Despite the continuous advancement of spark ignition engine technology, further solutions are under investigation to improve fuel efficiency and meet worldwide CO2 emissions regulation. Currently, engine manufacturers have focused on engine downsizing to improve efficiency of fuel combustion with the introduction of turbocharger or higher compression ratio engines that have the drawback to run the engine at delayed spark advance or richer mixture due to the knock occurrence. In this paper, the water injection technology was examined with the aim to reduce the air-fuel charge temperature to enhance the knock limit. The potential benefits of this technology were investigated on a downsized PFI twin-cylinder turbocharged spark ignition engine, by installing an additional port fuel injection system. The engine was run at high load condition (~15.5 bar IMEP) within the engine speed range from 3500 to 4500rpm. For each test case the effect of the injected water quantity on combustion and exhaust emissions was investigated by sweeping from 10%w to 30%w the water to gasoline amount ratio. The water was injected at the same timing as the gasoline by a low pressure injection system external controlled, able to handle the main engine parameters. Tests were performed changing the spark timing up to no knock and in-cylinder pressure peak restriction. The water injection resulted in a spark timing advance without knock occurrence that produced an improvement of IMEP and a reduction of ISFC. Increase in the injected water mass resulted in reduced turbine inlet temperature and advanced combustion phasing.