Why my RPM so high when driving on HWY?

[matt_fulghum]

yeah, like they're saying.... it's just the way that the car's tranny is geared... there's almost nothing you can do about it... pissed me off too, when I first bought my car, but luckily the engines are pretty small in our cars, and they're light, so their gas milage isn't too bad.

[jschrauwen]

Yeah, thats normal. Mines been that way since driving it home from the dealership back in 93. Truth be told I often try to shift from 5th to some none exstistent next gear, I just laugh at myself and put it back into 5th.
at 80 mph I'm at 4000 rpm.

Andy P. 93 GS

Been there, done that, way too many times...LOL, ever since I got it in '95. At 4k rpm also, but I like Yoda's remedy, something to seriously consider.

[TsiMiata]

Why is everyone complaining about the rpm's? It's not damaging the engine, Its not effecting fuel mileage. That is just the way it is. The tranny is geared to get the most effective use of the engines power. High rpm's on the highway is what you end up with.

Overdrive on any transmission is any gear where the output shaft of the transmission is spinning faster than the input shaft, IE: and gear ratio lower than 1:1. Just look at the word- overdrive. It means to over drive the output shaft.

The bolt on overdrives are doing just that. Taking a transmission that doesn't have overdrive(a ratio lower than 1:1) and further increasing the output shaft speed in relation to the engine to make the transmission an overdrive unit.

Someone mentioned are low final drive negates that. That's not really true. The output shaft of the tranny is still spinning faster than the engine and thats all that overdrive means. It maybe is a bit more confusing to think about on a fwd car. On a rear drive car the driveshaft would be spinning faster than the engine, Then the rear end ratio reduces that speed to something reasonable. On a fwd drive car the "rear end" and "driveshaft" equivelants to a rwd car are all in the transmission housing.

[matt_fulghum]

of course it affects fuel milage... every combustion cycle requires a certain amount of fuel, based on the AFR. By that logic, unless you lean the combustion out to where the engine would completely be destroyed, the higher RPM you go to, the more fuel you burn. Simple physics.

[Aston Wards]

6 hrs to see your girlfriend?
ahh, 18 i see!
good on ya mate

[TsiMiata]

of course it affects fuel milage... every combustion cycle requires a certain amount of fuel, based on the AFR. By that logic, unless you lean the combustion out to where the engine would completely be destroyed, the higher RPM you go to, the more fuel you burn. Simple physics.

Completely untrue. An engines effeciency changes across the entire rpm band. Things like compression ratio, intake runner lengths, cam timing etc.. all effect the fuel requirements at different rpm's. High RPM's does not always equal rich fuel mixtures. The other thing your not thinking of is that as the rpm's are going up so is your speed. Even thouhg you maybe using more fuel per second, your speed has increased. So you can end up getting the exact same fuel mileage. That is assuming the drag on the car hasn't gone up. That is where asll the power is usually going at highway speeds, overcoming air drag.

[matt_fulghum]

ah yes, but if you can attain the same speed at a lower RPM with a better gearing, then you're still reaching equilibrium with the same forces, but without pumping as much fuel into the engine for combustion

[Gro Harlem]

high rpm DOES mean it burns more fuel. it makes perfect sense why.

Even if its running leaner when you are just cruising, the fact you are seeing 3500 rotations per minute means you are seeing 1750 combustions per minute which is about 290 combustinos per cylinder per minute. Each combustion uses fuel.

If you lower the RPM to say 2500, you are looking at 208 combustions per cylinder per minute. Thats a savings in gas for sure.

I put larger diameter tires (205/60R15) & swapped out my differentail for the LSD with a 4.11 final drive to lower my RPM's. My 70mph highway rpm is now about 3250 instead of 3550.

The main reason i used larger tires wasn't gearing, it was b/c of how cheap 205/60's are compared to 205/55's. The Falken ZEIX 512's were about 66 each in 55's while only 42 each in 60's. Screw paying a shitload more for the proper size when the 60's are like 2% off. Saving money in tire costs & fuel costs as a result

[matt_fulghum]

Thank you Gro.... I don't understand why the idea of fuel burning faster at higher rpms is confusing

The gearing in the RS totally sucks. I was driving back from Connecticut College yesterday in my mom's Subaru, and at 90 mph it was only going about 3400 rpm or so I recall.... hell, at that rpm in my car I'll be going somewhere around 60

Too bad there aren't any direct bolt on 6-speeds (with decent gearings that is) that we could throw in the MX.

[TsiMiata]

I still dont agree. I have another thought about it too. It takes a certain amount of power to move your car down the road, regardless of rpm. You will hold the gas pedal in a position that the engine will make just enough power to maintain the speed you want. Lets say it takes 20hp to over come drivetrain losses, 15hp to overcome air drag, maybe a few more for the drag of the tires. Im just throwing out made up numbers here. So it takes 40hp to make your car go 65mph. When fuel burns it releases a certain amount of energy. No matter what the rpm you will only use 40hp worth of fuel. If you used 45hp worth of fuel the car would accelerate up to the speed that takes 45hp to maintain.

Its not just the rpm's that matter here, its the load on the engine. The more load there is on the engine the more fuel that will be injected per combustion cycle. If the car was in neutral and you floored it the engine would not use nearly as much fuel at 7,000rpm as it would if you were in 4th floored on the highway. It's all about how much load is on the engine.

[IanL]

Engine design is a complex thing. One engine may be most efficient ( i.e. most hp per unit of fuel) at 3000 rpm, another design at 4500. The only way to make a meaningful statement is to put the thing on a dyno at various loads and speeds, and measure the fuel consumption.

I'm afraid broad brush assertions that low revs are/aren't obviously more efficient are futile. Only test results have any validity.

[Gro Harlem]

its true that the efficeiency of the motor has to do with it but still.

We are comparing if the stock engine rev'd lower at speed than it does from the factory. IF you take the SAME engine and have one rev a constant 2500rpm at speed while the other revs at 3500 it is obvious that the 3500rpm one would burn more fuel.

[matt_fulghum]

besides, don't forget that it takes a lot more force to accelerate than it does to just keep going at a certain speed. the reason for gear ratios is that they allow the engine to run at reasonable speeds in comparison to the wheels. They do this with a thing called mechanical advantage. At higher gear ratios, the advantage is greater, but it makes it harder to spin it faster (http://stage.itp.nyu.edu/~tigoe/pcomp/machines.shtml)

Simple physics... I don't understand why you can't understand this.

[TsiMiata]

What I tried to explain in my last post is that the rpm doesn't matter. It takes X amount of energy to maintain a certain speed. regardless of engine rpm the engine will only use enough fuel to make the amount energy needed to move the car that speed. The tuning of the engine components will have an effect on this too. Usually an engine is most effecient at its torque peak. One reason why big american V8's cruise at super low rpm's and high revving imports cruise at much higher ones.

In your example of 2500 vs 3500 rpm this is what will happen- The throttle plate will be more open at 2500 than at 3500 rpm. Since the engine is spinning slower you will have to move more air into the cylinder to light off the required amount of fuel to make the exact amount of power you need to propel the car at what ever speed. At 3500 rpm the throttle plate will be closed more than at 2500rpm. Since your doing more combustion cycles per second it will take less fuel(in turn less air) per combustion cycle to make the same amount of power you were at 2500rpm. The Amount of fuel the ecu puts into the engine is not just dependent on the rpm. The throttle position and air meter have huge effects on this. T

[matt_fulghum]

the point is, that it takes far less energy to keep up a certain speed than you make at say 70mph, thus, you shift up, so that you make less power, but due to mechanical advantage keep the same speed... why do you have a hard time with this issue? XD

Look, you want proof? Buy a full tank of gas, and redline it EVERYWHERE until it burns away, then buy another one, and drive it as low an rpm as you possibly can, and you'll find that you are sorely mistaken.

Yes, an engine is generally most efficient at its torque peak, thus it can take in the most fuel and air as it can burn. Notice: as much AIR AND FUEL AS IT CAN BURN.

Bam.

You burn far more fuel at higher rpms than you do at lower because despite the increase in engine efficiency, that simply allows it to take in even more fuel and put out even more power. Because of the mechanical advantage of higher gears, and the fact that to coast at a constant speed you simply need to equalize torque output (acceleration) vs. wind resistance and such (deceleration), often that power is entirely wasted in a lower gear, because the mechanical advantage, the actual total efficiency of the drivetrain, is far lower.

Higher gears trade torque for mechanical advantage. Simple f**king physics

oh, and....

At 3500 rpm the throttle plate will be closed more than at 2500rpm.

Nope.... not at all. No way. The throttle plate (when in the proper position for giving exactly the right air flow required by the engine to maintain a speed) will be more open at 3500 rpm than at 2500 rpm, regardless of what gear you're in, because the air requirements of 3500 rpm are higher than at 2500 rpm.

Since your doing more combustion cycles per second it will take less fuel(in turn less air) per combustion cycle to make the same amount of power you were at 2500rpm.

Do you realize exactly what you're saying there? Alright, an engine has a certain amount of displacement. After a combustion cycle, that displacement creates a vacuum in the cylinder, which pulls in a certain amount of air, which requires a certain amount of fuel. The amount of air pulled in per cycle does not change, by the nature of pressure equalization. An engine, when the intake valves are opened, pulls in enough air to attain atmospheric pressure (ever notice how cars don't perform as well at high altitudes? this is why they put turbochargers on airplane engines, because the pressure up there is so low that air needs to be forced into the cylinders in order to attain high enough power to maintain flight). After a combustion cycle, the gasses inside the cylinder are at very high pressure, due to the cooling of the exploded gasses, and shoot out of the cylinder, then the piston starts pulling down, creating a vacuum again, ad nauseum.

Now, it stands to reason that if every combustion cycle pulls in pretty much the same amount of air (it all really depends on the way a cam is ground, so it's not necessarily a complete pull of air, but for our purposes, let's pretend), and that air is mixed with enough fuel to keep the combustion at, say, stoichiometric levels, then at higher rpms, the amount of air displaced and fuel used is greater than at lower rpms.

Jeez, this shouldn't even be an argument. This is high school physics.... I should know.... I'm in high school (helps that my dad's a physicist though )

I just gave strong, hard reason on why Gro and I are right.
Just stop trying to argue it.
Physics man... Physics.