[Cycle]

My plan is to use the exhaust heat to preheat water to just below its latent heat of vaporization. I'll then use a new ECU that'll control all the normal engine components (fuel injector, spark timing, etc.) but also control a water injector in the intake manifold.

That injection water, being at such a temperature from the exhaust that it's just about ready to flash to steam, will absorb heat in-cylinder and flash to steam.

That has a couple advantages... it'll act as an octane boost, it'll slow down the lean burn so it's less explosive, it'll act as internal cooling and it'll contribute to cylinder pressure (and thus engine output). The trick is to get the water hot enough that it's just about ready to flash to steam before it's injected, so it flashes quickly once in the cylinder, to inject it in a fine enough mist that it can absorb heat quickly, and to inject enough to contribute to cylinder pressure while not so much that we're washing the oil off the cylinder walls.

The new ECU will have a fuel map and a water map, so it'll be easy to tweak things just right.

I'd use direct injection of the water, but my cylinder is so tiny that there's not a lot of room in there. Direct injection would let me time the water injection to just after combustion initiation, so the water follows along behind the flame front, absorbing the heat and flashing to steam. But unless I can find an exceptionally tiny injector, there's no way I can fit it in.