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Originally Posted by KiwiBacon
There is no difference between what you call "work efficiency" and what you call "fuel mileage efficiency".
For example, running a car in 4th gear at 100km/h will give more vacuum than running a car in 5th gear at 100km/h.
5th gear has a wider throttle opening and less vacuum, yet the engine is using less fuel, less air and turning at lower rpms.
If you want to know how much air (and/or fuel) is entering your engine then a vacuum gauge cannot help. You need to read from the air flow meter or fuel computer. Manifold pressure alone is useless.
I agree that a small engine running at higher load will give better efficiency, but not running at the 90% top speed that you claim.
The engine has to be sized to run near maximum load at a speed below peak torque for best BSFC.
Making an engine last while doing this isn't a problem. Convincing the consumer to buy a car with no reserve power will be.
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I don't know what kind of car you're driving, but i'f im tootling down the highway in fourth gear, i most definetley have to hold the throttle wider than in fifth gear.
*edit* Anyway, as i said before, your thoughts aobut BSFC does not relate to fuel mileage. Simply put, the best BSFC you'll get is when your at peak torque RPM at WOT. This means you will get
the most power per pound of fuel = work efficiency. This does NOT mean you will get better fuel mileage by running at max BSFC, UNLESS you need that amount of power. Anymore and you're wasting gas. Simple as that. Auto's are inherently inefficient because of the range and loads that the engine must go through. Most of the time people only need 80hp to cruise down the highway at 70MPH, so what would be the point of running the engine (v8 for example) at 4000RPM to make 280 pounds of torque and 200 horsepower?
Prime movers on the other hand are designed to go two speeds most of the time, Stop and Go. Go means WOT at torque peak = best BSFC = best fuel efficiency.