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Ok, I think that I asked a similar question about a
year ago here. I'm starting to work on my bus againg
and I have forgotten everything I learned a year ago. So, forgive me for asking twice. :)
My bus is a '71 Chevy C/50 with a 2 speed rear end.
At 65mph my 350 turns at about 4000rpm and sucks 5mpg.
Is it easier (possible) to replace the 2 speed with
another 2 speed with different ratios or should I just
go to a single speed rear end.
Is there a standard ratio that is good for highway driving?
I saw someone mention that they were looking for 4:10 gears.
I realize that by changing the ratio I may loose some power,
wont be able to accelerate as well. So, my next project would
be to ditch the 350 for either a better 350 or a big block.
If anyone has a similar bus with a different gear ratio or engine,
I would be interested in hearing what you get for gas milage and what
your top speed is.
Ok, I think that I asked a similar question about a
year ago here. I'm starting to work on my bus againg
and I have forgotten everything I learned a year ago. So, forgive me for asking twice. :)
My bus is a '71 Chevy C/50 with a 2 speed rear end.
At 65mph my 350 turns at about 4000rpm and sucks 5mpg.
Is it easier (possible) to replace the 2 speed with
another 2 speed with different ratios or should I just
go to a single speed rear end.
Is there a standard ratio that is good for highway driving?
I saw someone mention that they were looking for 4:10 gears.
I realize that by changing the ratio I may loose some power,
wont be able to accelerate as well. So, my next project would
be to ditch the 350 for either a better 350 or a big block.
If anyone has a similar bus with a different gear ratio or engine,
I would be interested in hearing what you get for gas milage and what
your top speed is.
Thank you,
Brent
The last guy had some good advice. See how your engine pulls (i.e. using the vacuum guage. I would take it a bit further. If you engine pulls well, you are in great shape because you have a two speed rear-end. There are hundreds or thousands of Motorhome users out there who would kill to have the ability to split gear ratios with a 2 spd.
Basically it comes down to this:
Does your unit pull well on hills etc. I would guess that by using your 2 spd in combination with your trans. that you can find a gear that pulls almost any size hill. If the jump between most gears isn't too large, i.e. most shifts give you a 3-500 rpm change without great power loss then you should seriously look at putting higher ratio gears and dropping you rpm's when cruising. Turning 4000 rpm's at 65 mph has got to the be main reason you're down to 5mpg. My motorhome turns 3000 @ 55 mph. It's got a 4.10 with a 3 spd automatic. I'm looking to find a gear vendors overdrive unit to it. The main reason is to get a gear in between 2nd and 3rd (to pull hills) and to have an overdrive (cut the engine rpms at cruising). The 3rd Overdrive gear would equal a ratio of 3.20. Perfect for cruising @ 65mph around 2700 rpm.I have a 440 engine and my unit weighs about 11,000 1bs. dry so I think it would handle it power-wise).
You already have the two speed so you need to match up your ratio with the horsepower you have. You might be overgeared right now. Buses were designed to go slow - it may have more highway stretch available to it. I would check with a used truck shop or auto wrecker to see what ratios can be put in your specific rear-end. If you feel you have the power and you can extend your splits between gears then you can handle higher ratios. You'd notice a lot of difference in reduced noise). Less wear on your engine and alternators, a/c pumps etc because of lower rpm's. You'd also be perfectly set up for when you get to that bigger engine. It sure won't need the high rpm's to pull you around.
Dig up the torque and horsepower - rpm specs for your engine. I would think that you don't want to be pushing the max horsepower rpm on the highway because that would be hard on engine life, but I would want highway rpm to be above peak torque so I don't have to downshift at every slight incline.
Ok, I think that I asked a similar question about a
year ago here. I'm starting to work on my bus againg
and I have forgotten everything I learned a year ago. So, forgive me for asking twice. :)
My bus is a '71 Chevy C/50 with a 2 speed rear end.
At 65mph my 350 turns at about 4000rpm and sucks 5mpg.
Is it easier (possible) to replace the 2 speed with
another 2 speed with different ratios or should I just
go to a single speed rear end.
Is there a standard ratio that is good for highway driving?
I saw someone mention that they were looking for 4:10 gears.
I realize that by changing the ratio I may loose some power,
wont be able to accelerate as well. So, my next project would
be to ditch the 350 for either a better 350 or a big block.
If anyone has a similar bus with a different gear ratio or engine,
I would be interested in hearing what you get for gas milage and what
your top speed is.
Thank you,
Brent
Unfortunately, there isn't any simple quick fix to improve MPG. Basically, an internal combustion engine is a thermodynamic device for turning heat energy(combustion) into mechanical energy(rotary motion). How efficiently that is done depends on the engine design, ideally, the higher the compression ratio, the better. However, you reach the point where oxides of nitrogen are formed and must be dealt with before the exhaust enters the atomosphere. The high ratios demand a high octane fuel to control detonation so that knocking caused by multiple flame fronts doesn't occur. All that said,your MPG is going to depend on how much "work" the engine must do. All things being equal, a smaller engine is going to burn the same amount of fuel to produce the same power output as a larger engine. It's like removing a 100 watt light bulb fixture and replacing it with a two 50 watt bulb fixture, the energy use remains the same. There is no way to trick or lie to your engine to increase your MPG, it is, regardless of size, going to have to produce the same power to move the same weight against frictional loses and wind resistance. In light of this, you'd have to A reduce the weight of the vehicle, and B streamline it. This is just what the auto industry has done. I'm assuming your two speed is an under and direct unit, a good combination. The ratios have been worked out by engineers, so they're pretty good for all-around use. Unless you live in one of the plains states and always have a tailwind, I'd stick with it.
All that I can find is the horsepower and it is 170@4000. The max recomended rpm under load is 4000.
Someone mentioned that I could probably find a gear to climb any hill... that's true. When I drive this bus, I start in 2nd gear High (that is the fourth gear combination from the bottom). I almost always leave the rear end in high gear. About the only time that I use the low range is if I really want to shift through all 5 gear combinations between 2nd high and 4th high. Normally I just go 2H-3H-4H.
I looked up my rear end ratios last night. They are 6.50 - 8.28
I looked up my rear end ratios last night. They are 6.50 - 8.28
I would agree your gear ratios are better suited for picking up and dropping off students than for highway cruising.
As it has been pointed out by others, use a vacuum gauge to know what your manifold vacuum is doing.
If you are pulling less than 10" of vacuum while going 4K RPM, you won't be able to pull a higher speed rear end without using more gas.
It takes torque to get you going and HP to keep you going. The real problem of small blocks in larger vehicles is they don't have enough of either to really be effective.
A 4.10 would be about as fast as you would want to go but in order to pull such a fast gear ratio you are going to need the torque and HP of the big block.
Be aware the 454 would not be a good choice. The big truck 366/427 is what you want. The 454 is not designed for the severe service the 366/427 engines were designed for.
Also, be aware that 5 MPG is pretty good. Regardless of what you do you will not see much, if any, improvement in fuel mileage. It goes back to the physics of the problem.
The only way you will see any real improvement in fuel efficiency would be to put a Cat 3208 or Detroit 8.2L in place of the gas engine.
I chose those 2 V-8's mostly because they have generally the same RPM range as the small block. The 8.2L was installed in a lot of medium duty GM trucks and buses. Off the shelf hardware should be available to install the 8.2L into your bus. Better yet, if you rescue the engine out of a wreck you should be able to get everything else you need to install it into your bus.
The IHC 6.9/7.3/Powerstroke is a little on the small size for your size of bus.
I realize that gas mileage is a difficult thing to improve. I know that reducing the forces due to kinetic friction is the best way to improve mileage. Since I can't remove any weight from the bus, and I can't change the shape of it to reduce wind friction, I'm kind of stuck.
I do live in an area that is very flat in most places(Minnesota). I know that with a different gear ratio It will take more power to accelerate and to climb hills but most of my driving is on flat ground and at highway speeds. With the engine turning at 4k at those speeds, it has to be sucking down more gas than it needs to. At this point, I don't think that it is so much a question of power as it is the speed that the engine must turn. I picture someone riding a bike in a low gear and peddling very fast. Then someone else in a higher gear, going the same speed, but peddling half as fast. I believe that the second person is doing less work. It may have taken them longer to get to that speed, but maintaining it is easier.
Even so, I can't imagine that it is good for my engine to be spinning at 4k rpms for 4 hours at a time on the road. If I can get the rpms down and still maintain a reasonable speed, I wouldn't be too sad if my mileage didn't change.
You are correct that my bus was designed by engineers who knew what the best combination of parts was. However, they never planned on my bus being driven without a load of passengers, and they thought it would rarely be driven on a highway.
I realize that gas mileage is a difficult thing to improve. I know that reducing the forces due to kinetic friction is the best way to improve mileage. Since I can't remove any weight from the bus, and I can't change the shape of it to reduce wind friction, I'm kind of stuck.
I do live in an area that is very flat in most places(Minnesota). I know that with a different gear ratio It will take more power to accelerate and to climb hills but most of my driving is on flat ground and at highway speeds. With the engine turning at 4k at those speeds, it has to be sucking down more gas than it needs to. At this point, I don't think that it is so much a question of power as it is the speed that the engine must turn. I picture someone riding a bike in a low gear and peddling very fast. Then someone else in a higher gear, going the same speed, but peddling half as fast. I believe that the second person is doing less work. It may have taken them longer to get to that speed, but maintaining it is easier.
Even so, I can't imagine that it is good for my engine to be spinning at 4k rpms for 4 hours at a time on the road. If I can get the rpms down and still maintain a reasonable speed, I wouldn't be too sad if my mileage didn't change.
You are correct that my bus was designed by engineers who knew what the best combination of parts was. However, they never planned on my bus being driven without a load of passengers, and they thought it would rarely be driven on a highway.
I appreciate the response! :)
Brent Rader
The two bike riders would be both producing the same amount of energy to do the same work. The laws of physics being what they are, that's how things work. To determine just how hard your engine is working the majority of the just tee a vacuum guage into one of the hoses connected to manifold vacuum and connect it with enough hose to get it to where you can see it from the driver's seat. As you drive, watch the gauge. The lower the vacuum, the harder the engine is working. Inversely, the higher the vacuum, the less lightly loaded the engine is. This will tell you if the engine can handle a numerically higher gear ratio without the carburetor "tipping" into the power circuit, hence feeding a richer mixture to the engine, dropping the MPG .
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