I have to disagree with the higher RPM less throttle using more fuel.

Fuel use is directly related to load place on an engine, Particuarly in an EFI controlled engine, Higher the load=low vacum=higher injector pulse width.(pulse width is also controlled by throttle position, TPS sensor)

At higher RPM with little throttle, yes the injector must cycle more than at a lower RPM, but the pulse width is much lower. (I am assuming just cruising, and not a WOT blast. )

So if you are going 40 mph in say first gear the RPMs are at ,say 8000 and the throttle is only open a fraction. High vacumn and very little fuel is needed to maintain this. But Go the same speed in 6th gear with the RPMs very low and the throttle wide or almost wide open is going to create low or no vacumn (higher load) with will raise the injector pulse width and use more fuel.


Here are a couple other examples.
*Copied from another site*
1- How Engines Use Gas

When you press your
gas pedal, your gas consumption goes up, right? Many people believe
the gas pedal is connected to the fuel pump somehow, but this is
technically incorrect. Instead, the gas pedal simply opens a valve
called the throttle. All this valve does is to allow air into the
engine. The more open it is, the more air enters. So, the term "gas
pedal" is really a misnomer...it's more of an "air pedal". Anyhow, as
the air entering your engine changes, sensors pick up this info on the
fly and adjust your engine gas intake electronically. In older cars,
this was done through a carburator, a rather ingenious device. This
means simply - the further you push your gas pedal down, the more gas
you burn. It does not matter what gear you're in; how fast your
engine is revving, or really anything else. Gas pedal goes down; gas
cunsumption goes up.


#2

Any engine is at its most efficient, when its RPM is HIGH, and
the load on it is LOW. When you are in a high gear, the load on the
engine is greater, and a slight up hill can easily reduce the RPM,
meaning that there is a greater load on the engine. By shifting down a
gear at that moment, you will effectively reduce the load on the
engine, and the RPM is able to come backup. Yet you have not pressed
the accelerator down any further, thus your fuel (energy) consumption
is still constant.

 

ok, average joe can get confused at this point.

Lower octane=more power if you're not pinging/detonating.

 

They're talking about effieciency in relation to power, not to fuel consumption. If engines were most fuel efficient at higher RPM, we'd have no reason for transmissions. At higher RPM, even though you have not introduced any additional air, you're introduced additional RPMs. In an 4 stroke engine, you always fire on the ignition stroke and always intake on the intake stroke. You need fuel to do both, and more RPMs = more intakes and ignitions = more fuel delivered. Test it yourself. Drive to work in 1st gear one day and then drive to work the next day in high gear. See which one uses more gas.

Also, higher RPM = higher vacuum. In a carbureted engine, that's going to suck more fuel, probably through the lager main jets vs. through the smaller, more fuel efficient pilot jets. In a fuel injection car, your MAF is going to sense more air or you MAP is going to sense less manifold pressure, and provide the car with more gasoline to compensate for the RPM.

Engines generate more power at higher RPMs, but are certainly not most fuel efficient at high R. My 307 cubic inch Cadillac motor sits at ~1000 RPM all day with its low gearing and gets the same mileage as a V6 and some I4 motors that run higher RPMs.

 

Octane is less volitile than hectane, which is the other component in gasoline. Hectane is much more combustible than octane and burns out much more quickly, so it's technically most performance-wise to use the lowest octane mix you can run without pinging if you're looking for performance. Running 93 octane gas in a car rated for 87 octane doesn't do anything except prematurely wear out your cat converters since they have to work extra hard to burn off all of that excess fuel.

 

But we are talking about high RPM with little throttle. A maf sensor can only sense the amount of air that passes it, you can only get so much air past the butterfly when its only a quarter open VS wide open. And yes a MAP sensor will provide more fuel based on lower vacumn.
A carb is going to be the same, Its not going to suck the fuel past butterflies that are not open very far, specially if its a 4 barrel. Also look at the power valve, which is vacumn operated, it open allowing more fuel in when vacumn reaches a certain point, Ie 6.5 inches which most Holleys, Demons Etc come with out of the box, So at a higher vacumn it remains shut.

Another way to show this is a car that has a A/F ratio gauge, At higher RPMs with little throttle the A/F will read higher, 12.1 and higher, Being at wide openthrottle at lower rpm, the A/F gauge will be alot richer. 11.0 down.

 

I have no idea what in the world you guys are talking about

 

I do agree about the transmissions and cruising at a higher RPM is NOT the most fuel effecient, A lower rpm for a given speed with the throttle only partially open will give you the best gas mileage. But Im only answering the question of the two extremes he posted.

Just because you have more RPMs does not mean you need more fuel, An engine is still just an air pump, Example most modern cars will shut the fuel injectors off all together under decaleration regardless of what RPM the motor is at. Although thats not really pertanaint to the questions he posted.

 

If we're talking about the same exact engine, if you have more RPMs, you're going to use more fuel. The vacuum differential between the pressure in the manifold and the outside pressure changes based on RPM. With more RPM, you'll see less pressure in the manifold, thus the engine is going to suck harder and use more fuel. That fuel is gonna come from either the jets in the carb or the fuel injectors, but it's still gonna come out. You can't raise RPM without consuming more fuel than at low RPM.

Think about a garden hose. It's always open the same amount, but if you only give it so much pressure, by turning the spout on halfway for example, only so much water comes out. The higher pressure the water, the more water is going to come out of the same hole. Same goes for air sneaking past a slightly open butterfly. When you have more pressure coming in, due to a pressure drop in the intake manifold, you're going to be able to squeeze more air in.

When you WOT an engine, do you think it gets maximum air instantly? Nope. You've gotta wait for the RPM's to build up before it realy starts to max out the throughput of that venturi. That's why carbs have accelerator pumps

 

careful on the garden hose example, when you squeeze it the hole size (diameter) changes it is now smaller, which increases the pressure with same amount of flow, you are not passing more water thru it

 

I'm lazy and didn't read any previous posts, but anywho.... I've always wondered the same thing as you are asking. I have a little module in my racecar that shows the injector pulse lengths in realtime and when you are at high rpm but just holding partial throttle, the injector pulse length is short as all hell, but at low rpms, to get it to maitain a steady speed you have to give it a lot more throttle and therefore the injector pulse length and way longer. I guess it just depends where the motor is comfortable and happy. Oddly, my truck will get about the same mpg at 75ish as it does at 65ish even though the revs are higher and there is more wind resistance, because the motor is happy where its at and I'm not using as much maintnance throttle.

Oh yeah, I thought that the higher the octane number, the smoother the burn. Smooth means less violent, and therefore a less violent burn will result in more knock-resistance.