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Old 03-16-2017, 11:56 PM   #1
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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 wont 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 Ill 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 03-17-2017, 12:01 AM   #2
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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 03-17-2017, 12:35 AM   #3
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6.50 rear end with 40" diameter tires are whats limiting you...

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Old 03-17-2017, 10:08 AM   #4
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Western NC school route bus?
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Old 03-17-2017, 10:35 AM   #5
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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, 10:38 AM   #6
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BTW, Fuel cut(redline) on a 6.9 idi is about 3500 rpm.
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Old 03-17-2017, 11:06 AM   #7
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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, 11:09 AM   #8
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How does one remove a road governor?
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Old 03-17-2017, 12:37 PM   #9
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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, 01:12 PM   #10
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Gee...thanks.
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Old 03-17-2017, 01:35 PM   #11
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if your bus Never revs above 2400 regardless of speed, then id say there is an RPM limiter on it, which on a mechanical engine like yours limits the bus speed..

I dont know alot about the 6.9 IDI, but i believe there is info out there on how to adjust the top limit RPM governor on the fuel pump itself, there are usually springs and screws which limit this on mechanical engines..

the 7.3 IDI is similar.. perhaps this may get you started.
More RPMs!!! - PowerStrokeNation : Ford Powerstroke Diesel Forum

-Christopher
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Old 03-17-2017, 03:13 PM   #12
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Quote:
Originally Posted by Booyah45828 View Post
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.
A 6.9 with 6.50 rear will go 60-65?? My 7.6 with 4.44 does 62 wide open!
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Old 03-17-2017, 03:51 PM   #13
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Quote:
Originally Posted by Abbadackerygirl View Post
How does one remove a road governor?
It has to be done by a mechanic... they have to hook a computer up to it to change it. Mine was set at 48 mph and I had it put at 65 mph.

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Old 03-17-2017, 03:56 PM   #14
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Quote:
Originally Posted by EastCoastCB View Post
A 6.9 with 6.50 rear will go 60-65?? My 7.6 with 4.44 does 62 wide open!

booyah says you can spin those 6.9s at 3400 RPM safely... much more than the 2400 or so that a DT-466 will do or the 2700 a DT-360 does..

wanderwoman: the 6.9 IDI is an all mechanical engine, theres no computers.

-Christopher
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Old 03-17-2017, 03:56 PM   #15
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I too have a 6.9 and it doesn't like going much over 50 (give or take), I've never minded it much since I feel like a dog going for a car ride. But it would be cool to see you resolve this and maybe do the same for mine .
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Old 03-17-2017, 04:03 PM   #16
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I'm not trying to argue- but if 6.9's with LOW gears can do freeway speeds why are we all so worried about having Allison unlock that extra OD, why are we looking for 8.3's?
Sounds like we should all be on the hunt for 6.9's with rock crawler gears.
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Old 03-17-2017, 04:08 PM   #17
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It's good to note my bus has gone as low as 10mph up some of the big hills, but going flat she'll run around 50mph.

Quote:
Originally Posted by EastCoastCB View Post
I'm not trying to argue- but if 6.9's with LOW gears can do freeway speeds why are we all so worried about having Allison unlock that extra OD, why are we looking for 8.3's?
Sounds like we should all be on the hunt for 6.9's with rock crawler gears.
I lack any knowledge to discuss mechanics. But as a spectator, I just want to verify if this statement contains sarcasm?
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Old 03-17-2017, 04:14 PM   #18
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Quote:
Originally Posted by TAOLIK View Post
It's good to note my bus has gone as low as 10mph up some of the big hills, but going flat she'll run around 50mph.



I lack any knowledge to discuss mechanics. But as a spectator, I just want to verify if this statement contains sarcasm?
Honestly, yeah there is a *bit*, but I'm half serious. If something could be had with cheap components, low gears, easily available, and would cruise at 65 that would be a no-brainer for cheap buses to convert that would be interstate worthy.
I have my doubts, but I'm genuinely interested in this subject.
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Old 03-17-2017, 04:15 PM   #19
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FamuosInternetJesus turbo'd his 6.9 and added a brownie box, so I KNOW his is fast as $%^.
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Old 03-17-2017, 05:02 PM   #20
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Back to topic, have you checked your fuel filter? I was always taught to check the simple and less expensive possibilities first.

I'm not saying it's not the govenor or that you shouldn't go to a shop to have a mechanic adjust it. Even a shop is going to check the possible basic maintenance issues first if you haven't had the vehicle for very long.

I've been buying auction equipment for about 25 years. Maintenance isn't always that good depending on which shop maintains the equipment prior to public sales. For instance auction farm tractors usually have the fuel filters plugged up to the point that they don't run well and have limited power, but they do run. At times it's a cheap fix.

Or I could be completely off base.
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