It’s very simple. You don’t change your engine or ECU (car computer). A quart-size (95O cc) container, usually made of glass but can also be plastic or metal, is placed somewhere under the hood. You fill it with distilled water and low-cost household electrolyte (causes electrical current). The device gets vacuum and 12 Volts from the engine, and produces HHO gas (Hydrogen+Oxygen) as shown in the video
