Recently installed new exhaust manifold because of exhaust leak/crack on manifold weld (also had precat fail) with precat delete, new manifold gasket and donut gasket of course. Bolted everything back up as it should (without the springs for that connect the manifold to down pipe) along with new up and down stream 02 Senors. NOW the leak is gone BUT I have no horse power on acceleration. Engine sounds like it's struggling, lost of back pressure, hard to describe noise.
Could the stock O2 be to long sticking inside the Exhaust manifold and blocking the exhaust travel to the cat and out the back??? Also I don't think I did but could over tourq on exhaust manifold bolts on engine or down pipe cause back pressure?