View Full Version : MAKG, explain something for me
12-05-2007, 11:47 PM
i'm having a debate with a couple buddies here about fuel mileage. Now what i have been reading about all the different types of RBV motors, most people would say that running at a higher rpm will save on fuel mileage. However my buddies are saying that if you run at a higher rpm (say 2800-3000) it will burn more gas then saving it. Now i am sure there alot of factors involved with it, being friction, gearing, etc. For example i was saying how if i drive around in 3rd gear, at about 2600 rpm, it would be better then driving around in 4th at say 1700 rpm.
So i guess what i want to know is, will a vehicle get a better gas mileage running at a higher rpm, vs a lower rpm.
Have your buddies considered the cost of a new engine after lugging it for extended periods of time? The cost of a new engine is a lot greater than gaining 1 MPG.
But I'll let Mike explain it...
12-06-2007, 12:07 AM
It's all a matter of running at or near the "ideal" rpm for the
particular engine in question.
Even running slightly below that "ideal" hurts mileage far
more than running slightly above.
AND you've gotta understand how the engine management
I've seen changes in gearing, going to grabbier, numerically larger gears
improve fuel exonomy too many times to have the least bit of doubt
about what I KNOW to be the truth.
If someone else doesn't believe, I feel bad for anyone that has to
deal their insistant ignorance.
But as an example, on my personal truck 1900rpm is all that's needed
to do 55mph in 5th gear and THAt is with 4.10's on 235/75-15's
So what exactly is a taller gear supposed to do?
12-06-2007, 12:19 AM
i also forgot to mention that they all drive autos so they have no idea what lugging can do or what its all about for a motor, if they want to pass on say a highway or something all they have to do is hit the pedal. etc.
12-06-2007, 08:46 AM
on aircraft the fuel consuption and flight range is directly related to how long you stay at the rpm it produces the most power for the least amount of fuel.if you have to put your foot further to the floor at a lower rpm in a higher gear you are making the engine work harder and waste more gas.
I've though about this a lot.
I went back to 1985 where some vehicles came with auto-over drives and some still came with non-overdrive.
This is a motor that makes peak torque at 2000rpm so it's reasonable to assume it will be most efficient around 2000rpm even though at highway speeds it certainly won't be making peak torque under normal conditions at that engine speed. These figures are the new EPA updated ones that take the newer speed limits into account.
F150, 300-6, auto overdrive (http://www.fueleconomy.gov/feg/2008car1tablef.jsp?column=1&id=814): 12 city, 15 highway
F150, 300-6, no overdrive (http://www.fueleconomy.gov/feg/2008car1tablef.jsp?column=1&id=813): 12 city, 13 highway
The non-overdrive motor is at 2300 going 65 and the overdrive is at 1600.
Why did everyone have to go to overdrive? Was it because of increasing speeds? No. The 55 limit was in place from 1974 to 1988 and overdrives became standard in the middle of that. It was for increased fuel economy. In 1975 congress ordered manufacturers to double the efficiency of vehicles by 1985. By 1985 everyone had overdrive.
But that was then and this is now. You need a vacuum gauge to see how your exact vehicle should be driven. You need as low of an rpm as possible while keeping the engine vacuum reading as high as possible. On a 1975 shitbox that meant slowing the engine way down. A large engine makes a lot of torque and at light loads has enough reserve that it can loaf without much throttle.
There are so many variables that it's not good to attempt a generalization. Get a vacuum gauge and see what is going on with your engine.
12-06-2007, 09:55 AM
good idea will.my mother used to have an old clapped out 71 dodge 4x4 with one of those vacuum gauges with its increments in "power","economy","cruise",etc.and while it was kind of a joke on that gas hog,it would be useful on my truck.does anyone know offhand where to get one marked like that?i bet it would co-oberate to some degree with what i've said above about finding an rpm where you use less throttle for the same momentum
I don't have all that much to add to what Will and Allan said. But I'll take it one step further. There NEVER HAS been any reason for overdrive. The engine, fuel economy, and all that don't give one whit about whether gearing lives in the transmission or rear end. And I suspect early 70s efficiency issues had more to do with carburetion than anything else. Carburetors don't really increase fuel at the right rate with throttle position, and as a result there are at least three, usually four, and sometimes as many as seven "circuits" within the carburetor, that kick in at various throttle positions. This means you can't optimize it for all engine speeds without making it expensive and difficult to tune.
The peak torque and maximum efficiency points on the engine need to be near one another for a basic reason -- fuel consumption per stroke is controlled mostly by the throttle position (hence Will's vacuum gauge suggestion), and the efficiency peak is the RPM where the highest fraction of the fuel energy goes into torque. It's not too hard to see that the torque will be highest there as well.
FYI, Rick, virtually all vacuum gauges intended to be mounted in the vehicle have markings like you describe. But as a rule of thumb, keep it as high as you can. Over 10 inches on level ground at highway speed is very good. But it's pretty sensitive to grade; even a modest grade or headwind can vary that substantially. I usually recommend people keep it above 5 inches all the time, except for SHORT bursts such as getting on the freeway (unlike what some of those annoying Prius owners think, flooring it for 10 or 20 seconds out of an hour trip isn't going to make a DAMN bit of difference to mileage -- so the MPG gauge drops to 5 for 10 seconds. BFD.).
12-06-2007, 11:09 AM
It's all a matter of running at or near the "ideal" rpm for the particular engine in question. AD
I agree....say you have a 2.3 the best place to run it is about 2,300 to 2,500 rpm give or take. The 3.0 likes 3,200 rpm for mileage. That's where their peak tourqe is. I'm sure the 4.0's best mileage is lower, since the tourqe is found at lower rpm's vs. the 2.3 and 3.0. I always drive in 3rd around town and 4th at anything between 45 and 55 and only use 5th on the interstate doing 70 or so. BTW I have a 2.3, 4.10's with 30in tires. which makes my gearing about a 3.83. I have also driven many 3.0's also.
12-06-2007, 12:22 PM
Actually exhaustive tests done long ago when testing marine, aircraft and automotive powerplants at "partial load" it was found that peak economy was at an rpm slightly above the rpm where the torque peak occoured while operating at part throttle.
In marine and aircraft engineering this information was used to determine how much engine to use for a given craft.
It was also determined that smaller engines didn't necissarily get better economy than a larger engine.
a small engine working hard can easily burn more fuel than a larger engine that can "loaf along" at partial load, presuming the same vehicle/speed/etc.
And there is a point to overdrive... it allows the use of numerically larger gears than could otherwise be used for best economy.
my 4.10 rear axle gears are converted into an effective ratio of 3.23 (compared to what gears I'd have to have to turn the same rpm with a 1:1 "top" gear) when I shift into the 0.79:1 overdrive gear of my M5OD-R1.
If you have 3.73's and a Mazda trans 5th gear is
"effectively" 2.94 compared to a 1:1 5th.
are you beginning to see the point of overdrive?
Are you beginning to see why I say 3.55's and 3.45's are wrong
and that 3.27's and 3.08's are a crime against reality?
Thus you get the best of both worlds, "diggy" gears for acceleration and a taller "Cruising" gear.
In a pure engineering sense I'd rather have a 1:1 top gear in a 6speed trans and a super granny gear at the bottom and run a taller axle ratio, because
that would eliminate the rubbing friction inherent in gear mesh while in OD.
if the "top" gear were 1:1 the transmission would be locked up straight
through and the only drag would be bearing roll over (insignificant in a
practical sense) and friction from stirring the oil in the trans.
But that would really only replace the rubbing friction in the transmission with
additional rubbing friction in the rear axle... another tradeoff.
Yes the engine doesn't care where the gearing is, but "short" gears built into the transmission have other issues... first that "Granny gears" tend to be mechanically
weak compared to gears with less reduction. and second a granny gear will either
force you to have large (or uneven) gearing steps OR force you to build a transmission with more gears. Space, weight and complexity all have their own problems.
All things in engineering are about compromises, the trick is choosing
the lesser of two (or more) evils.
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