My car has a catback and a header with a D16Z6 motor. When I'm in D4, the car fells like it accels slower then in D3. The sound that comes out of the cat between the 2 is different. The sound pitch is a little higher in D3.

Picture it in your head for me:

D4 = Brrrrr

D3 = Brreee

It's sounds like I go into some kind of overdrive or something.

How safe is this? The motor is properly tuned. Everyone that knows anything about motors tell me I have a great engine and accels good or better than average. I have know idea about the engine internals because the car was given to me. The person that had it passed away a few years ago.