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About 50 years ago we used a model T ford coil for spark at the plug. They had a set of points and when set right they would vibrate creating a series of sparks as long there was current to them. Rev the engine up and let off the accelerator, this would make a rich mixture in the exhaust and pretty flames out the pipes. Modern engines with the catalytic converters and computer mixture controls will probably not do that with out a source of fuel supplied to the tailpipe.
You're exactly right Charlie. Most show vehicles that have this type of thing on them now inject propane into the exhaust pipe just upstream of the ignition source. You also have to have a way of modulating the current to the ignition source (ie spark plug) to get a series of sparks to ignite the gasses. And fuel injected vehicles will almost never get rich enough to do this. If they do it usually gets trapped in the catalytic, like you said.
I haven't done anything like that in 35 years. I guess it would about the same, get the parts together and a lot of work. One thing different, would the ECU be compatible?[sm=patriot.gif]