If i replace the pre and main cats with mid and down pipes, what effect
does it have on the cars computer???
do u have to get a fuel cut defense or will it be fine with out making
any other mods to the car????

 

it will hve no adverse affect to the cars ecu. but yes you will need a fuel cut defencer. it will try to make somewhere areound 10-11 psi. at least that what i think i will make. i put mine on along with intake and cat-back and it made 13psi. you will almost definately need a fuel pump to keep up with the amout of boost you will make with the extra flow from the exhaust.

 

If you still have a stock cat-back, you wont really have to worry about overboosting the engine, but you will need a boost cut defenser, b/c boost will be over the fuel cut level. If you already have a cat-back, you might need to look into porting your waste gate(S4 turbos only, S5 seems to have a much better wastegate). But you will gets lots of power from it though.

If youre worried about fuel cut, just get the downpipe for now. then get the fuel cut defenser on, then go for the midpipe.