I found this on advrider, they were discussing coil oms and using ignition boosters on R65:
by Lou Dudzik;
Q: "...I asked him(Lou) why he specified at least 2.4 ohm coils in those diagrams, and whether I would get better spark if I used a lower-resistance coil, such as a 1.5 ohm or .6 ohm. Here is his reply:
"As you may know, it's the dwell that should determine what coil you should use. Or usually they choose a coil then set the dwell accordingly. Since most people won't change the shape of the points cam, it's the points cam that will determine what type of coil to use. Generally speaking, the lower the resistance of a coil, the lower the inductance of a coil will be. The lower the inductance, the faster the coil will charge.
People often think that a coil is fully charged when the current in the coil has reached it's peak possible limit. While that may be the case for slower coils, it may not be the case for faster coils. A coil is said to be saturated when the current has reached it's maximum based on the applied voltage and the DC resistance of the coil. But that should be specified as "current saturation". Another type of saturation is when the magnetism of the coil has reached its maximum. The magnetic saturation may occur long before the current saturation, especially on a fast coil. What this means is, for example, if a coil measures 1 ohm, and 12v is applied, the current saturation would be 12 amps. But the coil may be magnetically saturated at 6 amps. All of the current applied beyond 6 amps is wasted as heat. Also, once a coil is saturated, any time that the coil is held in saturation also only contributes to heat.
A GM coil in the .5 ohm range might get over 20 amps on it if left unchecked with 12v applied. So GM put a current limit on the HEI modules. The current limit depending on which type of module you have may be in the 5 to 8 amp range. During the current limit portion of the dwell, the module is heating up very rapidly. I wanted to avoid this. And it is avoidable because motorcycles don't need fast coils. Since you are only firing once per revolution (on most classic bikes) you can use a slow coil and avoid the current limit entirely. This leads to a very cool running module, which is always good for reliability. So by specifying a high resistance coil, I am actually specifying a slow coil. The 2.4 ohm limit just ensures the current limit is not reached since it would only be wasted on a motorcycle as heat.
It is important to note, slower coils, due to their higher inductance, often hold more spark energy than a fast coil, for a given current limit. Faster coils may get to a higher current, which may make it seem like the coil is giving more energy in the equation, but it doesn't. There are two reasons why... one is that the current will be limited by the igniter. That limit is what should be used in the equation. And secondly, once a certain current level is reached in the coil, the inductance relative to any increase in current reduces. This is because the coil is getting magnetically saturated. The coil starts to act more like a resistor once that happens, so the spark energy does not increase beyond that of the lower current value. You can see it on a scope by measuring spark duration. It's arguably the spark duration we are interested in once a certain amount of spark current has been reached, rather than actual energy..."
https://advrider.com/f/threads/what-happens-if-you-install-a-low-resistance-coil.1238304/
