How hot should a ballast get???

We recently connected a coil and ballast resistor to a test bench power supply to see what happened. At 13v it pulled 3 amps, which is 39 watts. Low power soldering irons are rated at 40 watts, so there is a lot of heat. A running engine will have a constantly cycling current draw and will not get that hot, but it is still going to take a good bit of current to create a 20,000 volt spark. The fact that the resistor is mounted in ceramic and has a small gap for airflow on the back side means that sucker is going to get hot. Don't know how hot is too hot.