To answer the question of octane rating. Higher octane burns better so it burns less fuel and the higher the octane the less the knock. Simple.
Two cars identical; #1 87 octane, #2 93 octane, # 2 will get more miles per tank than #1 because the fuel burns much better with more power which gives the need for less throttle, creating less load, in return less resistance from the engine. The offset here is the price difference in the fuel!
My Suburban runs so much better on 93 than 87 but the difference in cost doesn't warrant the use of it.
Most foreign models from Europe will recommend a higher rating of fuel as the engine both runs better and will have more power. They rate there cars horsepower with high octane fuels. Not our low grade petrol. If your car has ran on 91 octane for x number of years you will instantly notice a difference in power and performance if you drop to a lower octane fuel.
HHO doesn't "bring" the octane rating up, it helps to burn the fuel(whatever it be), at a faster rate which gives you more power with less fuel consumption. Be it as your burning less fuel with a higher ratio of power, your wasting less fuel and decreasing the hydrocarbons that are unburnt, which gives you a much "cleaner" exhaust.
Its done right or its not done !
Hail HHO.