Future diesel engine legislations are focused on further improvements in green-house gas emissions, such as carbon dioxide while additionally pushing for lower NOx emissions levels. These are being achieved with a combination of base-engine, fuel-injection system, air-system and after-treatment system improvements. In this paper, the effect of one injection system characteristics, namely injector flow-rate was investigated on engine performance and emissions using both numerical and experimental techniques. The phenomenon of increasing injector flow was first numerically investigated using commercial code Converge. Two approaches to increasing injector flow-rate were investigated. The first approach was by increasing the injector nozzle hole size while keeping the number of holes constant. The second approach was to change the number of the holes while keeping the injector nozzle size fixed. These simulations led to procurement of injectors to validate the simulation trends. Engine tests were performed with Navistar’s 12.4 L multi-cylinder heavy-duty diesel engine. The identified nozzle flow rates included a 66% increase from that of the baseline case. All the engine tests were performed at the typical cruising condition for this engine, at a series of injection timing and injection pressure values. It was observed that the crank angle for 50% of the integrated total calculated heat release (CA50) for the fuel burned was the most important factor that influenced the brake-thermal efficiency (BTE) and different injectors had similar BTE at constant CA50. With regards to emission, at higher nozzle flow rates, the combustion showed a slightly higher propensity for soot and increased levels of carbon monoxide.