Increased water vapor (higher relative humidity) in the incoming air to a reciprocating engine will normally result in which of the following?

Increased water vapor in the incoming air to a reciprocating engine generally leads to decreased engine power at a constant RPM and manifold pressure. This is primarily because higher humidity levels reduce the density of the air entering the engine.

As the amount of water vapor increases, the proportion of oxygen in the air decreases since water vapor occupies space in the air mixture. Since power output in a reciprocating engine is heavily reliant on the mass of air (and therefore oxygen) that can be drawn into the cylinders for combustion, less available oxygen results in a lower overall power output.

Maintaining a constant RPM and manifold pressure under these conditions shows the engine is still operating but with a reduced efficiency in combustion due to the diluted air-fuel mixture. Factors such as air density play a significant role in the engine's ability to produce power, hence the correlation between increased humidity and decreased engine power.