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Old 01-07-2008, 08:22 PM   #1
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Get my bus moving!

Hello -
I have an '88 Chevy bus with an 8.2L Detroit Diesel and it runs great. However it will only reach a top speed of 55 on a flat, but usually chugs along between 45 and 50. Now I know this 500 cubic inch beast can go faster. It was a fresh trade at a bus dealership, so it came straight from a school district and I know they put governors on their buses. How can I get it to go at least 60? I love driving my bus to and from the campgrounds but I hate having people backed up behind me. Thanks.

-Adam
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Old 01-10-2008, 10:25 PM   #2
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Re: Get my bus moving!


That information is here, somewhere! Start searching, and elsewhere on the internet also.

Here's a site I found when I was studying the same question: http://www.freewebs.com/nevrenufhp/index.htm It has instructions for increasing fuel flow on your Detroit 8.2 -- and many other engines.
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Old 01-11-2008, 12:55 PM   #3
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Re: Get my bus moving!

School buses are ordered new with a very low rear gear ratio. Most buses use a 6:30 ratio. The top speed is usually about 45-60 MPH. To change the top road speed, the ring and pinion must be changed. A good ratio is 4:10 or 3:93 and will allow about 70-75MPH. The diesel engines do have govenors based on design requirements. A diesel engine can run forever at the maxamine engine govenor speed. Primarily the end use decides top road speed. School buses get very poor fuel milage. Frank
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Old 01-11-2008, 01:28 PM   #4
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Re: Get my bus moving!


I can add that the rear end ratio of a vehicle depends greatly on the operating RPM of the engine. Gasoline engines spin faster than diesel engines. My former bus had a gasoline engine, which probably could be wound to 4.000, and the rear end was 7 point something. Millicent has a diesel, with a top RPM around 3.000, and the ratio is 4.78.

In addition, school buses mostly have the engine governor set quite low. I have not gotten around to installing my tach yet, but I would not be suprised if it tops out at 2.500, because my top speed is 55 MPH. You/I could pick up quite a bit of speed by "turning up" the governor a few hundred RPMs.

There are several web sites that will calculate all this for you in seconds. Here are two of them:

http://www.csgnetwork.com/rearendgearcalc.html

http://www.idavette.net/tech/ratioc.htm
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Old 01-11-2008, 01:49 PM   #5
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I disagree

Engines of all designs are govenored in relation to engine design.There are several engine maximine RPM designs as low, medium, and high rpm. The Typical Cummins 855 cu in engine is a medium RPM engine with a set speed of about 1800-2500 RPM. The Int/Ford diesel is a high speed RPM engine at 2800-3200 RPM. Each engine designer decides engine RPM. There are several methods of gaining more power from a diesel engine but usually the RPM remains at or near design limits. Engine max RPM is not usually changed to gain a higher road speed. Frank
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Old 01-11-2008, 01:59 PM   #6
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Re: Get my bus moving!


Quote:
Engine max RPM is not usually changed to gain a higher road speed.
Agreed. The MAX engine speed is not something we want to fool with. It's when an engine is governed at a lower-than-max speed, that we can adjust it back up towards design limit revs. I could have made that clearer the first time around.
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Old 01-11-2008, 02:15 PM   #7
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Re: Get my bus moving!


I just calculated Millicent, using http://www.idavette.net/tech/ratioc.htm

At 55 MPH, Millicent's engine is turning only 2200 RPM.

At 3.000 RPM, which is about the design limit for the 5.9, she would go 74 MPH. She would drink fuel like I drink Gatorade in August, but she would go 74 IF the power output is enough to overcome wind resistance.
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Old 01-11-2008, 02:15 PM   #8
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Engine maximine RPM???

Please describe how engine max RPM can be adjusted? Tell also any criteria for determining max engine RPM? Do diesel engine have greater longivity after engine RPM changed? My opinion is that modern diesel engines should not make any max rpm changes. My Cummins NT855 is set to 2250 RPM, and will be changed to 2500RPM. This is a common RPM change, but not 3000RPM. You may change engine speed but the gains are minimal. Frank Old
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Old 01-11-2008, 02:57 PM   #9
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Re: Get my bus moving!


Frank, engine MAX RPMs, also referred to as redline, cannot be adjusted. Redline is, as you correctly point out, decided by engine design. The only people who can increase the safe redline of an engine are serious big buck engine builders who build custom engines such as for racing. That would involve a stronger crankshaft, more precice balancing, and so forth -- not something we deal with here. The consequenses of reving an engine beyond redline can include picking up pistons and pieces of crankshaft from the road. I overreved my IH gasoline engine and bent several push rods.(It still ran, sorta, so I drove it to the wrecking yard and gave it to them.)

Criteria for determining redline of an engine design are a bit beyond my knowledge! But I do know that the valve train and the connecting rods are often weak links. Inertia is the enemy. Typically, the valve train will "float", meaning that the valves fail to return to closed position "on schedule". Then the piston comes up and hits the still-open valve.
Connecting rods will stretch and even break. I have read that the highest force on connecting rods happens at the top of the overlap stroke, between the exhaust stroke and the intake stroke, when the inertia of the piston tries to stretch the connecting rod. This inertial pulling force is supposedly higher than the compressing force of combustion. That's about all I know.

An engine will always live longer when it is used well below design limits. It will also use less fuel that way. This probably explains why school buses often come with the governor set well below redline. Then the rear end ratio is determined to make it go "school bus road speed" at that engine speed.

Frank, you also bring up an important point about how gains from increased engine speed are minimal on a diesel engine. But we should emphasize that we are talking about power gain. Diesel engines make their best power at moderate RPMs. Winding them out to redline is often more a matter of reaching the desired road speed, or being able to "catch" the next gear while climbing a hill with a stick shift. The relationship between torque and horsepower is best explained with charts, and I don't have any. But with our modern 14- and 15-liter engines at work, we pull hills like gangbusters at 1200 RPM, and approach our redline of 2100 only on very rare occasions.
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Old 01-11-2008, 03:15 PM   #10
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Re: Get my bus moving!


Here is an example of custom built engines operating, successfully, far beyond original design limits.



(Bonneville Salt Flats, August 1997, Canon EOS Elan, 1000 MM elCheapo mirror lens, some exposure compensation for white salt.)

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