For the fuel injection vehicles several modifications have to be made to the sensors linked to the car’s computer, since it controlls the amount of petrol flowing into the injectors. In this case, the sensors coming from the exhaust pipe have to be fooled, so the computer doesn’t compensate the lack of gasses produced by petrol (because of burning hydrogen) with more petrol.
You must do this tune-up very carefully, no guarantees are made if something bad happens. You do it at your own risk. Anyway, the system is provided with a “bubbler”, a bottle or jar with water in it, in which the primary produced hydrogen enters, for separating it from the explosion place.
So the bubbles of hydrogen gas pass through that water before going to burn. The system is also provided with a safe cap, for protecting the installation from high pressures and explosions.
If you want more details, you can visit http://www.panaceauniversity.org/D6.pdf.