in working for the company i do right now, we did a lot of test on HHO systems as part of our other hydrogen fuel research and found that the HHO kits were completely garbage. on a smog dyno tests show that the engine runs lean, with added computer diagnostics, we also see that on computer controlled cars, that the engine actually retards timing to avoid pings and begins consuming more fuel. on actual hydrogen running motors, we have to jet them much differently than a petrol fuel motor. i cant get into too many details for now NDAs and such but we have a 69 chevelle running 500 HP dynoed... fuel efficiency isn't neccisary.... we burn about 5 pounds of hydrogen per 20 miles. car has a 100lb hydrogen tank, which cost about $23 to fill at airgas