I have been looking at videos and reading as much inteligent info as I can find.  That being said, if you have 1 cell producing 1 liter or so per minute, then 5 or 6 or even 10 would produce more.  Can your vehicle use as much HHO as you have units to produce or is there a limit caused by the amount of amp's needed?
I have built cells and have had good results but I want (like everyone) to get the max out of what is available.