[David_J]

Here is a quote on the subject from JPL..."The 2nd law of thermodynamics is a likely source of those doubts. Meaning ...the law -would lead you to believe that it will certainly take more power to produce this hydrogen than can be regained by burning it in the engine. i.e. the resulting energy balance should be negative. If the aim is to create hydrogen by electrolysis to be burned as a fuel, the concept is ridiculous."

So, it comes down to does the addition of H2 in the low volumes that could be created by electrolysis (in comparison to the liters of air that are flowing through the engine each cycle) cause a significant increase in combustion efficiency. I would suggest that a tail pipe smog test with and without would be a good starting point. If hydrocarbon emissions don't decrease at idle, it is unlikely that they would at higher rpm (lower H2 to air mixture).