What effect does increased water vapor in the incoming air have on a reciprocating engine at a constant RPM and manifold pressure?

Increased water vapor in the incoming air affects the density of the air-fuel mixture that the engine is using for combustion. When water vapor is present, it takes up space that would otherwise be occupied by air, leading to a decrease in the overall density of the air. This reduction in density means that there are fewer oxygen molecules available for combustion at a constant RPM and manifold pressure.

As a result, with less oxygen available, the combustion process is less efficient, which leads to a decrease in engine power. The engine may not be able to maintain optimal power output under these conditions, as the stoichiometric balance of the air-fuel mixture is disrupted. Thus, the correct conclusion is that increased water vapor contributes to decreased engine power.