I can tell you right now that HHO systems are nonsense. What they do is inject extra hydrogen and oxygen into your fuel system, effectively leaning out your car's ICE motor to 30:1 ratios. That is absolutely dangerous as Air to Fuel ratios are critical towards your ICE motors engine running efficiently within the set parameters that are controlled by your cars ECU (the computer that monitors the fuel injectors, spark plugs, ignition timing etc). Typical AF ratios are anywhere from 11:1 to 15:1. So doubling this creates a lot of problems, allow me to briefly explain.
Basically, you throw one of these systems on your car it might work for awhile but modern day cars with ECU's learn on the fly, meaning, they adapt to changing conditions in the motors. Sometimes sensors fail, bad gasoline gets into the fuel systems, people put in octane boosters to fix engine knock and all the other silly gimmick products that are out there. There are no magic solutions to fixing problems in cars with ICE motors. If you car is having ignition issues, change the plugs, make sure they are gapped properly, make sure to wires are corroded etc. If your car is sputtering, check the fuel filter, check the air filter, do all the common sense routine maintenance procedures that the manufacturer of the vehicle pleads with you to do. They are not trying to sell you extra parts or service through dealerships, these are simply the way cars are made today.
Getting back to "leaning" out your fuel system, the Air Fuel ratio is a vital balance between making ideal horsepower and a clean, complete burn process of the air fuel mixture. When you add in more oxygen to a fuel system and DO NOT add in more fuel, the combustion process in the cylinder head, top of the piston, the valves and tip of the spark plug all experience more HEAT because there is more oxygen being consumed than normal.
Heat is what kills motors, along with poor lubrication. So putting some $100 contraption on your car to lean it out to 25 or 30:1 is just asking for trouble down the road. If you want to start weakening or even melting valves, having the broken pieces of the valve fall into the combustion chamber and repeated get impacted into the top of the piston, by all means, roll the dice and put an HHO system on your car. What you think you're saving in gasoline or doing for the environment will only cost your more later when you need an entire engine rebuilt or rebuilt your cylinder head.
This is one of those cases where you have to ask yourself, if these things REALLY worked, why doesn't everybody do it? And why do the guys who claim to have the technology want $200 just for the information? Well, if you buy one of these "information packages" they also then hit you up mid way through the brochure that SURPRISE, we also sell everything you need to modify your car with this HHO system! Friend of mine stole a $200 info pack off a torrent site and this is what all I have written here is based off of.
Popular Mechanics recently announced they are going to debunk these systems once and for all because of all the damage they are doing to people looking for cheap fixes. Hopefully they get the article finished and published sometime in the next few issues.