Abstract: A performance modification/tuning kit for an internal combustion engine of the type comprises a kit control unit. An electrical sensor is adapted to detect the opening and closing signals of a control line to one of the injectors of the engine and connected through a sensor line to the control unit. An input receives the signal from a pressure sensor of the common fuel rail of the engine and an output report a generated pressure signal to the engines ECU. A selectively changeable modification map, comprising modification values for a range of different engine speeds and fuel loads demanded by the ECU is used by the kit control unit to generate the generated pressure signal. That is generated in dependence upon the value in the modification map for the engine fuel load Q currently demanded by the ECU and the engine speed currently pertaining from time to time.