The reduced end-gas reactivity is confirmed by the analysis of the knock tolerance function in the periphery of the combustion chamber, visible in Fig. 10 below. A 2 mm thick cell layer bounding the combustion chamber is reported: as visible, the knock tolerance function defined earlier exhibits higher values in the water/fuel case than in the fuel only one.The decrease of IMEP of about 1.8% can be compensated thanks to the increased safety margin to knock onset. In fact, for the water/fuel case it is possible to increase the SA in order to recover the in-cylinder pressure trace and so engine performance. Advanced spark timings are tested, ranging from 2°CA to 7°CA, in order to find the trade-off