This is my P5 .91 on the right, also bought from Paul, the muffler is rather large but high quality and not loud, the large volume most likely due to the way it is supposed to be run, turning large props in the 8,000 rpm range. It turns an APC 14x12 at 8,300 and does not strain doing it on 5% nitro. Eric, is the .55 supposed to turn large props or rev?