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Old 05-26-2016, 10:39 AM   #21
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Location: Houston, Texas
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Year: 1946
Coachwork: Chevrolet/Wayne
Chassis: 1- 1/2 ton
Engine: Cummins 4BT
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If you take a look at the horsepower and torque curves on a Cummins, or just about any diesel for that matter, you will see a spread between "peak torque" and "peak HP". Since it is torque that moves a load, their "Sweet Spot" is typically just above that peak torque RPM.

For example, my little 4BT produces a peak of 130 HP at 2300 RPM...but it's peak torque of 327 is at 1700. The sweet spot on my particular engine is about 1800 RPM. At that speed it moves the load most efficiently, producing less excess heat, the least amount of strain and consuming the least amount of fuel. I ran the numbers with both Cummins and Allison to create a driveline that will allow me to run 63-65 mph in that "spot". Plenty of speed for me. And the anticipated MPG? There are many factors, but based on existing systems and similar real world applications, I should easily get 25 MPG and possibly a little more. Coupled with a 44 gallon tank, that "should" yield a range of about 1100 miles per tank. Yet to be proven, but at least it was built towards longevity and efficiency.

Now if you want to race...that's a whole different deal. This same little 4 banger can be tweaked to crank out just under a thousand HP and torque in the 700+ range. But don't expect it to last very long or sip fuel.

Take note of the commercial big rigs rolling down the road. You never hear them screaming. In fact, they often sound as if they are practically idling at highway speed. That is precisely why they generally have so many gears. To keep their engines running as close to the sweet spot as much as possible.
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Old 05-26-2016, 10:40 AM   #22
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Quote:
plenty of peeps on the interweb say that you can run the cummins 5.9 pegged to the redline all day.
Suuuuure. No problem. Are they going to chip in for the rebuild?

Quote:
not at my weight of 20k lbs+. i can barely go uphill now.
Cubes is everything. There's no substitute for cubes. Otherwise work with what you brung to the party.

If you want to drive 75 or 85, maybe you should consider a different vehicle, like a Miata. I don't like driving a bus that fast.
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Old 05-26-2016, 11:50 AM   #23
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Join Date: Dec 2014
Posts: 447
Year: 1988
Coachwork: Ward
Chassis: International
Engine: Navistar 5.9 Diesel
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Talking Found something!

Quote:
Originally Posted by Tango View Post
On most units there is a plaque...somewhere. And it usually states the axle ratio but then, it could have been changed out.

Another way to find out what you actually have is to put it in neutral...jack up the driver side rear tires...align a mark on the tire and ground...and, put a mark on the driveshaft. Rotate the tire one full rev...and count how many turns the shaft makes. That will be your ratio or at least get you close enough to look up what is/was avail on your rig and get a match. I had to do that on mine and discovered the rear end was NOT what was indicated.
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Originally Posted by BlueBirdman View Post
There are usually two metal information plates attached above the driver's seat/windshield area. One of them will have the rear axle ratio.

There also may be a tag on the axle itself.
Found a plaque on the actual diff.: it says it's a spicer "something"455s, and it has full model and S/N. If gearing inside wasn't modified, i should be able to find some info starting from there....

Also, currently my bus has 11r22.5 tires, and comparing them to some pics that popped up yesterday on other threads, they look in very decent shape!

With that said, i guess i have to drive full steam to check max RPMs again, 'cuz i forgot!
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Old 05-26-2016, 12:07 PM   #24
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Join Date: Mar 2011
Location: Houston, Texas
Posts: 6,170
Year: 1946
Coachwork: Chevrolet/Wayne
Chassis: 1- 1/2 ton
Engine: Cummins 4BT
Rated Cap: 15
Or, instead of driving it, you can plug your numbers into the calculator below for the answers...

Engine RPM Calculator

I just use the bottom one. But you do need your tire diameter.
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