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I have read a lot of the "Highway speed" posts but have not seen any that will help. I have a 1992 Ford Blue Bird 72 passanger, it has a 5.9 cummins diesel, only 70k miles on it with a Allison OD transmission. I need to find out how I can make go at least 65 on the highway with getting a major cramp in my right calf... Any help would be greatly appreciated..
Since you don't mention how fast your bus will travel the way it is set up right now it is difficult to know where to start with some suggestions.
To start out, I don't think your bus has an overdrive automatic, particularly if it is an AT500 or MT600 series transmission. If you have an AT500 or MT600 transmission, in high gear you are in direct drive. If you have one of the newer World series of transmissions, you may have an overdrive transmission.
In either case, with a Cummins 5.9L you are limited to 2600 RPM's. At 2600 RPM's, the wheels are only going to go as fast as the gearing will allow. If you want to go faster, you will have change the gearing.
If you have 10-hole Budd wheels and you need to increase only 5-10 MPH, the least expensive fix is to change to larger wheels and tires.
If that doesn't work for you or if the change isn't enough, the only other choice is to change the rear gears.
Understand, when you trade to a higher top speed you are going to sacrifice hill climbing ability.
Some of the 5.9L's were rated as low as 160 HP and others as high as 250 HP. The most common ratings are in the 190-210 HP range. If you are on the low end of the power range you are going to be in the slow lane on the hills because you will be in at least one gear lower on every hill.
The 5.9L is a very good engine. It goes a long way with few problems and great fuel economy. It is built stout enough to take a lot of "hot rodding".
However, if you boost the power to get back your hill climbing ability you will be in danger of melting the pistons.
Only you can determine what will work best for you.
Mark, Thanks for the input. The bus does about 56 mph. I haven't looked at the rear end tag to see what gear ratio it has but I will do that this afternoon. I really don't want to "hot rod" it I just need a little more speed then what I'm getting.. I was thinking about going to a 24.5 or a 22.5 tire and wheel setup, maybe that will help a litttle..
The only assumption I'm going to make here is that 2600rpm represents a governed limit and you can set the engine on that speed "forever".
A lot of discussion has been posted here, but no one has ever broached the subject of Math. Here's what you need:
* Overall wheel diameter (from the top of the tread to the bottom)
* Maximum RPM
* Desired highway speed
* Rear end gear ratio (can be derived)
The formula for figuring speed based on RPM and rear end ratio:
MPH = [(pi)x(Tire diameter in inches)x(60)x(RPM)] Divided by [(12)x(5280)x(rear end ratio)]
Let's say you're stuck at 55 MPH, engine is limited to 2600RPM and your wheels are 28" in diameter. Solving for Gear ratio would show that you have (roughly) a 3.94:1 rear end ratio. Let's say you want it to cruise at 65MPH with the same set up. This would require the rear end ratio to change to (roughly) 3.34:1. Solving for tire size and keeping the 3.94 ratio would mean you need tires of 33" overall diameter to make 65MPH at 2600 RPM. I don't want to bore you further with Algebra; I can hear eyeballs glazing already.
This is a good estimate: a lot of the ratios you figure will not be available commercially, so you'll have to make due with an approximation. I also used 3.146 as a value for pi (easier math) because I am making an estimate.
Torque is the ability to do work. Horsepower is work per unit time. More horsepower, more work in less time. Here's the second half: Higher ratios (lower numerically) mean less mechanical advantage. This means you need *MORE* horsepower to do the *SAME* amount of work in the *SAME* amount of time. Conversely, the *SAME* amount of horsepower with less mechanical advantage will perform *LESS* work in the *SAME* amount of time. This means you *COULD* travel at 65MPH, but the motor won't move you that fast due to the load increase from giving up mechanical advantage. How much horsepower you need is based on factors such as weight, rolling friction and drag (wind resistance). Needless to say, if eyes glazed during ratios, they'll fall out if I go into horsepower. Suffice to say, if you gear correctly and you don't get to speed, you need to increase your motor's power output.
In reply to your question, dont worry about the power once you up the gearing.
The guy who talked about melting a piston is correctif you go to extremes, but that engine is offered at 360 HP right from Cummins.
If you want some details on increasing your power output without hurting the thing, send me an email. It is cheaper than most think, will not damage the engine in any way (you install a set of guages to monitor the EGT's to prevent any damage if you go up past 320 HP) and will not harm either of the two transmissions you could have, since they are both designed to take this power easily.
I run my cummins B series at 42PSI out of the turbo on a regular basis. I have 370 HP (at the rear wheels) and a torque rating that you would not believe. The motor just rolled over 392,000 miles. Trust me, they will deal with it and then some.
THE AXLE RATIO SHOULD BE ON A STICKER INSIDE OF THE BUS (PROB AT THE FRONT) MY 91 BLUE BIRD TC2000 WITH A 5.9 CUMMINS HAS A 4.33 GEAR RATIO. MINE IS A FLAT NOSE WITH THE ENG IN THE FRONT. MINE TOPS OUT AT ABOUT 57, IT WILL GO 60-61 DOWN HILL OR WITH WIND AT MY BACK.
Hello, I am thinking about swapping out my 534 ford engine with a 5.9 cummins in my motorhome. Currently @ 75mph it runs 3400rpm. Will I need to regear for this engine. How do you increase the hp of these engines? Is it expensive? Thanks Frank
email Robert G. Clark
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