Radio suppression capacitors were used in several places, voltage regulator (due to the contacts opening and closing), the + contact on the coil (primary voltage goes to 250 volts when the points open), alternator (produces a triple sinewave of dc current of varying frequency), etc. The biggest noise suppressor is the battery, the closer it is to the electrical the better. If you had a oscilloscope, you could see all of these variations on the dc in the car, it is also different at various locations in the car, it is always more pronounced at the source of the electrical noise. Dc generators do generate more electrical noise than an alternator, but a alternator does produce a whine, that is heard in some radios, this is removed or weakened by installing a choke (inductor) and a large electrolytic capacitor in line with the power to the radio. The suppression cap on the coil, has little or no effect on the spark to the spark plug. Good Luck.