There are two ways I have seen to make exhuast flames, the one with the fuel injetors just sounds like a horrible idea, but the old style where you use the engine seems safe as the amount of fuel is limited to how long your engine will turn after the ignition is cut, and the rear coil and plugs are powered and the flames don't get very far, the cars that I have see how do this get at the most a foot of flame for 10 seconds because they try to flip the engine ignition back on before the car dies.