Omen1

Hi all ...would like to know if back pressure is from the exaust or a certin amount of restriction is needid ...the reason i am asking i had a full 3"i.d pipe fitted to my STI7 for about a 3 weeks ...it was quite loud but fantastic performance ...swaped it out today with a similer size i.d pipe but this exaust has a 2 3/4 back box ...with this on my tubo seems to be spooling faster and it seems to be running better.