The combustion system with semi open combustion chamber (SOCC) was originally elaborated in Aircraft Engine Department of Warsaw University of Technology. In this system the original combustion chamber of the standard SI engine, was divided by partition in prechamber and main combustion chamber, but yet this division exists only when the piston is close to TDC, on the contrary by the rest of the cycle the chambers are fully open. The system operation mechanism, the visualization research results, the high speed changed of the pressure measurements, was presented in this paper. The influence on the system performances of the different combustion systems parameters: the prechamber volume, the nozzle hole diameter in the partition, the ignition place, the compression ratio, and the ignition advance angle (IAA), on the basis of the research results, using rapid compression machine was presented in this paper. All research results show, that the best results of the system operation can be obtained if the stream outflow from prechamber to main combustion chamber starts when the piston is at TDC, and if the stream energy will be so big to displace all main combustion chamber before the clearance between partition and piston crown was opened. If the system operated correctly, the combustion time shortening, the growth of the maximum cycle pressure, and the combustion efficiency increase were obtained.