The potential benefits of water injection on performance and emissions were investigated on a downsized PFI twin-cylinder turbocharged spark ignition engine. Experiments were carried out at high load condition (~15.5 bar IMEP) within the engine speed range from 3500 to 4500rpm with a step of 500 rpm. 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 ratio. The water was injected at the same timing as the gasoline by a low pressure injection system external controlled. Tests were performed at WOT conditions exploring, for each operating condition, a spark sweep from knock-free up to knock-limited operation.Compared to the full gasoline reference case, the water injection allowed to advance extensively the spark timing without knock occurrence. The 20% water to gasoline mass fraction gave the best improvements in terms of IMEP. The reduction of combustion temperature, due to the water injection, coupled to the spark timing advance without knock, led to a stable reduction of the temperature at the turbine inlet.