I though I’d add my experience with using hho in a Subaru, my test car was my 95 Impreza 1.8. the cell I built was better than some of the other ones you see people using and claiming great mpg gains it wasn’t some glass jar with wires and baking soda in it. I built a 4 cell unit that was wired in series so each cell ran at about 3.5 volts, splitting the 13.5 volts for the car 4 ways, this is more efficient because it’s the amps the do the work not the volts so keeping that volts low it best higher voltage just overheats the cell, and for my catalyst I used potassium hydroxide. The cell never overheated, ran at under 30 amps, and made about 2 liters of gas per minute, I also lean out the engine to, I used a rather primitive method I built a simple device that added voltage parallel with the O2 sensor to trick the ecu to think the engine was running rich so it world lean it out, it worked well enough that I could run it so lean the engine would almost stall at idle. But I never did get any change in my mpgs.
I’m not saying this to tell you to quit I hope you have success, it has been proven that adding hydrogen to an engine will make it run more efficiently and have a more complete burn of the fuel, even at a 14.7:1 fuel ratio not all the fuel gets burn so that equals wasted energy. But I think you will need to add more than something like 2 liters per minute, maybe 10 LPM would do it but it will be hard to produce that much efficiently.