Natural gas high pressure direct injection (HPDI) engines represent a technology with the potential for lower engine-out emissions and reduced fuel costs over a diesel engine. This combustion process uses a direct injection of natural gas, into the combustion chamber of a high compression ratio engine, to maintain diesel engine performance. As natural gas will not auto-ignite at typical engine conditions, a small quantity of diesel pilot fuel is used to initiate the combustion event.One potential technique to improve engine performance is the optimization of the intake and exhaust valve timings. To experimentally investigate these effects, tests were performed on a single cylinder engine based on Westport Innovation's 15L HD engine. The intake valve closing time was varied both before and after the standard closing (EIVC and LIVC). Early closing of the exhaust valve was also tested (EEVC). This work aimed to control in-cylinder residual content, equivalence ratio, and temperature to maximize performance and minimize emissions.The results showed that, due to pressure pulsations in the intake manifold and valve flow restrictions, LIVC was marginally effective at reducing charge mass. EIVC provided a larger reduction in charge mass under equivalent conditions. At loads below 50%, up to a 70% reduction in CH4 emissions is measured at fixed intake pressures. At high load (75%) a 19% reduction in NOx is measured due to reduced in-cylinder temperatures resulting from lower effective compression ratios. At 10% load, EEVC cams can simultaneously reduce NOx, CH4 and CO along with generating higher exhaust temperatures.