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Efficiency How does the efficiency of a piston aircraft change with altitude?

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Efficiency of piston aircraft with altitude

Turbine aircraft are more fuel-efficient, per mile, the higher they fly and this is for two reasons:

  • drag depends mostly on indicated speed, but the same indicated speed corresponds to higher true speed, and
  • the lower temperature increases efficiency of the engine.

Now both effects should also apply to piston-engined aircraft. Plus

  • spark-ignited engines are more efficient with open throttle, which again occurs near operating ceiling.

So I would expect piston aircraft to also fly as high as possible, and most piston aircraft should have no problem to get to at least 10,000 ft (where supplemental oxygen is not needed yet). But in various stories and documents I notice even long flights are often made much lower. Below 5,000 ft.

So is there actually some reason why piston-powered aircraft would be more efficient at lower altitudes? Or any other reason to prefer flying low?