If U run only on HHO, U must have stainless steel exhaust pipes and stainless steel valves - this are vulnerable to rust (burning HHO generates water) or apply a ceramic coating treatment to the engine.
Also, keep a close check on the engine temperature, because the HHO burn with higher temperature than fossil fuel.
Use a high quality engine oil.
In Otto engines-When U run only with HHO gas, U must adjust the timing of the spark so that it occurs after the top dead center, because the HHO burned faster than fossil fuels.
See U on forum