Reversed coil polarity requires about 20% more voltage to fire the spark plugs, since the spark is coming from the "cold" side, the ground electrode, and jumping to the center. Finding that out on a cold, wet morning when the thing won't start is quite a learning experience! I used to use that example for my students when teaching oscilloscope diagnosis- - - - -the difference in the firing line voltage on the scope is very obvious when the polarity is reversed intentionally.
Jerry
