you will need an EFIE and sometimes a map/ maf sensor enhancer, here is a link for some affordable hho components, they have this device called an ECU Genie that is supposed to control all sensors that work air/fuel and timing like map/maf,the engine coolent temp sensor, and theintakeairtemp sensor,it has been said that if one could retard the timing 1 or 2 degrees it would make a huge difference in millage,I dont know for sure but I bought one from them and installed it on my car, it seems to be working great, I know how far i can go on agallon of gasoline and I have went past that I will have a better millage percentage next week when my tank is empty and need to fill back up.
http://www.alternativeenergy2day.com