What is the effect of high humidity on piston engine performance?

• A. Humidity increases air density and engine performance.
• B. Water vapor is more dense than dry air and increases engine performance.
• C. Water vapor is less dense than dry air, and high humidity decreases air density and engine performance.
• D. Humidity has no effect on engine performance.

Answer: (C)

High humidity reduces the density of the air entering the engine, which lowers the amount of oxygen available for combustion and thus reduces engine power. Water vapor is lighter than the primary components of dry air, so when humidity increases, the average molecular weight of the air mixture drops. With the same pressure and temperature, that lighter mixture is less dense, meaning less air mass (and less oxygen) gets into the cylinders on each intake stroke. In turn, the engine can burn less fuel efficiently and produce less power, especially as humidity (and often temperature) rise. In other words, higher humidity makes air effectively thinner, akin to operating at a higher density altitude, which diminishes performance.