The guy's set up i saw didn't look much different to a jam jar! He had a pipe coming out of the lid and stuffed into the air intake pipe after the filter. Looked a bit scarey to be honest but he reckoned it worked. 
It doesn't take more than the size of a Jam Jar to produce the right amount of Hydrogen Gas to get the 20-60 miles extra that jikwan is talking about?
Dave inside the Jar did you see Plates or a coil? the coil type is made from stainless steel wire, the water is pure water and the additive can be baking soda, The actual Gas is called Browns Gas a little different from pure Hydrogen but non the less can still very dangerous if treated the wrong way.
Joeuk45 and I have the plans to make Browns Hydrogen Cells and if made correctly are no more dangerous than your Tank of petrol or Diesel.
Joe has already built his, Mine is still on the back burner (no pun intended
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jikwan and I have already had a good chat on the home production of Hydrogen cells and he is very knowlagable and If I remember correctly he built the 5 plate version and not the coil version, the info about the timing is also correct and with vehicles with computers to run them are a little more indepth and a few extra components are required.