I concur with Roland.
But, whether you use an EFIE or mod the ECU for maximum performance/economy, you need to be able to shut it off, or go back to factory stand setting when you are NOT injecting HHO. All the EFIEs I've used have an on/off switch making that easy. If you mod the chip, I'm not sure what you have to do. Volo claims their HHO chip can determine if HHO is being injected and then chooses the proper table to sent to the ECO.
1991 Plymouth Acclaim 3L V6.
1 dry cells with nineteen 6"x8" 316L ss plates, driven by constant current PWM set at 35 amps (13.3V at PWM). 28% KOH electrolyte. Total measured output 2.5 lpm. Mileage went from 18 to 26 mpg, all city driving (44% increase). EFIE set at .370 and I still need to play with ignition timing.