I am using the 8.5mm wires. This is what MSD had to say about the differences of their wires. Not sure how much of this is marketing hype.
"The MSD 8.5mm Super Conductor Wire is "the" performance wire. Engineered from the inside out, the 8.5 offers everything you have ever wanted in a wire; incredibly low resistance without electronic interference! In a single 12 inch length of Super Conductor Wire there is only 40 - 50 ohms of resistance! That is the lowest resistance of any helically wound wire. To accomplish this, we use a copper alloy conductor due to it's great voltage carrying capabilities. The low resistance results in less loss in spark energy so more reaches the spark plug. The conductor is wound extremely tight around a special center core. So tight in fact, that there is over 40 feet of conductor wrapped into a single foot of plug wire. This winding procedure, combined with a ferro-magnetic impregnated center core, produces an extremely effective Electro Magnetic Interference (EMI) "choke". This would be the best wire to use with the MSD 8 Ignition System.
The 8mm MSD Heli-Core Ignition Wires are perfect for passenger cars, tow vehicles and performance cars. The wires are 8mm and can be relied on to deliver all the high voltage your engine needs! Heli-Core wires feature a stainless steel conductor that is wound around a special glass core. The stainless conductor provides a low resistance path for high voltage to flow while keeping Electro Magnetic Interference (EMI) to a minimum. By winding the conductor around a special glass inner core, the EMI is suppressed inside the wire where it cannot interfere with other electronic devices on your vehicle."
