I have an EVO engine. I don't have my booklet in front of me. But if you look in the book it came with it it had some kinda reason. It said something about the shape of it effected the fuel or something. I think its just a stunt to try and get people to buy their plugs personally. I could be wrong. I do know that the plugs I have now are not the same kind that came with it and it runs the same.