Well, here's the basic question to answer first:

Why is this crap? Here's a discussion:

Where does that hydrogen and oxygen gas come from?

From electrolysis.

How do you perform electrolysis of water?

By using electricity.

How much electricity is required?

The electrical equivalent of the Gibbs Free Energy of the liquid. In this case, H20 creates 1 mol H2 and 0.5 mol O2 for each 231kJ energy at 1 atm.

Where does this electricity come from?

From the battery.

Where does the energy in the battery come from?

From the alternator.

Where does the energy in the alternator come from?

From the engine burning fuel.

How efficient is the alternator?

About 60% max. That means that for every 10 units of mechanical energy entered into the alternator, only 6 are converted to electricity.

The electricity which powers the electrolysis, right?

Yes.

So how much energy comes from burning the hydrogen and oxygen you get from electrolysis?

231kJ/mol at 1atm, the same as it took to create it the gas.

That's good. That means that the engine will make more power, right?

Yes, in addition to the 5400 kJ/mol that the gasoline provides.

Well, that's good right?

No. Because the engine is ideally 37% efficient.

What does that mean?

It means that for every 231kJ extra energy you put into the engine, you only get 85 kJ of mechanical energy back. And that's if you have a perfect engine.

85kJ!? Well, how much mechanical energy did it take to make the 231kJ energy from the water?

Well, remember that 60% efficient alternator? That means it took 369.6kJ of mechanical energy to make your engine put out 85kJ more power.

So what you're saying is that this water thing uses more electricity than it makes?

Yes. In an ideal engine, it would basically rob your engine of 80% of the power you put into it.

I don't understand what that means in terms of horsepower.

OK. Let's look at it this way. Let's say that you make a modest income and have bought a bright shiny sports car that puts out 200 horsepower.

Great.

Let's say that your alternator uses up 1hp to electrolyze water.

OK then how much do I get back from burning the hydrogen/oxygen gas?

Assuming perfect combustion, you would get 0.2hp back.

Screw this, I'm saving up my money for a $4000 turbo kit.