Let us know what you find out. Thread-jack is OP approved.
I've seen a racer-modded Porsche [picture needed].
I thought the original reference was suggesting thrust from the rearward-facing bells.
But I'm reminded of the
Moller Supertrapp. It was invented to muffle a snowmobile engine (actually
eight snowmobile engines.
) that drove a ducted fan.
http://8903e7f0897f2982f439-51d6683c...ite640x480.jpg
It would lose any additional thrust. Another possibility I saw but can't document was a muffler that was two back-to-back cones in a bellmouth.