[Marshall V. Daut]

You can accomplish the same effect described by nkaminar if as you are driving at road speed, you turn off the ignition key in gear and wait two seconds. Fully retard the spark lever and then turn the ignition back on while still in gear. BANG!!! The accumulated exhaust in the muffler and the increased back pressure in the cylinders will ignite into an explosion and shoot a flame from the tail pipe. Owners used to do this "back in the day" to shoo away dogs chasing the car and nipping at the tires.
A word of caution, though: Don't leave the switch off for more than two seconds or you run the risk of blowing out the insides of the muffler when the ignition is turned back on. This dog-be-gone trick is a bit hard on the muffler anyway. Just don't escalate the explosion by waiting too long before turning the key back on.
Marshall
ADDED: Have you ever been "bitten" by a Model T coil when it fires? If so, you may want to rethink hooking up a Model T coil to ANY source of ignition, lest it prematurely fires while hooking it up. YIKES!!! Once bitten by a T coil, you'll feel like you were hit by a sledge hammer and your hand will tremble the rest of the day. 'Been there...