Engine Car has no power under acceleration, but has no codes

Hello StangNet, and sorry for my first post being a help thread but I’m truly stumped here. My 1994 Mustang Cobra is running horribly. It has no power under acceleration and the exhaust has a horrendous sound. I dumped the codes but there was nothing there to indicate why it runs so poorly, just normal EGR and smog codes as the previous owner trashed it all and it ran fine with he stored codes so I never paid them mind. Cylinder balance test came back fine, with a 9. I already replaced all the spark plugs, but to no avail. I wanted some advice as to where to being now.

Thanks to all.