The thing about the 409s is that they use a carbon conductor sheath over their spiral wound core. Carbon conductors work fine for conduction and work well for suppression, but they wear out. Once they do, you're going to end up with the spiral wound conductor that's not suppressed as well. For those asking about aftermarket stereos, that's immaterial. When the wires emit RFI and EMI, they interfere with all of the electronics in the car. Just because you can't hear it doesn't mean it's not causing problems with all of the other electronic systems controlling the engine. Suppression is one of the more properties of spark plug wires, with measured DC resistance way down toward the bottom of the list.
