Depends what kit you get as there are a few. Mine is a devils own kit.
Taken from the website:
Increase horsepower safely by 10-15%.
Lower air temperatures by 50-200+ degrees.
Decrease cylinder temperatures up to 300 degrees.
Reduce the effects of heat soak in warmer climates.
Allows you to safely run more boost and timing.
Reduces carbon and helps maintain a clean combustion chamber.
Increases octane at user programmable boost levels.
If you use a 50/50 mix of water and methanol you can achieve an octane level of 116.
My friend ran the kit on an eaton'ed mx5 and said the aqua-mist kit made a big difference.
Im looking to fit mine in the boot as there isn't a lot of room in the engine bay for a tank, so i will have a gallon tank with pump in boot, run the nylon tube either through the car or underneath running along with the fuel lines, to the engine bay and put the injector either into the outlet of the intercooler or the big boost pipe as mine is alloy. But there are loads of ways to install them just depends on your preference.
The best way is to get your inlet manifold and drill into each inlet and put a nozzle into each inlet. That way the 4 aqua-mist injectors will spray equal amounts into each cylinder. As the least recommended place is the throttle body. It doesn't give enough time for the air to mix properly therefore it cant give an equal amount into each cylinder. Thats why im choosing to put it as far away as possible before the inlet except before the intercooler as....its bad.
But the main point of the aqua-mist kit is to prevent det.
