The objective of this paper is to investigate how the combustion chamber design will influence combustion parameters and emissions in a natural gas SI engine.Ten different geometries were tried on a converted Volvo TD102 engine. For the different combustion chambers emissions and the pressure in the cylinder have been measured. The pressure in the cylinder was then used in a one-zone heat-release model to get different combustion parameters. The engine was operated unthrottled at 1200 rpm with different values of air/fuel ratio and EGR. The air/fuel ratio was varied from stoichiometric to lean limit. EGR values from 0 to 30% at stoichiometric air/fuel ratio were used.The results show a remarkably large difference in the rate of combustion between the chambers. The cycle-to-cycle variations are fairly independent of combustion chamber design as long as there is some squish area and the air and the natural gas are well mixed.Geometries that give the fastest combustion give the highest NOx values at λ=1.2, but at λ>1.5, which is normally designated lean-burn, the differences are smaller. The lowest NOx values for lean burn were obtained with the geometries that gives fast combustion.The HC emissions display some correlation between high combustion rate and low levels of HC emissions, but combustion chambers with dead zones and large total combustion chamber areas give higher HC contents than the combustion rate alone would indicate.Indicated efficiency is reduced for combustion chambers with a large total combustion chamber surface area and thus large heat losses. High levels of turbulence also tend to reduce the efficiency for the same reason.