I am in electronics and thus have access to variable power supplies, etc for testing my homemade generators. I wanted to debunk a myth (or two) that seems prevalant with HHO junkies.

I used plastic bolts, washers and nuts to bolt two strips of SS together with 1/8th inch spacing between the two. I stuck theses two strips into a glass containing a KOH / water solution and connected a variable DC power supply to the strips.

With about one half of one volt applied, I could see bubbles of HHO being generated. Not many, but there was HHO being produced.
MYTH - It takes over 1.24 or 1.39 or ?.?? volts to produce HHO - BUSTED.

I began increasing the voltage. As I did, production increased. I did not measure production, I just watched through the glass that contained my test generator. With one volt applied, I had some production. With two volts, more production. With 4 volts, even more. With 6 volts, even more. With 12 volts, much more.
MYTH - After 1.29 volts or 2 volts or 2.5 volts or ?.?? volts, production does not increase - BUSTED.

Now, there is a magic point of tradeoff somewhere. 12 volts connected to only 2 plates wired +- generated a LOT of bubbles. No doubt why so many post on this forum wanting to figure out a way to do a cell that is wired +-+-+- without heat problems. It simply creates a lot more bubbles, than using neutral plates. But adding neutral plates decreases production. I will say it again. ADDING NEUTRAL PLATES DECREASES PRODUCTION. Please, lay down your ax and finish reading my post before you chop my head off for that statement.

The magic point of tradeoff is that you must use some neutral plates. 12 volts applied in a +- manner on two plates generates lots of bubbles of HHO, but it also generates way too much heat. The neutral plates reduce the output of the cell, but also reduce the heat generated in the cell. That is why they are added.

I feel that the number of neutral plates you need is contingent upon your particular cell design, it's location in your car, it's container... If it is in front of the radiator, where cool air can get to it, you may get by with less neutral plates because your cell can lose heat to the ambient air. If it is between your exhaust pipes, with no cool air around, you are going to have more heat problems. So how many neutral plates? Why do some recommend 4, some 5, some 6... some 100? Because it depends on a lot of factors. But realize this, less plates will generate more gas (and more heat.)

Opposites attract. Electrons have a negative charge. If you have a 12 volt battery, with two plates and a molecule of water between those plates, the negatively charged electrons will be attracted toward the positive plate. But the positively charged protons in the atoms of H and O will be attracted toward the negative plate. This causes them to split. and you just generated HHO. Because you split the molecule. Doesn't it make sense that 12 volts will apply more force to split the molecules than 2 volts would, therefore 12 volts will be more effective?

I don't know where these magic numbers of 1.24 volts, 2 volts... came from. I have read many places that after that certain voltage, production does not increase. If I had a video camera, I would post a video to disprove this myth. If you don't believe it, try it yourself. If you do not have a variable power supply, connect 12 volts to 2 plates, as I did and view the production, then add another two plates into another glass sitting separate from your first test generator, and wire the two in series so that each gets approximately 6 volts. Then view the production of your cell and contrast the amount of bubbles generated at 12 volts to the amount generated at 6 volts. 12 does more, way more. But generates too much heat. That is why we add neutral plates. They reduce the voltage between plates, thus reducing heat but also reducing output.

Now, I stuck my neck out, go for my head.