I used a ronded edge for strength and stability, knife is too far for me. This is to reduce the turbulence and allow free air flow. Remember the air is going at this point about 120 MPH and friction counts bigtime. Cat is prime assistant, especially the newest rescue RATT Bert, AKA RB. Enjoy, Woody Cleaning up the inlet passage is as far as I go till I have to take things apart further.WW