High EGR rates combined with turbocharging has been identified as a promising way to increase the maximum load and efficiency of heavy duty spark ignition Natural Gas engines. With stoichiometric conditions a three way catalyst can be used which means that regulated emissions can be kept at very low levels. Most of the heavy duty NG engines are diesel engines which are converted for SI operation. These engine's components are in common with the dieselengine which put limits on higher exhaust gas temperature. The engines have lower maximum load level than the corresponding diesel engines. This is mainly due to the lower density of NG, lower compression ratio and limits on knocking and also high exhaust gas temperature. They also have lower efficiency due to mainly the lower compression ratio and the throttling losses. However performing some modifications on the engines such as redesigning the engine's piston in a way to achieve higher compression ratio and more turbulence, modifying EGR system and optimizing the turbocharging system will result in improving the overall efficiency and the maximum load limit of the engine. This paper presents the detailed information about the engine modifications which result in improving the overall efficiency and extending the maximum load of the engine. Control-related problems associated with the higher loads are also identified and appropriate solutions are suggested.