Systems and methods for automatic calibration of large industrial engines in applications where the quality of the fuel supply is unknown and/or variable over time, particularly engines that drive compressors on a natural gas well site. A combination of throttles and an oxygen sensor including a mass-flow-air throttle and a mass-flow-gas throttle to determine the mass flow of air and mass flow of gas. As a response to exhaust gas oxygen level readings, the mass flow measurements are used to determine real time air-fuel ratios. An algorithm uses the air-fuel ratios as input data, wherein a microcontroller adjusts the throttles to meet engine performance demands. Additionally, using the air-fuel ratio data and suggested engine OEM calibration specifications as block multiplier inputs, particular fuel properties, such as British Thermal Unit (BTU) content, can be accurately interpolated, thereby enabling automatic calibration of the engine.