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Can maintaining a high RPM save fuel? I know this sounds dumb and i have always believed the oposite but i was watching the IRL race a few weeks ago and one of the comutaors said -
"Danaca is runing in 4th gear to keep those RPM's up so she can conserve as much fuel as possible. by running a higher RPM she is leaning the car out and causing less fuel to be used"
This doesnt make sense to me.The whole reason cars have a overdrive gear or a fith or six gear is in order to keep the RPM's down while driveing.
can someone shed some light on how this makes sense. i understand what leaning the car out means, but i would never use it to save fuel.

 
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hmm it may be applied to race cars only?

 
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It's all about your tuning.
I'm not sure what the stock A/F map is like on a Civic, since I come from the land of cars without pistons, but most of the time with the stock ECU a car will run rich throughout the powerband. This is a safety thing for the engine. It comes from the factory with a lean enough tuning to get the gas mileage that is advertised, but rich enough that if you push your engine hard, even on 87 octane, you won't damage it due to knock.

You'd need a decent wideband o2 sensor, an A/F controller, and either a dyno or someone who knows how to tune on highway pulls to get better fuel consumption.

What must be understood about A/F ratios is that it isnt a fine line between rich and lean, there is some leeway. So you could pull out some fuel from the powerband, leaning it out a bit, and save fuel. Most people tune for HP gains, not fuel economy though. Slightly leaning out a naturally aspirated car that isn't spinning at ridiculously high RPMs is generally safe provided you are monitoring your ratio and you know what you're doing. Severe lean conditions have more effect on forced induction applications. If you're worried about leaning out too much and getting predetonation you can run a higher octane gas, that will give your engine better protection against knock if you're really pulling alot of fuel out. As for running at higher RPMs to conserve fuel on a stock mapped ECU, I'd bet against it.

Predetonation and octane ratings are a discussion for another day though.

 
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Generaly speaking,
To achive best fuel milage is to drive in a highest possible gear at enough RPMs, where engine doesnt need to strugle to keep car going.
e.g. for Civic it is 5th gear at about 2000-2500 RPM (50-60mp/h).
My car (Acura EL) can get about 35-38 mpg, but I know some can get better than that.

 
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hmm.... ok, lets say in a Civic you want to keep the car at 80MPH in 5th gear.... reach a hill you may need to press on the gas some to maintain speed. On the contrary if you left it in 4th, the engine is using high RPM but it may not seem as stressed and you wont have to press on the gas as much to maintain 80MPH. Maybe something along those lines.

 
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^ this is just my observation.

duty cycle will be higher if you leave it in 5th gear ... however, if you put it in 4th, rpms will be greater, so duty cycle will be running faster ... I guess it depends which one of the two would save more gas? ... I'm guessing 4th.

bioweapon: true u can tune yer car lean, but using a higher octane gas would kinda negate the "saving gas" idea since you would effectively be spending more for a higher octane, hm?

 
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Yes but at that point I was not arguing for fuel economy. The only time you should be moving up in an octane rating is when you are lean enough, your cylinder temperature, and your intake temperature are high enough to cause predetonation. This is far more likely to happen when you are boosted. So yea, if you are leaning it out enough, it doesn't save you money, but you pay for the protection.

 
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hm, interesting