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Old 08-21-2010, 10:28 AM   #1
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Rpm!

Hey, its been awhile since my last post which was more about idling then anything (Worst MPG possible), I've since become a truck driver and I lease my truck which means fuel economy means everything to me as its my biggest expense (22 Cents per mile for last weeks settlement), and I was taught in school and have since learned this for myself, that RPM's are *everything* when it comes to fuel economy (at least with a diesel), if I shift progressively I.E. Low Gear to 1000rpm's 1st to just over a thousand, 2nd to 1100 3rd at 1100 or so, pretty much shifting at as low an RPM as possible without lugging the engine and keeping your oil pressure up, fuel economy goes up during the most fuel intensive part of driving (2 gallons when@80,000lbs) to get up to freeway speeds, as well as keeping your RPM's low when you drive, I avoid going above 1300 rpms while driving on a regular basis, if you pull a hill at 1800 rpms you might get up it faster, but if you keep it lower, you save *tons* of fuel.

Also its something alot of people might not consider is wind resistance, I average 5.8-6.1 or so at 65mph and 7.3-7.8 at 58 mph, may not seem like a huge difference in the world of hypermiling, but at driving 600+ miles a day, it is an insane difference in fuel cost.
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Old 08-21-2010, 12:01 PM   #2
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Re: Rpm!

Now we're talking. We have a member hypermiling an 18 wheeler! Just remember to keep it safe, and don't travel significantly slower than the flow of traffic, of you may end up in the middle of an accident.
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Old 08-21-2010, 02:01 PM   #3
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Re: Rpm!

Quote:
Originally Posted by Thellra View Post
5.8-6.1 or so at 65mph and 7.3-7.8 at 58 mph, may not seem like a huge difference
That's about 30-40% difference. That IS huge. It would sound more significant in the form of gallons per 100 miles (or similar measure). I'd go from 40 MPG to 65 MPG with the same improvement.

As for RPM, I've found the same to be true for gasoline engines in my vehicles. If you learn how engines work, how vehicles work, and where the energy goes, it makes sense. Because it's called a "gas pedal", people think that using more of it (which is necessary to produce the same power at lower RPM) means more fuel used; it does not. All it does is remove an air restriction, and at low RPM that just means the same amount of air comes in more easily rather than more air coming in. Then people go and spend loads of money on intakes that flow better so they will think they can close their throttle more...it makes no sense! They could just open the throttle for free and get better results.

Now, with diesels, I understand there is no throttle and the go-pedal is connected to the fuel pump. I'm glad to hear that the result is the same.

You mentioned lugging...is it even possible to lug a diesel? Lugging is severe pre-detonation caused (if I understant correctly) by high cylinder pressure.
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Old 08-21-2010, 08:17 PM   #4
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Re: Rpm!

Quote:
Originally Posted by Jay2TheRescue View Post
Now we're talking. We have a member hypermiling an 18 wheeler! Just remember to keep it safe, and don't travel significantly slower than the flow of traffic, of you may end up in the middle of an accident.
Heck, I don't think he needs to worry about that. Half the states out there still are double nickel on the interstates. (At least for trucks)
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Old 08-22-2010, 07:29 AM   #5
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Re: Rpm!

it makes sense that aerodynamics would play a huge part in semis..... only 13x8 front surface area and with few exceptions, not exactly smooth in front. far more than even a pickup truck which will only have a few planes much less a car which are usually somewhat aerodynamically shaped to start with (certainly relatively speaking)
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Old 08-22-2010, 08:49 AM   #6
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Re: Rpm!

My scangauge backs up your rpm observations.
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Old 08-23-2010, 07:23 PM   #7
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Re: Rpm!

Thanks for the tips. It's good to see how it works in other vehicles. I haven't heard of progressively higher shift points, I'll have to try it.
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Old 08-25-2010, 09:29 AM   #8
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Re: Rpm!

I guess its not lugging, but this is what I understand what is happening, when you are at lets say 1000rpms in 8th gear, and trying to pull a hill, your engine starts spitting, and the trucks computer is dumping more fuel then can be burned and with my trucks Regeneration system (My truck doesn't throw out black smoke I have a particulate filter that catches it and then burns it at an absurdly high temp once every 600 miles) gets all plugged up, and yes you can lug a semi, lets say you are trapped in slow traffic, and 2nd gear at idle is 5mp 3rd is like 7sh 4th is 10sh and its a really lazy way to roll but when you start up a hill your rpms will drop below 600rpm (idle) and it makes a hellacious noise.
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Old 12-20-2010, 08:22 PM   #9
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Re: Rpm!

rpm IS everything in a diesel.
Not so much for a gasser, here's why;
(I just deleted a long technical explanation...here's something simpler)
A diesel doesn't require vacuum to properly atomize the fuel droplets like a gasser. If you use 1/4 throttle and lope along, worste case scenario you lose speed/rpm on a hill.
With a gasser, (for milage) keeping the engine in it's peak torque band with the highest vacuum will give the best milage. What this means is, a gasser can get better milage at 1/8 throttle @ 2500rpm than it will at 1/4 throttle @ 1400rpm.

Actual example;
81 f-100 300 6 cyl,3 spd,2.75 rear gear. 19mpg and 1/2 throttle if a hill is on the horizon.
Pull engine out and put in another truck. (and the following day)
81 f-350 4spd (np435) 4.56 rear gear. 25mpg at 1/8 throttle @ 2500rpm or so.

So, what happened? Same engine put in heavier truck with lower gears?
The engine was so low in the torque band and was constantly using so much throttle it killed the vacuum. Around 9" of vacuum most of the time which was letting the power valve open and dump excess fuel in. If there were no hills then in theory it would get better than 19mpg but I never saw it.
The one ton was like driving in first gear, no throttle at all. 17" vacuum all the time and could bump to 20 if I let off the throttle at all. Even loaded down hauling a cord of wood it got 22mpg @ 60 mph.

My present truck (93 f-350 4 door,7.3IDI) gets 25mpg at 100kmh and about 21 at 120-130kmh. If I'm idling down a back road at 80kmh or less it seems even better but I've never done it long enough to actually check milage.
Oh, and furnace oil is about 22mpg@100 and 19mpg@130kmh.
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Old 12-21-2010, 04:11 AM   #10
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Re: Rpm!

dieselmech, what you said about vacuum is true in carbureted gasoline engines, but not so with MAF-equipped fuel injection. In fact, if you've got a lot of vacuum then you're wasting energy making that vacuum. Additionally, with each revolution comes the same reciprocating and frictional loss (or, if what I've read is true, friction goes up as a square of RPM); extra revolutions are extra waste.

Unfortunately I'm having some difficulty getting used to my carbureted car. With my previous fuel injected car, equipped with MAF and wideband O2, the best results came from flooring the accelerator and keeping the RPM between idle and 1500RPM. Now that I'm using a carburetor I'm feathering the accelerator and experimenting with different RPM, but this car seems to respond to ultra-low RPM too. If I let it get up to a little bit higher RPM it runs smoother but fuel economy suffers. However, perhaps this belongs in another discussion since this discussion is about diesels.
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