01 Firebird, slow to pick up speed, Is it a muffler?

2001 pontiac firebird, v6 3800L
120,000 miles. Auto Transmission.

My car is very slow to pick up speed at the beginning, but once it hit passed 30mph it's running fine and it can goes up to 100 mph with no problem.... The RPM pin didn't jump back & forth, so the speed MPH. But when I'm pressed the gas pedal I can hear the engine run louder but the car didn't go any faster till 30 mph..

I heard about transmission myth, but it run like a champ after 30mph..! Was is a symptom of bad trans?

I wonder if I have a blew muffler that lack of back pressure? Or a bad muffler.?

There was a Engine light code: p0420 (Cat effi bank 1), but after the new catalytic converter, the code is gone and runs a little smoother..

**I have been driving with a misfire and a leak of exhaust manifold for almost 6 months, didn't know then... Just got all fixed now, and wonder if those can messed up my catalytic converter, would it messed up my muffler as well...!

And if you think it is a Trans problem.., please explain why?

I have replaced: fuel pump, fuel filter, coil pack, spark plugs & wires, EGR valve, MAF sensor, Catalytic converter, O2 sensors (up & low stream), etc...

OK, LIGHTLY accelerate from a dead stop......do you slowly get up to 50mph and shift ok?

If yes, come to a stop, now LAY INTO IT! If it bogs down, doesn't shift, and possibly tops out at 3K rpms, it's an exhaust restriction....

Was this an aftermarket cat(usually smaller than original)?

Two things you can do, create an opening BEFORE the cat(remove O2 sensor), and see if problem is gone......if it is,

Put O2 sensor back in.....now create an opening just after the cat....if car runs ok, then there is a restriction in the exhaust after the cat....if it doesn't, it's the cat....

I will try it today, and let you know how it's going.. Thanks very much..!