Hi Octavian,

This is a two-part answer. Firstly, We are talking about HHO boosters, which are utilized to boost gasoline engine performance and increase miles per gallon. Boosters are efficent at doing this, but normally do not provide enough HHO to operate a vehicle engine at highway speeds without gasoline. It is said that Stan Meyer was able to run a small Volkswagen engine using 4 of his large water fuel cell units (WFC's), so that might give you some idea of how much HHO is really needed, as a bare minimum, to fuel a vehicle engine without using any gasoline. Stan realized that this was probably not the best way to get the job done, so later developed a water splitting injection system for his dune buggy. With such a device, water is typically pre-heated to a steam, or near steam state, and a very small amount (resulting from one or two water droplets) is injected to a cylinder and vaporized by a very high voltage plasma arc, which converts and combusts the vapor to a HHO burn. As you can imagine, that method would be far more efficient than attempting to utilize several booster type electrolyzers. S1r9a9m9 has used water fuel only, with a high voltage plasma arc, to run his 1974 El Camino V8 engine for several years. For more information about that method, see:
Water Car Technology From 1950 Resurrected

But that isn't the technology we are talking about in this thread, so I think that the second part answer, shown below, is more applicable:

We do not directly mix anything, and you definitely don't want to introduce the HHO to your gasoline line. Gasoline is fed separately, either to the intake manifold through a carburetor, or by fuel injection. An internal combustion engine also requires a large amount of air intake, in the proportion of a 14.7 to 1 (air to fuel) ratio. We simply introduce our HHO gas into the intake airstream, close to - and before - the throttle valve. This arrangement works fine on a carbureted engine, and also works fine on an electronically controlled, fuel injection engine, as long as you have "tricked out" the oxygen sensor. If you add additional boosters, or constuct a single Bob Boyce type electrolyzer, you will require additional controls, and will also need to retard the spark timing somewhat. In an engine running solely on Hydrogen as the fuel source, spark timing must occur after top dead center (TDC) of the compression stroke, not before TDC as when using gasoline as fuel.

I hope that helps,

Rickoff