I have used four different type HHO generators over the past three years...

   Some of the best advice  we have found is:

  1)  Use supply tanks with plastic welded connectors.... 

  2)  Usually a half gallon size is sufficient for most generators!

  3) Have used both KOH & NaOH ; the later seems to be least destructive! 

  4) Have used a couple different chopper  {PWM} and the freq is not so critical...

  5) One of the best improvements was to add an electrical (small) pump, so that it pushes the liquid through the Cell...

  6) Use of a foam trap from the supply tank.. This a 2 x 3 block of foam placed within a same sized Ploy - tank...

  7) Connect the Hydroxy feed tube into the filter box before the air filter!  This stops most back fire problems; plus if there is some form of liquid from the cell - trap, it will tend to drop into the intake air filter chamber before getting pulled into the engine intake manifold!

   8) Then we used a MAP / MAF controller to help lean out the engine..

   9) Plus we installed a Volo Control Chip...

   All this seems to be the best for most newer engines....