Q is larger than P , If you are flowing alot of air i.e. low 10's or better , the Q will lower Backpressure vrs a P trim . Q will not fit a garrett .63 hsg or .82 hsg without welding and remachining . Big bucks if anyone will still do it . You wil have to use a PTE exh hsg's .Thats about all I know about them .HTHs