What use are HHO plans that you don't understand or leave too much to the imagination? Maybe you could smack the dog with them, when the pooch gets into the trash, but other than that... not much use at all. The best plans are ones that not only work and are efficient, but they give you detailed instructions. The plans I'm talking about will have you scooting down the highway, saving fuel, in hours. Here are some things to look for.
Details details detail...
Look for how many details the vendor gives in the instructions. The advertisement manual is the vendors chance to put their best foot forward, and if they are jazzing about the fluff, and not showing details about the product... well, you might just buy some fluff. Look for pictures of the finished product, and see if there are detailed illustrations. Make sure the illustrations are in color. A decent set of plans should cut off with the ignition switch, and your going to want to know wire colors. Why would you want a cutoff with the ignition? You don't want to make HHO while your engine isn't using it. Freebie plans usually leave this detail out and are in black and white. Look for picture illustrated details, or you might be purchasing rehashed plans that have little to no use.
If You Can't Find Support To Build These HHO Plans... Walk Away
Anyone who advertises plans and is worth their weight in salt, will offer support. If you can't find support, welcome to the abyss of tinkering. You'll be tinkering and spending like crazy. You want to get down the road spending less fast. You don't want to waste time tinkering or wasting materials. Your return on investment will be within a few fill ups with good support. Unless of course, you like disappearing for months, to finally emerge smelling like burnt hair. These plans do work, but you can decrease your learning curve with good support. Look for support and you will get a faster return on investment.
The More The Merrier
The site or company for the plans should be littered with testimonials. When people use this technology correctly they can't wait to tell about it. Personally I like sites that offer plans over assembled fuel cells, because once you know how to make them, they really are simple to make and install. This also increases your return on investment quicker. See if the plans offer forums or groups of people who also use this technology. Every engine is different and you will increase your chances of finding a tweaked fuel cell for your specific engine. If you can't find your specific engine then fall back on the support I already mentioned.
Find plans that have detailed color pictures, offer support and have a group that you can share with. If you follow those suggestions you won't get ripped off, and you'll be saving miles per gallon faster. Above all else use this technology have fun with it, and pioneer a less dependent nation.