In the last few years, concern about the environmental impact of vehicles has increased, considering the growth of the dangerous effects on health of noxious exhaust emissions. For this reason, car manufacturers are moving towards more efficient combustion systems for Spark Ignition (SI) engines, aiming to comply with the increasingly stringent regulation imposed by EU and other legislators. Engine operation with very lean air/fuel ratios has demonstrated to be a viable solution to this problem. Stable ultra-lean combustion can be obtained with a Pre-Chamber (PC) ignition system, installed in place of the conventional spark plug. The efficiency of this configuration in terms of performance and emissions is due to its combustion process, that starts in the PC and propagates in the main chamber in the form of multiple hot turbulent jets. In this work, the benefits of the PC were assessed in a production small SI engine, fueled with gasoline, and equipped with a proper designed pre-chamber. The engine is intended to operate in steady state conditions as a range extender for a series hybrid powertrain. The effects of the PC on combustion process, engine performance and fuel consumption were experimentally analyzed at the engine test bed. Three different engine speeds were investigated, namely 2000, 3000 and 4000 rpm. All tests were carried out at full load, both in stoichiometric and lean conditions. A 1D model of the engine under study was developed by a commercial software. Combustion, heat transfer and emissions embedded sub-models were tuned based on experimental data, in order to accurately reproduce the engine behavior. Both numerical and experimental results demonstrated that lean engine operation allows to significantly improve thermal efficiency. The results of 1D model simulations are useful to support the energy management strategies of the hybrid powertrain in selecting the most suitable ICE operating condition, considering the trade-off between low fuel consumption and high power supplied to the battery charger.