If the charge is too low, the plug won't fire. It's also relative in that in a forced induction setup, you will 'blow out' the spark if you use 2-3kv. Regardless, of the setup, a large amount of energy is required to energise the particles between the electrodes for the spark to use as a 'bridge'. A pencil coil generates about 30,000volts when fully charged. A low voltage will not only risk the plug not firing it also limits the post firing 'duration of spark'.