Keys to the Perfect Marriage of Engine and Transmission

Steve Hirano, Editor
Posted on April 1, 2000

The marriage between engine and transmission is a key consideration when you’re spec’ing a school bus. A fleet manager who overlooks any aspect of equipment specification is not going to get the life expectancy and driver satisfaction he needs for safe, cost-effective transportation. The engine and transmission are especially important because they affect so many characteristics of the ride and performance of a school bus. Paul Cochran, fleet maintenance supervisor at Kyrene School District #28 in Tempe, Ariz., says there are three key considerations when spec’ing the engine and transmission of a school bus — application, performance and price.

Application is key
“The most important consideration is how you plan to use the bus,” Cochran says. Is it going to be used on daily routes or for activity trips that might include several-hundred- mile runs along fast-flowing highways? What kind of terrain will it encounter? Steep hills and winding roads? Straight, flat thoroughfares? Will it be used in the city, with a lot of stop-and-go driving? Or will be used in the country, with high mileage along unpaved roads? These questions are a good starting point for spec’ing the size and passenger capacity of the bus, which, in turn, will affect the choice of engine and transmission. Mitch Murray, tactical marketing manager for Allison Transmission in Indianapolis, says a school district that’s looking for an activity bus that might have to drive hundreds of miles through hilly areas should look at horsepower as the key determinant. “You won’t be using it all the time, but the higher horsepower can make all the difference when you’re stuck behind an 18-wheeler,” he says. Stepping up to a more powerful engine could require an upgrade in transmission as well.

Performance counts, too
Performance is a critical factor in choosing an engine and transmission. Some city driving requires good acceleration and low-end performance, while highway driving may focus on high-end performance. You’ll also want to consider fuel economy, especially in light of recent upswings in prices for fossil fuels. “Serviceability cannot be ignored,” Cochran adds. You need to consider how and where to get service. This is where local dealership satisfaction plays a major role. The repair history of the engine and the transmission should also be scrutinized. If you’ve ordered the same tandem in other buses and have had headaches, do you want to continue the marriage? Cochran says the final link in the quest for the perfect match of engine and transmission is price. Can you afford the equipment that you need? The answer to that question will vary from school district to school district and contractor to contractor. Murray says the retail list price of the 2000 series is about $600 to $800 more than the AT 545. Meanwhile, the MD 3060 (World) transmission adds a premium of $4,000 to $5,000.

More speed than need?
Murray says customers need to be careful when spec’ing the 2000 series, which was introduced to the school bus market last fall. The 2000 series is designed to replace the AT 545, which has been a standard in the industry. But Murray says customers should be aware that replacing an AT 545 with the 2000 series will produce changes in the top gear speed. “If you keep the same axle ratio and just put in the 2000, you could get in excess of 20 percent greater speed in high gear,” he says. A good reason to switch to the electronically controlled 2000 series from the mechanical AT 545 is that new engines are also electronically controlled. Thus, the interface between the engine and transmission provides excellent communication. Mating an electronic engine with an AT 545 doesn’t produce the same communication. “There’s no question that the interface [between electronic engines and transmissions] is better,” Murray says. If you’re uncertain about how to spec the engine and transmission, consult with your local dealers. You can find a dealer close to you by calling Allison at 800/252-5ATD.

Related Topics: engines, school bus specs, transmissions

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