OK, I need an answer to this fuel economy question...

"BJL" (simplifyandaddlightness) 10/17/2014 at 20:44 • Filed to: MPG

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Whenever I see a fuel economy run, I always hear low rpm, and go. However, wouldn't being in the power band be the most efficient way? Minimal throttle application for the most acceleration response?

Also, to use more fuel, giving full throttle at redline isn't going to do anything, as the car will just electronically limit the fuel amount poured into the throttle body/ carburetor. With that said, going into the highest gear possible and dogging the engine, in my mind, would give you the most fuel consumption, if I am not wrong.

What is the real deal I have had the question for a while, and it makes sense in my mind how it works, which is contrary to norm.

Forgetful > BJL 10/17/2014 at 20:52

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Think of it this way. Assuming a 4 cylinder, 4 stroke engine, doing 35mph. 1 minute at 9000 rpm is 9000 injections of fuel. The same speed for 1 minute at 2000 rpm is 2000 injections of fuel. The amount of fuel per injection might vary but rpm is the usually the biggest factor.

Zipppy, Mazdurp builder, Probeski owner and former ricerboy > BJL 10/17/2014 at 20:52

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I say low RPM, but it could depend on the car, like, probably not too low of an RPM that it stutters. The powerband on my brother's TDI is extremely low on the gauge, so it's ideal either way. A higher RPM gives less time for fuel to burn, so inefficiencies will happen.

Jordan and the Slowrunner, Boomer Intensifies > BJL 10/17/2014 at 20:54

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I would say you would rather stay in the low RPM, especially if that is where peak torque is.

davedave1111 > BJL 10/17/2014 at 20:54

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Google 'pulse and glide'. Sounds similar to what you're talking about.

BJL > Forgetful 10/17/2014 at 21:02

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OK, but doesn't your foot control the amount of gas being poured in the engine? If I don't push the throttle at all, there will be no fuel per injections, also known as engine braking. If I floor it in high gear, it will dump a lot of gas into the engine and give little response. Even though it is a dramatization, the same should occur for little amounts of throttle applied.

Forgetful > BJL 10/17/2014 at 21:15

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Yes, your throttle position matters, but the engine demands the fuel it needs to maintain the combustion cycle. That demand happens more often at higher rpm. Also the higher rpm the more instances of compression there are to overcome.

Every engine will have an efficiency curve, but don't confuse that with the power and torque curve.

OPPOsaurus WRX > BJL 10/17/2014 at 21:20

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my bro-in-law got a computer that could attach to the car and read all the stats as we were driving. it was absolutely fascinating. Even with my foot of the pedal it still had something like 12% throttle cuz 0 would be no gas and you'd stall. We also measured 0-60, boost, MPGs etc

samssun > BJL 10/17/2014 at 22:51

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More rpm means more frictional losses. In a perfect world with infinite gear ratios, your ideal engine speed would probably be down where it makes peak torque. Above that, you're making less torque, meaning the engine isn't pushing as hard at any given instant, but you're making back the power by doing it more often.

The other half of the equation is wind resistance, which increases nonlinearly. So if you drive at peak torque rpm you should be getting the most horsepower per unit of fuel, then you set your gearing as tall as that power level permits, where wind resistance becomes excessive.

Since you can't adjust your car's aero or gearing on the fly, you just end up going to top gear and keeping revs low and speed reasonable.

If only EssExTee could be so grossly incandescent > BJL 10/17/2014 at 23:40

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You have to rev to accelerate, but as far as holding speed goes low RPM is best. More economical to maintain 50mph at 2k RPM than at 4k.

BJL > Forgetful 10/18/2014 at 17:57

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ok, so a car with a low peak hp, say 2k will be more fuel efficient than one with a peak hp rating at say, 6k, correct?

In my mind, if the power band were to line up with the efficiency band, the mpg would be optimum, no?

Strange Noises Alou > samssun 10/20/2014 at 19:47

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and this is why I wish my car had a sixth gear.

at least I can't absentmindedly throw it into reverse!

samssun > Strange Noises Alou 10/21/2014 at 00:05

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My 5 speed 300ZX does 3500 rpm around 70-75 mph. Drives me nuts just thinking about it, and even more so on 5-8 hour drives.

Strange Noises Alou > samssun 10/21/2014 at 13:25

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completely nuts. I'm quite sure I could maintain 75mph at ~2200-2500rpm on level ground.

if I were significantly more of a gearhead, I'd swap in the 5th gear and final drive of a CRX HF into my SI. (the reason I got the '86 SI was for the EFI - I'm of the belief it's more reliable than a carb.)

biker > BJL 11/01/2014 at 19:11

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a bigger engine can at times be more efficient than a small engine, I could take a big block 454, & get better mileage than people would imagine, if you put it in a light vehicle, for example a s10, & use stock heavy flywheel, back it with a 5 speed, & a tall rear gear. & a carb designed for gas mileage & a RV type cam.

a great example take a vehicle with a V6, same one with a V8, in some circumsatances the bigger motor wont work as hard & deliver better mileage.