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The passive prechamber concept, known as jet ignition (JI), represents an effective way to promote mixture ignitability, reduce combustion duration and extend knock limits in spark ignition engines. These aspects allow the adoption of a higher compression ratio and the operation in lean conditions, thus increasing thermal efficiency. Despite the potential benefits, the literature typically shows that in port fuel injection (PFI) engines at full load a shorter combustion duration does not necessarily translate in a growth of IMEP. Despite this issue has been frequently observed, the causes have not been fully explained. In previous works some of the authors supposed that the gain in indicated efficiency could be counterbalanced by the lower adiabatic efficiency, as a result of the higher heat exchange coefficient and the additional heat transfer from the prechamber surface.