The details are over on Popular Mechanics, and you should head over there to read the whole thing, but the short version is that PM senior automotive editor Mike Allen has done his homework, and none of the HHO systems work as advertised. None. We'll crib just one piece of math from Allen's article: onboard hydrogen systems change much less than one percent of the air needed in the combustion process to hydrogen. And that means that it doesn't boost your MPG by 300 percent. PM and NBC spent over $1,800 to prove that HHO systems don't work. You can just read the article.
[Source: Popular Mechanics]
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