At best HHO would be a supplement to the motor fuel. There's no way you could make enough HHO on demand to run the engine in any driveable fashion.
When I was in high school, a kid did a lawnmower motor hooked up to a HHO setup. Yes, it ran on 100% HHO, but he had to have the HHO setup running for hours (on a 12V battery charger) to make enough HHO to run the lawnmower motor for 30 seconds.
__________________
|