, 1.6 inches wider and 1 inch longer, and weighing about 5 pounds less, the new 6.7 liter engine will slip right into existing production boat manufacturing lines without retooling engine rooms.

"The 5.9 liter engine is currently used in so many applications. We wanted to build a Tier 3 compliant engine to replace it in existing production boats," says Chris Ley, Cummins engineering team leader for the new engine. "Higher horsepower ratings are a new market that will come later."

Considering past launches from Cummins, anticipate noticeable horsepower increases over the coming years -- the QSB 5.9-liter diesel was introduced with electronic common-rail fuel deliver in 2004 at 380 hp. It increased to 425 hp in 2006 and finally to 480 hp in 2008. The larger 8.3 liter QSC electronically governed diesel grew from its initial 480 horsepower in 2003 to 540 hp in 2004 and finally 600 hp in 2007.

But even at the current rating, the new 6.7 liter is noticeably more powerful. Cummins reports their older 5.9 liter engines accelerated the company's 44-foot test boat from 0 mph to 20 mph in 14 seconds. When I was aboard in Charleston, South Carolina this past October, the new 6.7 liter engines drove the same boat to 20 mph in just 8 seconds.

Fuel economy was quite close between the two engines as well, even though pleasure-boat diesel engineers say EPA Tier 3 emissions compliance typically comes with a 10 percent penalty in fuel burned per horsepower produced. Better economy comes partly from newer fuel delivery technology coupled to smarter engines -- the new engine ECM is programed with over 10,000 lines of computer code versus a few dozen lines of code in ECMs just over a decade ago. This helped Cummins focus their efforts on fuel economy at cruise rpm, not wide open throttle.

When operated with Ultra-Low-Sulfur Diesel sold here in the U.S., the new engine only requires service every 500 hours, rather than the industry-normal 250 hours between oil changes. Smarter fuel delivery decreases transom soot to near zero, and a tiny "pilot injection" of fuel right before the main injection event makes the engine considerably quieter, too.