Years ago, J.C. Whitney catalog used to carry a water injection system for cars which just bolted on. It actually worked pretty good. A friend of mine installed one on a VW bus. Got better mileage and ran smoother.
Interesting.
My father described a system he had in a car sometime around the mid nineteen twenties. It had a water reservoir with a large wick similar to an oil lamp which was in the air intake stream. He said that you could tell when the water ran out because the car didn't run as well. I made something to use on a car in the nineteen fifties but couldn't tell the difference in performance. Maybe my device wasn't up to the job. Or maybe the humidity in Houston is so high that additional moisture didn't have any effect.