With environmental policies becoming ever more stringent, there is heightened interest in natural gas (NG) as a viable fuel for medium to heavy duty engines. Typically, the industry has seen minor changes to the base engine when converting to run on NG, which, in turn historically provides degraded performance. In utilizing the positive properties of NG, Westport Fuel Systems has developed the High Efficiency Spark Ignition (HESI) combustion technology that has been shown to significantly improve performance. The HESI technology leverages a proven combustion system that is capable of generating a knock resistant charge motion while cooling the flame face. In conjunction with high boost for driving high pressure exhaust gas recirculation (EGR), this technology demonstrates the possibility for downsizing strategies while maintaining performance. Throughout HESI development, industry standard design, analysis, manufacturing, and testing processes have been applied on both medium and heavy duty engine architectures. In carrying over the bottom end of the donor engine, this technology is extremely adaptable for packaging and vehicle interfacing. Measured performance is compared with numerical predictions, meeting the pretest targets and showing a robust combustion process. When run with stoichiometric combustion, measured emission levels are in line with the California Air Resources Board optional low NOx standard of 0.020 g/hp-hr (0.027 g/kW-hr) with a passive three-way catalyst.