The technique of liquid Water Injection (WI) at the intake port of downsized boosted SI engines is a promising solution to improve the knock resistance at high loads. In this work, an existing 1D engine model has been extended to improve its ability to simulate the effects of the water injection on the flame propagation speed and knock onset. The new features of the 1D model include an improved treatment of the heat subtracted by the water evaporation, a newly developed correlation for the laminar flame speed, explicitly considering the amount of water in the unburned mixture, and a more detailed kinetic mechanism to predict the auto-ignition characteristics of fuel/air/water mixture. The extended 1D model is validated against experimental data collected at different engine speeds and loads, including knock-limited operation, for a twin-cylinder turbocharged SI engine. The model predictions are compared with the experimental data, in terms of in-cylinder pressure cycle, burn rate profile and knock propensity. The numerical model correctly reproduces the experimental findings of fuel consumption, turbine inlet temperature and in-cylinder peak pressure. The combustion process, both with and without water addition, is predicted quite well, except for some inaccuracies in the early stage of combustion. Both experimental and numerical data confirm that the WI technology is able to improve significantly the fuel consumption of the tested engine under high-load knock-limited operation. Main drivers of the above advantages are a reduced over-fueling and a better combustion phasing. In a second stage, the validated model is used to build-up a complete engine operating map aimed at investigating the potential of WI technique to improve the fuel economy of a segment A vehicle. Engine maps with and without WI are introduced in a vehicle model to estimate the grams of CO2 per kilometer over a WLTP driving cycle. A reduced impact of WI is observed, since a knock-free operation occurs along most of the WLTP cycle. Nevertheless, some limited benefits can be still appreciated.