Full load in engine performance refers to the maximum power or torque an engine can produce safely without causing damage. It is determined during engine testing using a dynamometer, which applies load through a water brake system. As the engine warms up and RPM increases, water is introduced to increase load until the engine reaches its limit. At full load, the engine operates under steady-state conditions, typically at wide open throttle for gasoline engines or full fuel rack position for diesel engines. Understanding full load is crucial for accurate engine performance testing and optimization.