I don't think that there is going to be much "post-combustion" propane left, because the power behind the expansion of the gas is in the Propane combining with oxygen. There is tons of oxygen. So once the reaction starts theoretically the C3H8 is going to all be converted to either CO2 or H20. I really don't see how there could be any propane left as the amount of propane that is let into the chamber is controlled, but the amount of oxygen is not.
