In this paper, retrofitting of a hydroxy (HHO) cell which is coupled to a gasoline engine is performed by investigating the effects of electrodes’ shape and distances and the impacts of improvement technologies ultrasonic waves, DC pulsed power with various frequencies and magnetic field on the cell’s bubble pumping efficiency and rate of HHO gas production. Subsequently, an energy analysis was performed to provide a comparison between different improvement technologies. As a result, by applying DC pulsed power with 300 kHz frequency to the HHO cell, highest HHO gas production rate, energy efficiency and lowest energy intensity is achieved among other technologies, however, it will allocate lowest surface intensity which leads to requiring more electrode surface area. In addition, employment of ultrasonic waves improves HHO gas production rate from approximately 6.4% to 52.4%, and it is possessed the highest surface intensity which result in requiring lowest electrode surface among other methods. Finally, the effects of superior improvement methods on the overall cost and weight of an HHO generator and its impacts on engine parameters are studied.