a better explanation for the on again off again gains (and continuous loss of power) you get after "tuning" for HHO can be found by doing some research on water injection.

under the right condtions spraying a little water in the intake of a diesel will give you a milage boost, the HHO generators boil over and do just that, but its not controlled so they will reduce the life span of your motor, specialy if they bubble over enough to hydro lock you.