won't work very well since for this to happen you have to interrupt the ignition for a second and have it dump raw fuel through the exhaust or be running mega mega rich ... with a catalyst in the way of the fuel going out the tail pipe, you're more likely to clog your cat up or damage it in the process than fire a ball of fire...

not to mention it has the possibility of being very very dangerous if used at the wrong time (it happens) ... if you want a great show item, look into the zex rapid fire machine gun style nitrous purge, you do not even have to have the system hooked to the engine if you just want it for show ... and it makes a much better statement than flames coming out of tail pipes setting your bumper on fire making everyone laugh out loud as you try to drive faster and faster to put it out hehehe. (seen it happen).

good luck whatever way you decide to go ... but please go the safer route.