What is the difference between 7mm spark plug wires and 8mm? 7mm is the factory size, and Ive read on here recently that 8.5 is too big without an upgraded ignition system. I was just curious because I have read on here that everyone uses 8mm, so I went to several stores around here to get the 8mm ones and no body carries them, they have to order them in. I was just wanting to know the difference before I have anyone order them or I buy the 7mm ones. Thanks