[RedDevil]

I think the majority opinion here is that HHO is a synonym of scam.
It poses no benefit at all as there's no way to overcome the energy lost in generating it.

Some research suggests minute amounts of hydrogen gas can have a disproportionally large gain in some very specific conditions (IIRC purified octane in a 30:1 compression rate diesel type engine), but these conditions are not present in automotive engines.

Nor do performance air intakes generate any economy benefit at all, as the engine would run at reduced throttle anyway. The less restriction the air filter provides, the more the throttle valve will close for the same intake pressure at the cylinder head.

As for excess HHO venting out: even if the throttle valve closes, it will let several liters per second by. If you have instrumentation that shows RPM and air intake pressure you can calculate how much.
For instance, my Insight has a 1339 CC engine. If it idles at 1000 RPM it pulls 500 times its displacement in a minute (it is 4 stroke after all). The intake pressure typically drops 80% idling, so that would be equivalent to 100 times its displacement a minute, 134 liter per minute, 2.23 liter per second.
Your HHO generator will produce only a minute fraction of that amount, which should all get sucked into the air intake even when decelerating or idling. I expect no effect to be noticeable.

That said, there's nothing wrong with trying things out.
The best way to test the effectiveness, or lack thereof, is by doing a series of ABA test in constant conditions (warm engine, no traffic, low wind, constant temperature, same route).