Join Date: Jul 1999
Location: hastings on hudson, NY, USA
In AC the S means extended tip. The number signifies the heat range. Lower number is colder plug for hotter engine. For instance a 327/275 should run a 45S with standard compression ratio. You can try colder or hotter but then you'll have to check them after a few miles to see what color the porcelin around the electrode is. They should be a light brown kind of like bran flakes. If they are lighter than that go one range colder which is one number lower. If darker then go one number higher. You should always run a complete set of the same plugs. You'd hate to have one cylinder colder and fouling and another so hot that it goes off too soon.