Walter,
I get your point, but I don't think you could get the tailpipe to a temperature that would ignite fuel or even fuel vapors. Maybe at the header, maybe at a cat converter, but not at the tailpipe. Now, on the other hand, if one were to completely screw up the valve settings like...say...setting them 180 degrees out of phase on one side of a 4 cylinder 1967 VW engine and somehow be able to get the engine started and somehow get it to run for long enough to allow the muffler to actually serve as a combustion chamber and shoot flames out the tailpipe...well that might do it. Don't ask me how I know, it was too long ago. But I can tell you that the SOP for extinguishing a car fire is for the FD to break the rear window with an ax and then fill the car with foam. Who was it who said wisdom comes from experience and experience comes from making bad decisions, or something like that.
Best,
g
'64 230sl, fully sorted out...ooops, spoke too soon