Partial conversion of the large inventory of diesel engines to natural gas (NG) spark-ignition (SI) will reduce U.S. dependence on imported petroleum and enhance national energy security. This paper describes the methodology used to retrofit such an engine as well as the experimental setup used to investigate and optimize the conversion, including engine modifications, coupled dynamometer, engine control, and data acquisition system. Low-pressure gas injectors placed upstream of the intake valve produced a homogeneous combustible mixture inside the cylinder. The final setup was verified via experiments that changed the equivalence ratio from 0.7 to 1.0 at 900 rpm, using methane as a natural gas surrogate. The results showed that despite the higher compression ratio (which increased in-cylinder pressure and temperature at spark timing compared to conventional SI engines), a high-energy spark plug was necessary to produce robust and repeatable ignition. In addition, the moderate compression ratio of the converted engine (13.3) resulted in knock-free operation at all equivalence ratios. Finally, the reliable and stable operation at the investigated conditions (COVIMEP < 1.5%) and low rate of pressure rise (< 3 bar/deg CA) support this solution for converting diesel engines to NG SI operation, at least for the conditions investigated here. The trend of engine-out emissions agreed well with existing studies, which also validated the design of the test cell for optimizing engine efficiency and sampling emissions.