If i have a new header and a catless dp installed how mandatory is a tune, or will i just run a little funny and not be using it to its fullest? which its a CIA 4-2-1 i believe and a 3in dp

 

well if you dont want CELs you'll need a tune to turn those off.
Headers can have a big effect on combustion properties and volumetric efficiency.

W/o a tune you will probably lose performance as opposed to gaining it.

Is it mandatory? no. But its also non-mandatory to refrain from insurance fraud.

 

My buddy has a ZZP shorty header with their catless 3 inch dp to a custom 2 1/2 inch catback and is throwing a CEL so yeah you will need a tune to turn it off but as far as running bad, his runs fine and has been for a little while now.

 

well no ce light yet probably because i cheated and put spark plug non foulers for the o2. so its not getting correct readings on the rear.