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Old 03-16-2017, 10:56 PM   #1
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Join Date: Jun 2015
Location: North Carolina
Posts: 4
Year: 1985
Coachwork: Thomas
Chassis: International s1853
Engine: 6.9l IDI
What is limiting my bus to 45mph?

I know there's a lot of contention on this forum about wanting to go faster; I understand that it's a school bus. I'm not talking about going 80mph, I just want to keep up with traffic around here (55mph). As it is now, once I hit 45mph (at 2400 rpms) the gas pedal becomes unresponsive, this thing wonít even coast down the back of the hill faster than 45. The gas cutting out makes me think speed is being limited by the injector pump. Has anyone had success in adjusting this, or can you link me to some relevant info/tutorials? I could be wrong, so Iíll list a bunch info here from my search.

Transmission: Allison AT545, Acceptable full-load engine governed speed 2400-4000 rpm (diesel)

Engine: International Harvester 6.9L IDI V-8 diesel
Horsepower: 170 hp @ 3,300 rpm
Peak Torque: 338 lb-ft @ 1,400 rpm

Rear end ratio: 6.5
Tire size: 10R22.5

Some other thoughts here: Can this engine even hit 55 without operating at rpms that are too high? If I ever decide to put taller rear end gears in here, will I lose too much power?
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Old 03-16-2017, 11:01 PM   #2
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Join Date: Mar 2011
Location: Houston, Texas
Posts: 8,461
Year: 1946
Coachwork: Chevrolet/Wayne
Chassis: 1- 1/2 ton
Engine: Cummins 4BT
Rated Cap: 15
Get your tire diameter then run the numbers on a gear/speed calculator like the one at the bottom of the page below to see where you stand.


Engine RPM Calculator
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Old 03-16-2017, 11:35 PM   #3
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Join Date: May 2009
Location: Columbus Ohio
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6.50 rear end with 40" diameter tires are whats limiting you...

-Christopher
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Old 03-17-2017, 09:08 AM   #4
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Year: 1998
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Engine: Navistar 7.6L
Western NC school route bus?
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Old 03-17-2017, 09:35 AM   #5
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Join Date: May 2014
Location: West Ohio
Posts: 1,084
Year: 1984
Coachwork: Bluebird
Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
IF the rpms are at 2400 and you're only 45 mph then you have a road speed governor slowing you down.

I have a 5.57 rear ratio with the same engine/trans that you have and with a 10r22.5 (40.2" tall) tire I can do 70-75 red lined.

With your 6.50 ratio you'll drop that down to 60-65. You don't necessarily want to run red lined like that, but your rear gearing isn't what's limiting you.

Check your speed rating on the tires to make sure you don't exceed that. If it's something more then 45 mph then you can remove the road speed governor.
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Old 03-17-2017, 09:38 AM   #6
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Join Date: May 2014
Location: West Ohio
Posts: 1,084
Year: 1984
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Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
BTW, Fuel cut(redline) on a 6.9 idi is about 3500 rpm.
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Old 03-17-2017, 10:06 AM   #7
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Join Date: Jun 2015
Location: North Carolina
Posts: 4
Year: 1985
Coachwork: Thomas
Chassis: International s1853
Engine: 6.9l IDI
Yes I live in the NC foothills, that may explain the gearing. Good information Booyah, I was wondering what the redline was. The rpm calculator indicates I'll be at 3000 rpms at 55mph, that's as much as I'd want to do. I'll check the speed rating on the tires and post some pictures of the bus for y'all.
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Old 03-17-2017, 10:09 AM   #8
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How does one remove a road governor?
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Old 03-17-2017, 11:37 AM   #9
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Join Date: May 2014
Location: West Ohio
Posts: 1,084
Year: 1984
Coachwork: Bluebird
Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
Quote:
Originally Posted by Abbadackerygirl View Post
How does one remove a road governor?
Depends on the bus.
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Old 03-17-2017, 12:12 PM   #10
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Gee...thanks.
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