This is a question we are continually asked by Honda and other Japanese-engine owners in particular who previously attempted to fit other brand oversize spark plug wires to their engines. The reason Magnecor Race Wires are so flexible is that the insulating jackets are made entirely of high-temperature aerospace silicone rubber, which by nature is strong and extremely flexible, and, unlike all other performance wires, can safely be installed into the original 7mm size spark plug wire retainers despite the larger 8.5mm (Magnecor KV85) cable size.
All other ignition wires use multi-layered insulating jackets to reduce manufacturing costs, and all performance wires use either a thin layer of low-grade silicone rubber or a silicone blend as the outer jacket (sleeve). No oversize performance wires other than Magnecor KV85 can be successfully installed into the original 7mm retainers without the risk of breaking the retainers — if at all!
A consumer’s best indication as to the quality of a spark plug wire’s insulating jacket is the stiffness of the cable used; the stiffer the cable — the poorer the materials used to make the outer “silicone” or blend jacket. Of course, at extra expense, poor quality spark plug wires can be covered with loose sleeves designed to protect against engine heat, and this is the reason so many of these products have become so popular in recent years.
However, poorly suppressed, low-resistance spiral conductor wires with more flexible low-quality silicone outer jackets have been developed in an attempt to imitate Magnecor Race Wires, and appear in mass-merchandisers and speed shops.