bookworm... 20 GPH is the mixture setting!
No, it's not the mixture setting, it's the fuel flow. While in many circumstances the fuel flow is a reasonable surrogate, it isn't here.
Creating a specific power (ie. 45% 65% 75%, etc) requires a specific fuel flow for each. If you create 65% power at 2000', 4000', 12000', etc you will be burning the same fuel at each altitude.
Not so. The pumping losses, or exhaust backpressure as it puts it in your extract on the second page, is a real effect that changes the efficiency of the engine. At higher altitudes, you get more power for the same fuel.
I'm not sure how the constraints on my original post don't make sense.
But I now agree with you on this, I think. The constraints are OK, they're just hard to deal with.
Start with "at sea level, with 65% power set at 23"/2400RPM, and 20 GPH" and climb. At 8000 ft there are two effects that change the power to give you more than 65%: one is the lower temperature, which means that more air is going in at the same MP. The other is the lower exhaust backpressure, which means that the engine is more efficient.
So you can reduce the power in two ways: you can reduce the MP to maintain 65% power, but that will reduce the fuel flow below 20 GPH. So to restore it to 20 GPH, you creep the mixture control forward at the same time, to make it less efficient (assume we're on the rich side of peak). That will increase the power, so you keep reducing the MP and enrichening the mixture until you get that 65% power
and 20 GPH. You're correct in that only one combination of mixture
and throttle will give both 65% power
and 20 GPH. But it's not easy to calculate what the corresponding MP is.