"Where is the hho best place input into the air duct of the engine?"
As with any HHO system and any ICE (Internal Combustion Engine) gasoline or diesel, the point at which the HHO gas should be inducted is BEFORE and injection system, Turbo or other modification, but as close to the carburettor or injectors as possible.
The engine creates the demand for 'air' and 'sucks' the HHO gas into the system. The savings part (economy) of the HHO gas is how the onboard computer sometimes called ECC, ECM or ECU (if any) controls the 'demand' for fuel or air from the vehicle's injection or carbonation system, i.e. rich or lean.
Performance is great and the savings are immediate!