more HP + less skinny pedal = better MPG, why not?

[prosnomonkey]

Why is it that if an engine has more power it always uses more fuel? I was driving my boss's Toyota Tundra to Salt Lake City and back from Idaho Falls (about 600 miles round trip) and only got about 11 mpg the whole way. It has a supercharger and (he says) puts out about 500 hp. I stayed out of the skinny pedal (boss's truck ) and kept the RPMs at 2500 or so. Now my ranger with a 4.0 gets 20 mpg on a trip like that. It seems to me that if I have more power doing the same amount of work, I should use less fuel. Assuming of course that I am not accelerating hard or racing. Why am I wrong?

 

[Accord4tehloss!]

Couple thing Weight, Aerodynamics, gearing Plus at 2500 rpm that bigger motor uses more fuel then your 4.0 at 2500 rpm even if both the motors were free wheeling

 

[prosnomonkey]

Right, I understand aerodynamics and weight play a factor, but why when I add more HP to the same engine, body, etc I still get the same result... less efficiency?

 

[Jason]

You are getting more power by moving more air and fuel through the engine. That lowers mileage.

 

[Simple_serf]

It's not less efficiency.

Engine 1 makes x amount of power. Engine two makes twice the amount of power(2x)

Your big motor is making twice the power of your little motor. Gasoline has a fixed amount of stored energy per gallon. If engine two makes more power, it has to be using more fuel to make that power.

Now, this is a grossly simplified version of what is going on.

Efficiency is how much of that stored energy goes to being used to turn the wheels as opposed to how much is wasted as heat. Everything from Aerodynamics to transmission efficiency is going to make a difference.

 

[alaskan155]

its the supercharger, vs aspirated., the supercharger requires more fuel and air and is constantly going, so i would expect a aspirated or turbo(depending how its setup) engine to get better millage than a supercharger

 

[MAKG]

You guys are all missing the point that this is at part throttle.

The two hypothetical engines are producing the SAME power (even though one may produce more peak power) because the vehicle is presumably cruising at the same speed.

The problem is that the 500 HP engine has been optimized to produce more peak power, and almost no one optimizes part throttle behavior when doing this. So, you really do get less efficiency. Changing one part of the operating range changes all the others, often in very counterproductive ways.

Part of that is the supercharger -- it ALWAYS takes some energy to turn it, though that energy is in proportion to the power produced.

There are also differences in aerodynamics and weight, as you have noticed. A Tundra is a full-sized truck, significantly larger in both of those than a Ranger.

Another possibility is an incorrect odometer....

Mistake #1 is assuming that the engine produces 500 HP all the time. It's MAYBE half that at full throttle at 2500 RPM. Under conditions specified, FAR less, probably well under 50 HP.

 

[TRX501RYAN]

More displace takes more fuel plain and simple


IMO that tundra must have some low gears meant for towing,


That thing should only be reving at about 1500-1700 rpm at highway speeds in overdrive, atleast thats what a half ton will normally rev,



JMO if it were geared better it would be better on fuel imo, 2500 spinning 8 cylinders all day long is gonna suck back fuel!

 

[colinrmitchell]

More weight requires more force.

If you have more weight to move, and also more friction based on weight, and more wind resistance to fight, you need more force. I am sure that Newton, Euler, Lagrange, , Fourier, Gauss, Galileio, Plato, Socrates, and MAKG would agree that this would require more fuel (which is the source of the energy required) to produce the same average and instantaneous velocities throughout the trip.

But maybe I don't know what I'm talking about....

 

[Loanranger]

The real savings here would be in maintenance, not fuel. A higher horsepower motor spinning 1400 rpm to achieve say 120 hp, is going to have far less maintenance than a low hp motor spinning 6500 rpm to achieve that same 120hp.

 

[colinrmitchell]

Loanranger is making sense, but miles is miles, I think, when it comes to drivetrain components. I definitely feel like engine reciprocating components would require less maintainence with less revolutions, but are we yet completely sure that these costs would overwhelm fuel costs as we take miles to to a sufficient engine life (infinty )? (A little math humour, I am sure KELLY88 would like).

Or maybe I'm drunk. Time will tell...

 

[Simple_serf]

The real savings here would be in maintenance, not fuel. A higher horsepower motor spinning 1400 rpm to achieve say 120 hp, is going to have far less maintenance than a low hp motor spinning 6500 rpm to achieve that same 120hp.

Bull....

If an engine is DESIGNED to run at 6500 rpm all day it will last just as long.

My 2.5 isn't happy below 3000 RPM... It has 200,000 miles on it. Compare that to all of the old school low speed engines (That redline around there)... Seems like they all need to be rebuilt around 100,000 miles. hell...compare that to the "beloved" 2.8/2.9/4.0. They make power lower down (esp the 2.8/2.9), But don't last for shit. It took HOW long to build and engine that will Occasionally do that?? (the 4.0 which seems to be a crapshoot, sometimes you can't kill them, sometimes you can't keep them going)

In a properly designed engine, rpm doesn't greatly affect it's lifespan, so long as it says within the specified operating range.

 

[Sevensecondsuv]

I know why:


It's cause Toyotas SUCK!!!!!!!!

 

[snake]

Your theories don't hold water when it comes to diesels though. Turn one up and drive it "normal" and you will see an increase in efficiency. This applies to the pre-EGR engines primarily.

 

[Original_Ranger84]

Your theories don't hold water when it comes to diesels though. Turn one up and drive it "normal" and you will see an increase in efficiency. This applies to the pre-EGR engines primarily.

That is because diesel fuel has alot more stored energy, and when you increase the power the fuel still has that much extra energy never gets used in a regular diesel engine, and you unleash it with a power upgrade, (IE you could fun less fuel in a stock diesel and it would be able to run just as well with better mpgs. This is why a 460CID diesel can get around 10mpgs better than a comperable 460CID gas engine, the diesel fuel contains more energy.

I think Sevensecondsuv solved this delema!