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Old 04-22-2018, 05:00 AM   #1
Bus Nut
Join Date: Jan 2016
Posts: 252
Difference in power requirements

What is the difference between the power requirements for a bus that is 30' long and a bus that is 40' long? Obviously, one is 10' weight shorter, plus the weight of all of the stuff that would be put in the 10'. When planning, is this significant enough to consider a different size engine/transmission? Gas mileage is probably not that much different because the cooefficient of drag is the same for either bus.
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Old 04-22-2018, 10:39 AM   #2
Bus Geek
Join Date: Sep 2017
Location: Owasso, OK
Posts: 2,627
Year: 1999
Coachwork: Thomas
Chassis: Saf-T-Liner MVP ER
Engine: Cummins 6CTA8.3 Mechanical MD3060
Rated Cap: 46 Coach Seats, 40 foot
It's the wrong question. All of the engines are, at times, put in all of the buses. It depends on what the original purchasor wants it to do.

So the question is "How much power do I need in "X" bus, to do this?"

The choices are narrow, in terms of actual engines, but if, for example, you ended up with a DT466 tuned to 190 hp in a 40' bus, and it wouldn't hold a decent speed up hills, then you can fairly easily increase the power. The same goes for most of the engines you will find in buses although some are easier (cheaper) than others.

One factor to consider is that the transmissions also have input torque restrictions. Another good reason to avoid the Allison AT545.

Most buses were specc'd to the budget of the School District that bought them. That means they often got the cheapest combination that would get the job done. Back in the late '90s, the Cummins 8.3L was an $8000 option over it's smaller sibling, partly because it needed a better transmission too. That's the entire, per pupil funding for one year.

Speed on the flat is not really a function of engine power. It is more directly related to gearing, but when the terrain gets hilly, or you want to tow, then the more power the easier it will be, and the length of the bus is a small factor.
Steve Bracken

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