I recently purchased a dc sport header for 30 bucks from some dude parting out his wrecked car and the header inlet is 2" in diameter. With that being said my exhaust piping is 2.25" in diameter, now what size hi flow cat will I need to join the two? 2 or 2.5"

 

2.5

 

you could use a 2.25 or 2.5......i would use the 2.5 for better overall flow, your header will just sit a tiny bit inside the cat