if you do this at high temperatures then you are going to get water vapor in your gas output, when you are measuring your output be sure to bubble through cold water, this will take water vapor/steam out of the "gas", and give you a more accurate reading, also hot gas is less dense, so after your run it through a cold bubbler to take out the water vapor, the gas volume will shrink because of the lower temperature. if you then figure hot gas is more efficient in burning in the engine, then you can always reheat the gas with the heat from the exhaust.