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Gearing and Spec’ing for Success: B6.7

Cummins six-cylinder B-Series engines are one of the greatest success stories in the transportation industry, with over 30 years in production, millions of engines built and billions of miles logged through continuous improvement that today makes the B6.7 the productivity leader in its class.

While some would like you to believe that downsizing your engine can provide the equal productivity of the six-cylinder Cummins B6.7, there’s simply no replacement for displacement. And with proper gearing and spec’ing, there’s no better choice than a Cummins B6.7 for Class 4, 5, 6 or 7 truck customers. Fact is, with ratings from 200 hp to 325 hp and peak torque ranging from 520 lb-ft to 750 lb-ft, the B6.7 offers a wider range (on both the lower and higher ends of the power spectrum) than the competition. This gives greater flexibility in matching specific equipment and duty cycles, no matter which application your customer is running.

The key to helping a customer achieve the optimal combination of fuel economy and performance in any work truck is spec’ing the right engine, horsepower rating, transmission and the proper drive-axle ratio – also known as the rear-axle ratio.

The first step in getting the ideal spec is to determine which kinds of routes will be run and the weight of the load to be carried. A refuse truck that rarely exceeds 25 mph while picking up trash in neighborhoods, and has to negotiate muddy, steep ramps in a dumping zone is going to have far different demands than a pickup-and-delivery (P&D) truck operating in a city or the suburbs, or a beverage distribution truck carrying a heavy load. The more work you are asking the engine to do, the higher the horsepower and torque that any vehicle will need.

The second step is coupling the engine with the right transmission and spec’ing the optimal axle ratio. A high rear axle ratio provides maximum towing and payloads, and may be needed for hilly terrain with steep grades. The mid-range ratios deliver flexibility for operating on varied terrain with moderate loads, while lower ratios are the best choice when running a flat terrain, with lighter loads or running at consistent highway speeds.

Cummins PowerSpec provides a simple way to enter these operating parameters into a spec’ing tool. Your customers determine whether they want the vehicle spec’d for optimal fuel economy, maximum performance or somewhere in the middle. That decision will determine the overall drivability of the vehicle, which makes a big difference in driver satisfaction. Cummins PowerSpec uses that input to provide optimized vehicle gearing, with recommended electronic features and settings to achieve operational goals.

Let’s say you and a customer are spec’ing a P&D truck for delivering baked goods to grocery stores and restaurants in a city. These trucks “cube out” before they even come close to 33,000-lb Gross Vehicle Weight (GVW), so a B6.7 rated between 200 hp and 250 hp (149-186 kW) is the right engine to choose. In a situation in which the truck will be operating in very hilly terrain, a balanced spec will most likely meet driver expectations. This vehicle will need to be geared to operate at intended cruise speeds in the 1900 rpm-to-2100 rpm range.

Regional-haul vehicles that carry heavier loads (between 33,000 lb and 50,000 lb), such as suburban delivery and beverage distribution trucks, will utilize higher horsepower ratings – 250 hp to 325 hp (186-242 KW) – with gearing in a lower rpm range. If your customer were running the same truck in a relatively flat terrain, he or she could consider spec’ing the vehicle for maximum fuel economy with a maximum rpm in the 1800-to-1950 range, and get comparable performance. This is illustrated in the second chart below.

The third chart shows vocational applications. The term “vocational” applies to any vehicle used for a lot of stop-and-go functions and operating on soft road conditions at times, often with a varying load. The highest rpm for the 2017 B6.7 will be 2400, used only with the maximum performance setting.

Fuel Economy, Balance, Performance Rating Charts

For more information, download the 2017 B6.7 brochure or visit

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