The research presented in this paper focused on the simulation analysis of the pre-chamber neck diameter on in-cylinder processes in a large gas engine. The investigation was conducted using AVL Boost software and the implemented PCSI combustion model. The scope of the analysis included different neck diameters from 17 to 26 mm and variations of the ignition timing in the range of 0 to 40 degrees. Using the narrowest ignition chamber neck resulted in the largest inter-chamber throttling effect. This translated into an increase in the maximum pressure in the cylinder, obtaining more heat release and the heat release rate. Further reduction of the constriction below 17 mm led to a rapid pressure increase in the ignition chamber at the start of combustion above the maximum pressure in the cylinder, for which the indicated constriction value was considered limiting.