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 knocking onset. The new features of the 1D model include an empirical correlation for the prediction of the water evaporation rate, and a newly developed correlation for the laminar flame speed of a toluene reference fuel, which explicitly considers the presence of water vapor in the surrogate fuel/air mixture. The latter correlation is combined with a fractal model for the estimation of the turbulent combustion rate. In addition, a more detailed kinetic mechanism is introduced in a previously developed knock sub-model for a more accurate prediction of the auto-ignition characteristics of fuel/air mixtures containing water. The extended 1D model is validated against experimental data collected at different engine speeds and high loads, up to knock limited spark timings, for a twin-cylinder turbocharged SI engine. The model predictions are compared with the experimental data in all the considered operating conditions, in terms of in-cylinder pressure cycle, burn rate profile and knock propensity. The numerical model correctly reproduces the experimental trends of air flow rate, indicated specific fuel consumption, and Turbine Inlet Temperature (TIT). Main inaccuracies only regard the prediction of the early combustion process, while its core is described in a very reliable way, both with and without water addition. Both experimental and numerical data confirm that the WI technology is able to improve significantly the high-load ISFC of the tested engine. Main drivers of ISFC improvements are the reduced over-fueling and the increased spark advance, compatible with the absence of knock and allowable values of maximum in-cylinder pressure and TIT. 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 along a modern driving cycle. Strategies employed to identify control parameters all over the engine operating range are discussed for both dry and WI operations. 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 indeed observed in this case, since a knock-free operation occurs along most of the WLTP cycle. Nevertheless, some limited benefits can be still appreciated.