The fuel mix must be very potent to ignite with all of the exhaust fumes in it. the only time i could get fire out of a muffler is if the engine missed and unburned fuel was vented into a hot enhaust sysyem. but in that case there was no burnt fuel exhaust combined with it. if my chemistry is correct, mixing a pure gas (fuel/air) with an impurity (exhaust) denatures the pure mix and changes its properties (ie wont burn). i sayits a hollywood effect, not a real-world effect
also, if theres flame in the exhaust, there will be flames in the compression stroke of a 4 stroke engine thus causing pre-ignition and stalling.
thus i argue that sustained flames in the exhaust is impossible in normal operation. i could be wrong.