Well in V4 there would be NO concern or problem at all IF the valves of the unused cylinders remain closed.

In the one system I am looking at there is:
1. a manual, safety, ON/OFF switch (which has to be ON for the system to work at all).
AND
2. is triggered off the ignition and only works with the ignition ON.

I suppopse in the Hybrid it would be smart to trigger this circuit off the "Autostop" circuit rather than the keyed ignition....therfore the HHO generator would shut off when the engine did and resume when the engine restarted.
Any idea on how to identify and tap that circuit?

Another concern is cold weather operation. The water/electrolyte solution is said to be good down to -15F. I'll have to question the manufacturer more closely on this. Possibility of using antifreeze etc.

There are also larger throttle bodies and better intakes for the 6.0 L available. Not sure if the cost/benefit is there MPG wise....I'm sure it would be performance wise.....but then you would probably need to add headers and a good CAT back system

 

Be careful on antifreeze and electricity. Some is rather conductive and will just steal power, make heat, and little HHO.

 

Update: Manufacturer recommends adding isopropyl alcohol in cold climates.