Hello every one .
i just got invidia mid-pipe+borla Alex-back.
I have some questions

when I push the gas and loss the gas it has pop sound from back of my car.
is the sound come from muffler or it comes from mid pipe?

when I installed mid pipe I did not use oem spring bolts connected muffler.
is that safe for the car or I have to change spring bolts with it?

the oxygen sensor is ok for me run this system ?

thank you guys.