An apparatus includes an engine module, an in-cylinder content module, and an engine out NOx module. The engine module is structured to interpret engine in-cylinder data regarding an operating condition within a cylinder of an engine, wherein the engine in-cylinder data includes an engine torque, an engine speed, a rail pressure, and a start-of-injection. The in-cylinder content module is structured to interpret at least one additional in-cylinder data point regarding the operating condition within the cylinder of the engine. The engine out NOx module is structured to determine an engine out NOx amount responsive to the engine in-cylinder data and the at least one additional in-cylinder data point.