I just recently bought an 87 RX7 and was driving it quite a bit until I found out that the cat had rust holes all over it and the mufflers were rusted OFF. Okay to my problem.. I got rid of the mufflers and I replaced the cat with a cherry bomb (Iwas hoping for a cheep temporary fix) now the car runs like crap, tops out around 45 MPH and I am wondering if the extreme lack of back pressure is causing this or is there a more serious engine problem showing itself? Please, any help will be appreciated!

 

Probably much more likely the Cherry Bomb muffler has had its glass back melt out and clog the exhaust.

Or they removed the Cat feed/ air pipe and the aux ports are not working anymore.

And back pressure is never a necessary or good thing on a rotary engine