I'm wondering if there is any reason, be it emissions, fuel economy, or whatever, that an auto manufacturer would recommend a hotter plug than is ideal for performance. Here's my case, I have a Saab 9-5 aero and on the recommendation of other saab owners I tried a plug that is one heat range colder than the stock plug. The car seems to run a lot better now. It doesn't seem to go any faster, just a lot smoother than before, even more refined. So what do you guys think, is there a reason they wouldn't recommend the colder plug right off?