The optimization of SI engines fueled with syngas requires a deeper understanding of the related thermal mechanisms in engine-like conditions. Therefore, to improve current knowledge and provide reference data for modeling and simulation of internal combustion engines, experimental and numerical investigations were carried out on a port fuel injection (PFI) spark-ignition (SI) optical research engine fueled with pure syngas (CO/H2). Methane and methane-H2 blends was considered for comparison. The analysis of different air dilution, spark timing and fuel dilution was performed. The engine was operated at fixed rotational speed (900 RPM) at wide open throttle (WOT) as representative condition of energy production applications. In-cylinder pressure and related thermodynamic parameters were analyzed as indicators of combustion behavior. Exhaust emission was measured in order to compare the different operative conditions. 2D cycle resolved digital visualization was performed to follow the flame front propagation. Custom image processing was applied to estimate the flame speed and others morphology parameters. A quasi-dimensional model was validate to simulate the first phase of the combustion process with experimental thermodynamic parameters (In-cylinder pressure and mass fraction burned) and then a comparison with the optical results was performed. The hydrogen addition improves the flame propagation speed, reduce flame distortion and center displacement for syngas and methane blends. In addition, was seen a decrease in terms of cycle-by-cycle variation and extension of the flammability limit for both fuels. Also, the advance of spark timing shows an increase in terms of flame propagation speed and flame distortion. The optimum spark timing (MBT) was found in the expansion stroke for pure syngas in stoichiometric and intermediate air-fuel ratio. Moreover, spark timing re-calibration is required in order to fully take advantage of fuel properties such as higher laminar flame speed and increased stability. Lastly, air and fuel dilution were found to be a good strategy for decrease NOx emissions without losing efficiency and good flame propagation properties (speed, distortion and flame center displacement).