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Old 03-10-2016, 09:13 AM   #1
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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, 09:34 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, 10:46 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, 11:11 AM   #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, 11:12 AM   #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, 12:00 PM   #6
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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, 02:04 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, 03:31 PM   #8
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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.
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, 03:39 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, 03:48 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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Old 03-10-2016, 03:52 PM   #11
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Awesome!....love the people on this forum.
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Old 03-10-2016, 04:07 PM   #12
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80 would be cool! i've driven enough across wyoming and montana, that i only dream of 80.

i have a 4.7 rear end, and am stuck at 67mph @2500 rpm.


if i had a 4.1 rear, i'd get 77mph, but best economy around 65mph.

if i had a 3.8 rear, i'd get 85 and best economy in the 70's.

i just dont think my cummins 5.9 has the power for either of those rear ends. going up hills is slow as it is now.

i'm beginning to believe that the best answer is just getting used to slow.
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Old 03-11-2016, 12:34 AM   #13
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Originally Posted by Booyah45828 View Post
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.
I agree , and will add " what will your application be " ? I plan to drive 300 miles a day and stay to the main HWY , my coach would be a Motorcoach.
When I think of using park systems ,and some off road adventure , I want a springer shoolie . I want great ground clearance , and low range gears , so what if I go slow up a mountain .

So determine how you want to use the bus and then , the motor heads here can advise , the best running gear for us .

Just my .02c
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Old 03-11-2016, 06:36 AM   #14
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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.
Stock form is turbo, inter and/or after cooled.

But yes, once you start carving out injectors to resemble garden hoses, you'll have to be watching the EGT gauge quite close.

The new engines out now in most any brand are fully computer controlled and can pump out some great power with automatic safety features to keep you from damage.
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Old 03-11-2016, 08:31 AM   #15
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Thank you all. I am more convinced that I should buy a skoolie - the question I am dealing with now is "when". I will need a whole summer to do the main upgrades - especially the roof raising. I teach, so have summers off, and during the school year, have NO free time
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Old 03-11-2016, 09:09 AM   #16
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I wouldn't necessarily say when as far as your schedule permitting. It's more of a when the right bus becomes available. It's far easier to get one with all the desired options from the get go then it is to try and fix the right gearing, or the add right power, or the right brakes.

Schools are trading off busses for new all the time, have patience and you might be rewarded. If your bus sits all winter because you bought it in the fall I think you'll be farther ahead.
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Old 03-11-2016, 09:46 AM   #17
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I absolutely agree with Booyah --- Read up and decide what size & type of bus you want along with all the features you need (engine, trans, gearing, storage, etc.)...then hunt until you find 95% of what's on your list and in good condition at a reasonable price. Unless you go in planning on replacing/rebuilding just about everything on board, the extra bucks spent can get into the Twilight Zone very fast. Better to be patient and spend a few more bucks up front than spend many years and pesos trying to fix the basic platform.

I had a pretty good idea of what I was getting into when I decided to start with a WWII vintage bus, but even having accepted that virtually every system would have to be replaced or upgraded...it has STILL become much more of a challenge than I could ever have imagined. A FUN challenge, but...
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Old 03-11-2016, 12:12 PM   #18
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I absolutely agree with Booyah --- Read up and decide what size & type of bus you want along with all the features you need (engine, trans, gearing, storage, etc.)...then hunt until you find 95% of what's on your list and in good condition at a reasonable price. Unless you go in planning on replacing/rebuilding just about everything on board, the extra bucks spent can get into the Twilight Zone very fast. Better to be patient and spend a few more bucks up front than spend many years and pesos trying to fix the basic platform.

I had a pretty good idea of what I was getting into when I decided to start with a WWII vintage bus, but even having accepted that virtually every system would have to be replaced or upgraded...it has STILL become much more of a challenge than I could ever have imagined. A FUN challenge, but...
expensive one
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Old 03-11-2016, 03:36 PM   #19
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How'd you guess!?
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