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Actually I look at it differently. Here is my example.
Take an engine that has no throttle plate and produces 20 horsepower (under load) at sea level, then move it to a higher altitude.
As the atmospheric pressure becomes lower the power developed becomes lower, regardless of the fact that there is no throttle plate.
At a certain altitude depending on the mechanical compression of the engine, the atmospheric pressure becomes so low that there is not enough compression for combustion to occur.
You control the power developed by gasoline engines by restricting the atmospheric pressure. The pressure available for compression, in the cylinders, is the difference between the atmospheric pressure and the manifold vacuum.
Diesel engines are different, because they will run with very lean mixtures, compared to throttled gasoline engines, which require a specific range of air-fuel ratios to run properly.
Regardless of whether the engine is gasoline or diesel, the power developed is directly proportional to the available air pressure that enters the cylinders to be compressed and ignited.
I call this effective compression.
BSFC is (at 2000 RPM typically) a direct relationship with effective compression. The higher the effective compression the more efficient the engine. Supercharging increases effective compression by increasing the available pressure beyond atmospheric.
Without supercharging aircraft engines lose power at altitude, supercharging restores lost effective compression at altitude.
It's not so much the energy required to pull air past a throttle plate that creates inefficiency as it is the lower compression available due to throttle restriction.
I used the diesel example to illustrate the point about atmospheric pressure reduction creating lower available power at the same RPM.
regards
Mech
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