I had a friend do it, not exactly sure how.... but he had another ignition coil, hooked it up to a sparkplug he put about a foot from the end of his tail pipe. He would turn on a switch so the sparkplug would spark, rev up the car and turn off the key which would shut off the normal ignition and therefor push unburt air/fuel mixture through the engine and down the tail pipe. it would ignite and make an 8" blue/red flame.
I believe hotrodders do this kind of stuff and very well could have a kit that might show you atleast what you would need.
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First problem, the engine used up all the oxygen in the exhaust. Second problem, the engine needs the exhaust to flow out or it will stop (think the potato in the tailpipe). Third, this would be very illegal and your car would be impounded and sold for scrap with no chance to buy it back at auction.
That said, you could use the exhaust to drive a venturi style blower with a carburetor action and ignition source, flame size would be determined by engine rpm.