disco: "Here's the link that Pale provided"
Thanks for reminding me where to find that article. I think it's very helpful.
"Are you sure you are using 70% throttle?"
I think there's a magic number, and I think 70 is probably about right, but I don't know if 70 is exactly right. I don't mean to imply that it is.
The issue is that at full throttle, the computer opens the injectors 100%. I think that's what's called open-loop mode. In the absence of a SG or similar instrument (like a DMM), it's hard to know exactly when this starts happening.
Open-loop mode wastes fuel. So the most efficient setting is somewhere slightly short of full throttle. And the right answer probably varies for different cars. What you want is the maximum throttle opening (because this reduces vacuum and therefore reduces pumping losses), just short of sending the ECU into open-loop mode.
"It's pretty clear from that graph that the most fuel efficient operating condition, regardless of load, is between 3,000 and 3,500 RPM."
I think you're misinterpreting
the graph. The graph doesn't say that 3200 RPM is highly efficient "regardless of load." The graph says that at any
given load, 2800-3200 RPM is highly efficient. Here's the key to understanding the graph: "load" basically means "throttle opening."
You are noticing, correctly, that given a load of 100%, that the engine is most efficient at 3200 RPM. This simply means that if the engine is running full throttle at 3200 RPM, it is more efficient than when it is running full throttle at, say, 1000 RPM. Fair enough. Here's what that means in terms of driving.
I'm climbing a steep hill. I'm in top gear. I have the throttle wide-open (for the moment, let's set aside the open-loop issue). The engine is at 1000 RPM. My speed is probably about 20 MPH. The speed is constant; i.e., I'm producing exactly enough power to fight gravity and travel up the hill.
Now imagine that I downshift to 3rd, or 2nd. RPM is now 3200. The throttle is still wide-open. Speed is still 20 MPH, and is still constant. I'm still producing exactly enough power to fight gravity and travel up the hill. But I'm not accelerating, because the hill is very steep.
The graph shows that in this circumstance, I am indeed better off in the lower gear. In other words, when heavy throttle in a low gear is not enough to raise my speed (assuming I am not already at the redline), maybe that means the hill is so steep (and/or the trailer is so heavy, and/or the headwind is so great) that I actually belong in that lower gear, and shouldn't upshift.
But this is an unusual circumstance. For almost all modern cars (even with low-power engines), if we assume a flat road and a vehicle carrying only one or two people, then full throttle (or 70%, if we want to take the open-loop issue into account) is enough to produce acceleration, even in top gear. Unless we're already at a high speed (like 90 mph), where air resistance is providing the equivalent of a steep grade.
In other words, if we're using the throttle to keep the engine at high efficiency, that probably means we're accelerating. That's the essence of P&G: acceleration, alternated with coasting.
Back to the graph. Yes, given a fully-open throttle, 3200 RPM is more efficient than 1000 RPM. But our cars do not stay at 1000 RPM, if the throttle is fully-open. Even in top gear. They accelerate. Unless you're climbing a grade, and then it indeed makes sense to downshift. But the important thing is to keep the throttle fully open (70%), except for when you're coasting. If I'm climbing a hill in 3rd gear, with full throttle, and this is not producing acceleration, then you're right, I probably shouldn't be in a higher gear, even if that would let me lower the revs.
"If I'm using 1/2 throttle, the acceleration is very brisk"
Life is much more complicated with an automatic. It's probably brisk because your tranny decided to downshift. That's what an auto trans usually does, when you give it a large throttle opening. A stick is much better, in this regard, because it lets you choose a large throttle opening without downshifting.
Theoretically, we want full throttle 100% of the time, except when we're coasting. I say 'theoretically' because we have to back off just enough to avoid open-loop mode. And with an automatic, you also have to back off just enough to avoid unwanted downshifting.
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