All calibration and tuning parameters in a factory ECU are fixed by the manufacturer to suit the engine in its standard form and it is often impossible to adjust this for engine modifications. However, if the factory ECU is replaced by a programmable aftermarket system, adjustments can be made. An engine tuner can connect the ECU to a PC to tune the engine for optimum performance at each load and speed condition. This requires dedicated tuning software and the sophistication of this software is one of the key differentiating factors from one brand to another.
These days, aftermarket ECUs not only control injection and ignition timing, but also manage more complex functions like boost control, idle speed control, intercooler spray bars and traction control. Many ECUs provide extra auxiliary outputs that can be used to activate devices such as pumps, fans and even servo motors.