Water injection decreases the possibility of detonation by lowering the combustion chamber temperatures and increasing the effective "octane" of the fuel. This allows the engine to run a higher boost and full spark advance without the use of race gas. Additionally, if the water is atomized in the intake stream, there is an additional cooling of the intake charge. Both effects are increased with the inclusion of 50% alcohol to the mix (don't spray the exterior of an intercooler with alcohol!, this refers to internal injection only). The additional fuel of the high octane alcohol will in turn allow boost beyond what the factory fuel system can deliver.
Water injection also has the effect of "steam cleaning" the combustion chambers and exhaust valves. This removes the carbon build up that decreases efficiency and leads to pre-ignition(detonation). It will also keep the throttle body and interior of the intake manifold much cleaner than normal.
Water Injection FAQ:
Will water injection increase my horsepower?: Yes, but not directly. Water injection works to eliminate detonation, also known as knock. The big gains come from the ability to increase boost as well as the full ignition advance allowed by the ECU when it does not detect knock. While the cooling of the intake charge will produce some power increases, those increases are cancelled by the charge space occupied by the water vapor. In other words, there is slightly less room for air and fuel. However, by using up to 40 percent alcohol with the water, additional cooling takes place before the turbo, and the alcohol works as a fuel in the charge. This results in an increase in power.
Is water injection new?: No. Water injection was used during WWII to surpress detonation in fighter aircraft and increase their service ceiling.
Does the water "burn" in the engine?: No. The water simply converts from a vapor to a gaseous state which absorbs huge amounts of heat, due to the energy required to transform it.
This product would obviously be most advantageous for the turbo or blown guys. I utilize it simply because after the initial cost (which is relatively low depending upon which brand you choose), water is cheaper than 93 octane gas for the predator tune.
Look around at some of the sites such as coolingmist.com or the snow site (maybe the same as boost cooler--can't remember) and for the do-it-yourself guys, go to http://www.dawesdevices.com/water.html
where I plagarized the above explanation from.
Have fun!