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Old 03-10-2016, 10:13 AM   #1
Skoolie
 
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Engine and speed potential

Has anyone ever thought to make a chart showing engines and speed potential?

For example, assume a 40' bus, fe type D bus with a

5.9 cummins 4.11 60 mph
etc
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Old 03-10-2016, 10:34 AM   #2
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There is more to it an just the engine. Tire size, transmission final ratio, rear axle ratio, etc.

Here is a tool for playing "what if" with the relevant numbers.

Engine RPM CalculatorEngine RPM Calculator
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Old 03-10-2016, 11:46 AM   #3
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You also have different power levels, torque ratios, and the like per engine. A 5.9 cummins from 1990 is vastly different then one from 2007.

That's not even counting transmission options either. Most of the old stuff will have a 1:1 final ratio. Sometimes you can get one with overdrive but it still will only help so much.

The main thing you should be concerned with is rear gear ratio. I don't care what engine you have in it or how much hp, something with a 6.50 won't likely go down the highway at the 70. And if it does, it's going to be redlined.
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Old 03-10-2016, 12:11 PM   #4
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Yes, it all becomes numeric soup. I live up in the northern plains states..speed limits up here are 75 or 80 mph. With a good choice of rear end and transmission, can a Cummins 6bt keep up with the pack without blowing up? I know they are durable engines - have tons of mods - and also have a reputation for being able to move very heavy loads, i.e. lots of torque. The next question, of course, is would the brakes be able to stop a 40' bus going 80 mph?! I am guessing that a rear end with less than a 4.11 is not going to do well on even moderate hills.
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Old 03-10-2016, 12:12 PM   #5
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That's where a gearing calculator really comes in handy. If you can gear for your desired MPH at the engines "sweet spot", it will deliver peak MPG's and longevity.
For example, my little Cummins 4BT is happiest at about 17-1800 RPM and that is what I geared it to for 63mph (enough speed for me). The sweet spot is typically close to or at any given engine's peak torque output RPM.
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Old 03-10-2016, 01:00 PM   #6
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Quote:
Originally Posted by Tango View Post
For example, my little Cummins 4BT is happiest at about 17-1800 RPM and that is what I geared it to for 63mph (enough speed for me). The sweet spot is typically close to or at any given engine's peak torque output RPM.
That is the #1 thing I like about what you've done. You've got it at the perfect RPM and a very acceptable speed. Doesnt get any better than that and you did it the right way!
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Old 03-10-2016, 03:04 PM   #7
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In stock form, the 5.9 can run against the governor all day. It does so in equipment like farm equipment and construction, logging etc. Not to mention water pumps and generators.

If your bus is reasonable weight, and has all the ratios and tire diameters factored, aerodynamics will be your next level or mod to gain speed. Skirting abound the bottom and a ducting system of some kind on the tail of the bus to reduce the suction drag effect.(I forget the actual name of it just now)

IMO.
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Old 03-10-2016, 04:31 PM   #8
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Quote:
Originally Posted by Deezl Smoke View Post
In stock form, the 5.9 can run against the governor all day. It does so in equipment like farm equipment and construction, logging etc. Not to mention water pumps and generators.

If your bus is reasonable weight, and has all the ratios and tire diameters factored, aerodynamics will be your next level or mod to gain speed. Skirting abound the bottom and a ducting system of some kind on the tail of the bus to reduce the suction drag effect.(I forget the actual name of it just now)

IMO.
Stock, yes, but what about with a turbo, an intercoolor, larger injectors and a few other modifications? Other things equal, HP helps with the top speed.
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Old 03-10-2016, 04:39 PM   #9
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Is it more reasonable to say that a 40' type d would need about 300 hp to do 65 or 70 mph? to make that kind of generalization? Assume geared correctly.
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Old 03-10-2016, 04:48 PM   #10
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I have 250hp and can probably do 80. Would never try it though.

I'm 4.10 gearing.
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