It is applications like forced induction and/or nitrous oxide that overwhelm the stock ignition system. Supercharging and turbocharging ups cylinder pressure through boost while a nitrous oxide system uses a chemical reaction to deliver the additional oxygen that results in higher cylinder pressures. Either way, lighting off the entire air/fuel charge efficiently is the key to realizing the optimum power output of your engine combination. Cylinder pressures can also be increased by upping the compression of a naturally-aspirated engine. Increasing the rev limit reduces the margin of error when it comes to the ignition system producing the voltage needed. Precise timing and sufficient energy are of prime importance as the time between firing cycles is shortened. The amount of boost, nitrous, compression and increase of rev limit will dictate when a performance coil will need to be added to the mix.
As the combustion chamber pressure rises, it gets more difficult to get the spark to jump across the plug gap. The reason is the gas in the gap is an insulator. If you stuff more molecules in there (higher pressure) you need a higher voltage to jump the gap due to pressure resistance. It isn’t resistance like in a resistor – it’s ionization resistance. The voltage has to ionize the gas in the gap to jump it.
So, if you raise the boost you are also raising the pressure in the combustion chamber and the requirement for higher voltage to jump the spark plug gap goes up. You can reduce the gap but this is just a compromise – the shorter spark will have less chance of igniting the mixture, so very quickly you will run into the same problem – flame front blowing out and your engine starts missing.
Coils do wear out. They will often show the proper resistance, but they will start to “break down” internally. What this means is at some voltage level the energy will flow through the internal insulating material, effectively acting like a short and absorbing the energy you need to produce a spark. When the combustion chamber is at low pressure the spark will jump the spark plug gap at a voltage that is not high enough to flow inside the coil. The voltage appears to rise immediately to its’ max. value, but it actually takes up to a few milliseconds, and halfway to the voltage levels you need something starts to break down.
Bottom line is, if you want to run high boost/nitrous combinations you need an ignition system in good working order – that is, capable of generating very high voltages without breaking down.