To maximize payback for operators, it is important that natural gas engines for heavy-duty on-road applications minimize fuel consumption. To directly replace a diesel engine for a given vehicle mass and duty cycle, the natural gas engine also needs to match the diesel's power and torque characteristics. This paper reports the results of a development project to increase the torque and power of Westport's 15L 356 kW pilot-ignited, late cycle direct injection of natural gas engine by 10%, while matching or improving efficiency and maintaining emissions compliance. The strategies evaluated to achieve these objectives were to recover some of the exhaust energy with a power turbine, to increase the injector flow area to avoid excessively long combustion durations and to reduce the compression ratio to keep peak cylinder pressure below its maximum limit. The use of simplified aftertreatment, including a partial-flow DPF and smaller exhaust oxidation catalysts, was evaluated to take advantage of natural gas' lower emissions. Of these, reduced compression ratio, higher flow injectors, and a smaller oxidation catalyst were selected for inclusion in the final engine hardware. The modified engine was then recalibrated to achieve the higher-torque targets. Engine performance was demonstrated over the ESC 13-mode steady-state cycle. The target torque and power were achieved, with a maximum torque of 2570 Nm between 1200 and 1500 RPM and a peak power of 401 kW at 1490 RPM. Peak efficiency was 42%, with cycle average efficiency of 39.4%. Tailpipe emissions were not significantly increased compared to the US EPA 2010 emissions certified 356 kW engine over the same duty cycle.