Any coils you want so long as you build or buy appropriatley to your hardware, dont exceed battery ampage, etc etc, im not a mech user, but you do really need to understand Ohms law before building your own, and make sure as above to understand the risks involved, Regs protect you from nasty accidents, on a full blown mech you dont have that, so you really do need to understand the basics, even down to simple things that making sure the 510 pin on your rta, rda, is protruding out eliminating the risk of a short, there is so much to consider, it took me a while to understand but i got my head around Ohms law, even tho im not a mech user, i figured it was worth learning.