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Old 02-10-2016, 04:58 PM   #51
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Join Date: Mar 2011
Location: Houston, Texas
Posts: 8,453
Year: 1946
Coachwork: Chevrolet/Wayne
Chassis: 1- 1/2 ton
Engine: Cummins 4BT
Rated Cap: 15
Not familiar with that particular engine, but most diesels have a fuel screw somewhere on the injector system that can be turned up. It will add a few ponies without having much effect on MPG. The next item is the turbo. An upgrade there can again gain HP with very little MPG effect. Larger injectors will add still more but will reduce MPG somewhat. As long as none of these moves are too radical, they can collectively yield a pretty impressive increase in performance.

But...keep in mind the Allison 500 series are only good to about 250 HP. Any more and the engine will eat it. Likewise the torque and vehicle weight. See the specs below.

That's on the engine side.

Then there is gearing and tire size. Larger diameter tires will increase MPH at any given RPM.

Gearing, both in the tranny final output and the rear end can give you several more MPH. But here too there are limits. Gearing that is too tall or tires that are too big will require a significant increase in torque to gain any benefit. There are also aftermarket overdrive units that can put a few more MPH on the speedo.

A good place to compare these numbers is on the calculator at the bottom of the page.

Nearly all diesel engines can be easily tweaked to gain HP, just don't go overboard without updating all the other systems...including brakes. Remember they were matched to the other specs as well.

Allison 545 Specs:

Gearing Calculator:
Engine RPM Calculator
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Old 02-10-2016, 06:36 PM   #52
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Join Date: Sep 2014
Location: Eustis FLORIDA
Posts: 17,429
Year: 1999
Coachwork: Thomas
Chassis: Freighliner FS65
Engine: Cat 3126
Rated Cap: 15
Originally Posted by Pablo View Post
so my bus has a DT 360 navistar w a 545 allison transmission, the sticker on my rear end shows ratio 5.11 my top speed is about 60 at about 2900 rpm What are my options for increasing speeds about 10 miles per hour. I've read about hurting the overall engine performance so looking for a little help.
Go with a higher rear axle ratio or larger tires if possible.
I'm damned impressed you do 60 with that setup!
My tops is 63ish.
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Old 03-09-2016, 08:59 PM   #53
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Join Date: Feb 2016
Location: Boerne, Texas
Posts: 2
Year: 1992
Coachwork: Bluebird
Engine: Navistar Dt360, 545 allison Trans. 5.11 rear end
Tango, Eastcoast.
Sorry for the delay I've been out of reach. I appreciate all your words of advice. Tango you went all out, I appreciate that, and thanks for the links as well.
EastCoast it is a 47 passenger if that helps.
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Old 03-09-2016, 09:20 PM   #54
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Join Date: Jul 2011
Location: Winlcok, WA
Posts: 2,108
There are some gradability charts online that let you know how much HP and Torque you need to go certain speeds with a certain amount of weight.

The charts go up exponentially the faster you want to go up a hill.

What I do know is the single most limiting factor on any bus is the frontal area. Pushing air out of the way is the hardest thing any bus has to do. Reducing your drag can also help you get down the road--perhaps not a lot faster but at the same speed you will use less fuel.

I own two different Avion travel trailers. One is a 26' tandem axle that weighs under 5,000 lbs. fully loaded. The other is a 34.5' tri-axle that weighs right at 7,000 lbs. fully loaded. I have towed both with two different tow vehicles enough to know their differences. Yes the bigger one slows me down faster and longer on hills. But the really odd thing is with both tow vehicles I get the same fuel mileage towing either trailer. The only thing that makes a real difference in fuel mileage is speed. The difference between averaging 65 MPH and 55 MPH is about 1.5-2 MPG. One tow vehicle is a 1965 IHC D1200 Travelall Custom 4x4 with an SV304 V-8 and 4-speed. The other tow vehicle is a 1993 Chevy K2500 Suburban Silverado with a 5.7L and 4L60E.
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