I made one up a number of years ago just for something else to play with. It held about a gallon of water. Installed it in my old '89 Eliminator and it was actually a noticeable bump in millage! I don't remember off hand, but the truck was a 4.0, manual, 3.07s on 33s. All depends on the amount of power you put to it, how many plates are submerged in the water, how close they are, so on, so on. I found that I had the best results when I went through and cleaned/replaced all the sensors.
There's a ton of information online at most of it looks like smoke and mirrors. Do some real research on the differences on what might work and the magical "do-it-yourself and save millions" kits.