Me and my friend just changed the plugs and filter on his GTP. He was running stock NGK'* at 96,000 km it looked like, anyways so we changed them to G-Groove NGK TR55'*, one heat range colder ( his car is stock ). We also got rid of the stock airbox setup and used stock hose and put on a k&n conical filter. It whines now like a b*tch with even the stock pulley but, it seems almost as he lost power. Is it possible that the one heat range colder plugs made that big of a difference? I mean, wouldnt changing the old plugs alone outweigh the fact of the heat range change? Or do you think its the open filter problem? Whatever it is, im just interested. All the spark plugs we tightened hand tight, then 1/4 of a turn. I dont know, maybe im just feeling things wrong. It just feels slower.....