There are some free HHO plans floating around but there are a few crucial things they miss including the cheaper plans. On some engines you may have to deal with an oxygen sensor. This sensor is there to help create an ideal air fuel mixture. If you do not add this one item then you will never really see increased gas mileage on some engines. In fact what you may see is a decrease in mileage and a rough running engine. Many cheap or free HHO plans miss this item.