The resistance of the coil determines how much current flows through the ignition system. The 3 ohm coils, because of their lower resistance, pass more current and therefore run warmer than the 5 ohm coils. This is most likely to occur under worst case conditions, such as creeping along in traffic on a hot day in Arizona (this isn¹t so great for an air cooled engine either!). Rather than get into the specifics of every application we make the general recommendation - 5 ohm for street, 3 ohm for race. NOTE: DO NOT use the DC6-5 coil with the DS6-2. Damage to the coil and ignition will result and will NOT be warranted.