Okay i'm at home wondering about this, i found an articale in wikipedia:
http://en.wikipedia.org/wiki/Hydrogen_Fuel_Injection
There might be merit to HHO, but not as the way they advertise... 50% is probably based on higher concentrations of Hydrogen, than what HHO generators can produce. But that small about might still give an improvement.
This part got my attention:
Automotive fuel enhancement systems work by injecting a hydrogen rich mixture, or pure hydrogen into the intake manifold of an internal combustion engine combined with air/fuel ratio, and timing modification. A small amount of hydrogen added to the intake air-fuel charge permits the engine to operate with leaner air-to-fuel mixture than otherwise possible.[3] As the air/fuel mix approaches 30:1 the temperature of combustion substantially decreases effectively mitigating NOx production.[3]
Under idle conditions power is only required for extraneous components other than the drive train, therefore fuel consumption can be minimized. A 50% reduction in gasoline consumption, at idle, was reported by numerically analyzing "the effect of hydrogen enriched gasoline on the performance, emissions and fuel consumption of a small spark-ignition engine".[4]
Under most loads near stoichiometric air/fuel mixtures are still required for normal acceleration, although under idle conditions, reduced loads, and moderate acceleration Hydrogen "addition" in combination with lean burn engine conditions "can guarantee a regular running" of the engine "with many advantages in terms of emissions levels and fuel consumption".[5]
Increases "in engine efficiency are more dominant than the energy loss incurred in generating hydrogen".[3] This is specifically with regard to use of a hydrogen reformer. Overall computational analysis "has marked the possibility of operating with high air overabundance (lean or ultra-lean mixtures) without a" substantial "performance decrease, but with great advantages on pollution emissions and fuel consumption".[4]
Overall comparing "the properties of hydrogen and gasoline, it is possible to underline the possibilities, for hydrogen fueled engines, of operating with very lean (or ultra-lean) mixtures,[6] obtaining interesting fuel economy and emissions reductions".[5] The "concept of hydrogen enriched gasoline, as a fuel for internal combustion engines, has a greater interest than pure hydrogen powered engines because it involves fewer modifications to the engines and their fueling systems".[5]
[edit] Engine Control
ECU or carburetor modifications are required to establish lean or ultra lean burn engine conditions; where the hydrogen permits leaner conditions than possible with solely hydrocarbon fuel.[7][3]