Alright so I have a 88 Fc decided to take the mufflers off and still pretty quiet, sooo decided to go for the mid as well, I still haven’t put any pipes on yet(going for a straight pipe) but went to turn it on just cause I was curious on how it sounded. And I idled at a lower rpm then usual, and gave it a little gas and it backfired pretty hard, just wondering if it’s normal? Or could it be that I haven’t put any of the pipes on ???

 

I don't know anyone who has described an un-muffled Rx-7 as "quiet". Even most aftermarket exhausts are loud.

That being said, it shouldn't affect how your car runs. The car doesn't even use the O2 sensor except in 5th gear under certain conditions. Did you do any other work at the time?