Conductivity improvement by heating of water

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that's why conventional electrolysis seeks to hold the voltage at a minimum.
see, the amount of gas produced with conventional electrolysis is only determined by the current, not the voltage. you had the same current both times and the same amount of gas created.

the voltage was lower because of the higher conductivity, that's why large electrolysis devices use electrolyte to get a voltage of ~1.4V (if i remember correctly) to get to a point where it's >70% efficient. of course higher temperature will enable better conductivity at lower voltage and therefore a higher efficiency. but you must note that you need energy to heat the water too.

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...but you must note that you need energy to heat the water too.

Engines are 25% efficient at max efficiency.
35% of the heat energy blows out bhe exhaust.
30% of the heat energy blows out the radiator.

ie:
For a 100HP engine; about 100 HP of heat  energy blows out the exhaust, while another 100HP ends up in the coolant and is cooled by the radiator.
Thats 200HP of waste heat for every 100HP of shaft power!  :'(

There is no problem finding waste heat; the problem is safely increasing the pressure, to keep the electrolyte from boiling.
Do-able but expensive and heavy.