[gone-ot]

All AIR has some amount of water vapor (humidity is never zero), thus all internal combustion engines (ICE) have, since day one, been operating with water vapor (of obviously varying amounts) in the AIR consumed during combustion. That has BEEN and will CONTINUE to be a fact (pending global warming effects).

Adding additional water vapor into AIR is functionally equivalent to driving through air with extreme humidity, akin to "...a foggy London day/night..." That increased water vapor in the AIR does not itself produce ANY additional heat energy (power), rather it allows the engine's EXISTING ignition-mapping (new cars) or ignition-advance (older cars) to simply USE more of the already existing timing advance without fear of detonation from the same octane-rated gasoline. It is the advanced timing--not the water vapor--that produces more power, by extracting more thermal energy from the combustion process before the opening exhaust valve(s) cause the in-cylinder mean-effective-pressure (MEP) to cease.

And current ICE do not have intentional operating combustion conditions sufficiently hot enough to dissociate water vapor back to free H and O atoms...specifically to avoid combusting N2 in AIR into its oxides (smog).