Rallyfinnen use the TCI-board and this board is specialy for using a low Voltage bobine / sparkcoil.
But, the sparkcoil can be fired to ground but also to power plus batterie.
I don't know if he use the enginepower for the sparkcoil.
If he is only grounded the TCI to the engine there can't be a potential differends......there can't be a potential differends on a one wire powersource.