Problem is engine design. They are trying to use water for an engine designed for gasoline. It would be better if they used an engine designed for hydrogen oxygen. That way it would be more effishent. Gasoline alcohol and hydrogen oxygen burn at different rates so the engine has to be designed for that type of fuel.
Having a problem with the water injection idea. Sure the heat would make steam and lower engine temperature but everyone knows that sticks and leaves burn better if it is dry so the water would be moisture that dampens everything making ignition more difficult.
Fun look up sterling engines that run on heat differential. Put one under your car seat and expose you butt and sit on it so that it would power your automobile from the heat of your seat. That way you could have a bumper sticker that says
455 or well just 455 nobody rides for free