6 mm spacing is on the large side, and will need fairly high concentration of electrolyte for the current to push through. 2-4 mm is normal. Smaller gap needs less concentration of electrolyte and 2mm is considered the minimum to allow hydrogen bubbles to build up and escape easily.

If you are only using one cell, it receives the full 12 volts available. Only about 2 - 2.5 volts is needed for electrolysis. 2 cells wired in series will halve the voltage but still flow 20 amps for double the production. You will need to use more catalyst in the electrolyte to bring the current back up.

With 4 of your cells running at 20 amps you would be producing 1 litre per minute which is a reasonable amount of hydroxy for a 3 litre diesel.

With 4 cells wired in series the voltage would come down to about 3 volts per cell which will reduce heating, I would guess your current single cell gets way to hot to handle.

For an electrolyte I would recommend either Sodium Hydroxide (NaOH) ( available easily as drain cleaner, but check the label for 100% NaOH ) or Potassium Hydroxide (KOH) which can be obtained from a soap makers. May need to sign a Hazmat Waiver to get hold of it though.

Use either of the above in steam distilled water to minimise foaming and residue. You would be amazed at the amount of crap that comes out of tap water when you subject it to electrolisis.

Any other points you want clarification on, feel free to ask here.

Rik