check out this post... viewtopic.php?f=6&t=3199&start=0&st=0&sk=t&sd=a
I don't think you can completely run on just hydrogen without some major modifications... When using gasoline you're still feeding the correct ratio of fuel/air to the engine - and adding hydrogen in the air intake (similar to adding nitrous oxide to gain HP) With fuel when you step on the throttle you feed more fuel to the engine... with HHO isn't there just a generator feeding the same amount of hydrogen to the engine at idle and at higher RPM's?
I'm no expert on HHO but that's one major point you'd need to address.
Are you seeing significant mileage increase? Smitty on here tried it, but I don't think he seen very good results and he gave up on it. Made me very skeptical of HHO add ons....
And now that gas is less than $2 a gallon (diesel around $3) I haven't heard much buzz on it.