Assuming it works (an assumption of which I've seen no meaningful proof despite seeing plenty of activity), there should be no need to add fuel to use that extra oxygen since the hydrogen comes in along with the extra oxygen...and we already know that's at the right ratio since it was water when it started.

Thanks a lot everyone for enthusiastic responses. Any discussion helps not only to crystalize a point, but raise more questions.
On HHO. Beef and HolyCow.
Alternator argument have been dodged long ago by Stan Mayers and by many of those who have been working HHO systems using their alternators as power supply in straight forward electrolysis, not even Stan's circuits. Somehow, physicists can't explain why 2nd law of thermodynamics don't work here. Well, there is Tom Bearden, go argue with him, if you have time. Recent research by prof. Bolotov suggests viability of Stan 'electrical bombardment' method if I can call it that.
2) I brought HHO just as an example of additional power from other fuels/mods. As Holy Cow quotes me "Even if it doesn't see it, why keep injecting extra fuel if there is already increased power from 'extra' HHO, this is asking for leaning the mixture, no?"
I want to remind us that this is not the Racing forum, but a Fuel Efficiency one, so when I mention extra power from outside, I assume that to keep the same level of output we now need less original input, AF mixture that is. This is why I am concerned with leaning the mixture. Keep in mind the mixture is not actually lean, since it is supplied with HHO, for example, the ECU might figure it is 'lean', but the combustion may be as stoich as before.
3)Remapping. Thanks for cautioning me, BEEF, I appreciate it, I know of dangers involved in lean burn and all the blown engines out there, but this is why I am on this forum, to find out how it is done right, not that it is impossible. Also, not least a factor - even though not born in US, I am very much an American, so don't tell me there is something impossible to do.
Mr. BEEF, isn't remapping nowdays simplified enough so we can change the x,y tables, or does it still require coding? I've seen FORD and GM software that uses simple spreadsheets to change the number parameters.
Thanks, everyone.
All the best.

ok, after retyping and erasing and retyping and erasing several times. let me try and just write simple things.

-Your country of origin doesn't matter. The majority of the people I work with on a daily basis are not US citizens. Many of them are from China. We do have quite a few from India as well as the Middle east (Pakistan, Israel, and Palestine). Most of which hold at least a Master's Degree and a hand full of PhDs around.

-You say you have seen the software...Have you ever priced it?

-It is much more complicated than a simple map as well. There are several maps depending on conditions and you have to know what the engine can handle at a given condition. Lean burn only kicks in during certain conditions where it is sustainable. You can change a parameter here and there and see if it works but if you are wrong, BOOM!!!!

-Also, any fuel map/curve is just X and Y coordinates... it always has been. They encrypt the hell out of it so that regular people can't just do it themselves. That is the reason most people take it to the dealership if they have an ECU problem... and the reason that people that have figured out how to tune them charge an arm and a leg for a retune. (they are usually very specific towards performance and certain upgrades so the engine will react better to those upgrades)

-HHO is a whole different argument as I would think if you could increase efficiency by 40% or better, you would have one hell of a business case and be making millions over night. I would think the government which is cracking down on car companies about EPA MPG numbers would jump all over this sort of technology. I am still waiting for someone to "silence the skeptics" as I am a very science oriented person and thus I am waiting eagarly for the evidence.

-furthermore, I am not trying to discourage you from experimenting. Be my guest. Just realize what work has already been done and what information is out there... real information. I don't have magnets on my fuel lines or an obstruction in my intake because I know they don't work. There are marketing teams devoted to changing that thought though evidence doesn't change. There are products all over the internet that just flat out don't work. There are people that get rich off of selling products that don't work and never have.

Good luck to you in all your experiments. Please do be careful and keep a fire extinguisher handy.

Thanks BEEF!
I have not priced the software, not yet, not before I figure what needs to be done and not before I have enough recommendation and advice. Maybe people you work with may suggest best? (and cheapest or free to download. do you have info on ECUEDIT and WINOLS?)
HHO. The problems with HHO are not the production of enough gas. HHO people agree half a litre of gas a minute per liter of engine displacement is enough to deal without ANY fuel. the question that so far nobody answered me is what is the combustion temp with HHO, too hot burns rings and valves and HHO is a very peculiar gas concerning temps.
in any case, thanks. I still need advice on software and tuning. I will look on other forums, more specific to tuning.
You mention that you work with many people. What exactly do you do with engines?

There is no need to lean the mixture. Since the driver is not racing, any extra power from HHO will be compensated by the driver reducing accelerator input.

Reduced accelerator input = reduced air intake = reduced fuel injected.

I do nothing with engines but realize that people of a certain mindset tend to gravitate towards certain hobbies.

I actually work for a communications chip design firm. We have the largest GaAs fabrication facility in the US (and maybe the world).

Many of my colleagues are on the power side of cars. I have a friend with a supercharged mustang (that he supercharged), another with a 1980 corvette, another with a 4cylinder turbo mustang, another with a race ready corvette that has more money in the suspension than my car cost new.

A few have contemplated HHO themselves but always come up with a negative equation and thus drop the idea. There are just too many losses. I have lived on a gas saving forum for 5 years and have never seen anyone positively gain from it. There was one guy running an old carbed vehicle that claimed some gains but there aren't too many carbed engines still around.

Engineering types are skeptic by nature, it's what we do.

I just can't see an engine running purely on water as you so state. the idea of an engine running just on electrolycized water would be a great one but I have no idea how you would actually achieve it. Has anyone came up with a current number for that sort of output? You could actually cool the engine with meth injection or water injection. that is a trick that the turbo guys use.

You may want to broaden you research on HHO to sites that are less favorable towards it. There are too many sites that talk about the good it does but focus little on the fact that it doesn't really work. I am still waiting to see a gas log of unbelievably high numbers running HHO. I have seen a few claiming better numbers but their start numbers are ususally well below EPA ratings for the given car and the end numbers usually aren't as good as a comperable gassaver member with the same car. around here, beating EPA is almost a given.

Ecu
 

@BEEF.

I suggest checking out Stan Mayers and his way of turning water into HHO.
Arabs offered him a billion dollars to stay quiet. He is dead now. Maybe it is the conspiracy theories...I don't know, but the method resembles research of russian professor Bolotov. Bolotov, as a teenager noticed that radio lamps give out more heat than they use power. Among million other things this man invented, he bombarded electrodes with huge amounts of current.
IN any case, check out Stan Mayers and his story. Wikipedia article, I am not sure you can trust.
here is youtube vid .com/watch?v=S44eRCr-O7o
but there a lot of stuff to read.

HOLY COW.
There is no need to lean the mixture. Since the driver is not racing, any extra power from HHO will be compensated by the driver reducing accelerator input.

Reduced accelerator input = reduced air intake = reduced fuel injected.

Can you prove that computer not add extra fuel?

extrordinary claims require extrordinary evidence -carl sagan-

It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong. -Richard Feynman-

Without repeatability, there is no Theory. -Me-

In God we trust...All others bring data. -My former Engineering Manager-

The only method of using hydrogen to improve fuel economy I saw that might work was a slide show on a plasma generator that cracked the hydrogen free from a fraction of the vehicle's gasoline. It might work because it should take less energy than freeing the hydrogen from water and it might side step some of the conversion losses of electrolysis.

Another method is just use a tank of hydrogen to see if there is any synergistic reaction going on to improve the engine's efficiency. I haven't seen a post from a HHO proponent that has done so.

Running a car on hydrogen alone is going to require a fair amount of fuel. The Honda Clarity is certified 240miles of range on 4.1kg of hydrogen by the EPA. That's a fuel cell. For an ICE there is the hydrogen RX8. It can go 62miles on the 2.4kg it carriers, and that might be by the more optomistic Japanese test.