Though analysed by a few researches, the practice of water injection in Spark Ignition Engines (SI-ICE) does not yield homogeneous results, owing to various typologies of engines used for experiments.In this paper the effects of water injection in the intake pipe are investigated from both a theoretical and experimental viewpoint. Pressure vs. time diagrams were recorded on a single-cylinder CFR engine at AGIP PETROLI, Priolo (CT).Tests were performed according to Research and Motor Method (ASTM). Water was supplied by a continuous injection system inclusive of comparatively high pressure pump. The engine was fed with low O.N. base gasoline (cheap products, intermediate of refinery processes). The water to fuel mass flow rate ratio was varied in the range 0 to 1.5.The NOx emissions measurements confirm the tremendous effectiveness of water injection in reducing the engine environmental impact.Test data have been used to implement a detonation model that allows to predict water injection effects.Results have shown that water injection really represents a new way to avoid detonation, to reduce compression work and to control NOx formation in SI engines.