get a spark plug and mount it about 5cm from the back of mufler tip. run a spark plug lead to a second coil (mounted under the car as leads are fucking expansive!) then hook the coil up to the positive, a switch between the coil and the earth and away you go. if you want mean flames, unplug one plug from the engine so a little bit more fuel runs through you exhaust.
remember, if your car is tuned nicely, it should/will not have flames!!!