If you use the vacuum of the engine, then you're losing power. Even if it somehow doesn't reduce the engine's efficiency, that vacuum could be utilised driving a generator itself.
The HHO is still generated at a higher pressure because it's at the bottom of a column of fluid. Doesn't matter if it's HHO, air, water, mercury, helium, or whatever, it's still got density, so it still applies a pressure.
I thought that air could be allowed to fill the container and pipe while it's generating. Then as HHO appears, it 'bubbles' up the pipe through the air. But that means you have the even higher pressure of a column of air for the generator to work against.
And the bar has been raised. I found that industrial scale hydrogen electrolisis plants only have about 80% efficiency, although they do produce compressed gas, so that might account for some of it. Also, apparently the theoretical maximum efficiency is something like 94%. To get that you also need a 100% efficient engine (miles above the theoretical upper limit), and you need to recover the thermal energy present in the exhaust steam - ie you have to condense the exhaust into water, extract the heat and run another generator with it. There's an even lower theoretical upper limit on that efficiency because the exhaust is at a lower temperature than burning HHO.
Fuel cells are more efficient than combustion engines. So why not use one of them? Not that i'll help because it's still constrained to much less than 100%.
