[EchoOnefox]

2009 v6 limited. i have a straight pipe where my rear cat was, and since the change, my acceleration and performance have gone down considerably. theres no o2 sensor, so why the low performance?

did i melt the manifold cats

 

[tomw]

If you had a misfire problem, it is a good possibility. Many have had a COP fail, ignored it, and found they had destroyed their converter(s).
One test is to remove the upstream O2 sensor to relieve backpressure caused by a melted converter, a second test is to monitor the exhaust gas flow from the tailpipe with the engine running, rev it up a bit and note how well the gas flows. The flow should increase quickly and noticeably.
Do you have any check engine or other warning light lit? Do you have any codes? What caused the removal, or was it taken by someone who thought they needed it more than you?
tom

 

[dwightbresemann]

While, too much back pressure in no good. The same for too little back pressure is no good. Engines are designed to run with some back pressure. 1-2 psi at idle and 3 psi at WOT. Maybe the missing component has reduced the back pressure to an unacceptable level.

 

[tomw]

What Is Exhaust Backpressure, And Why Is it Bad for Horsepower?

And is too little backpressure just as bad as too much?

www.roadandtrack.com

read the article if you think you have too little backpressure. I would look elsewhere for a different cause for performance decrease.
Have you checked fuel pressure and delivery volume? Pressure increases as soon as you close off the outlet, with zero fuel being moved. Open the spigot(injector) and a flow restriction will drop that pressure like a rock.
An air filter that is clogged will still allow the engine to run - at lower rpms - but will not allow it to develop as much power. The O2 sensors will choke off the fuel delivered to match the airflow indicated by the MAF, so no matter how much you open the throttle, it still will not go any faster as the incoming air is being throttled by the filter. Unusual to be severe, but can happen.
tom

 

[JohnCT]

did i melt the manifold cats

If you're asking, I'm wondering what the whole story is.

If the bottom cat was replaced because it was plugged, there's a good chance one of the two upper cats started breaking up, but they can also melt together and become a blockage. Misfires will overheat these cats so fast you won't believe it.

As Tom said, removing the upper O2s and running it will tell you if you have a stuffed cat. The opening of the O2 sensor isn't very big, but if the cat is mostly blocked, then the small opening of the removed O2 will give it more power, and if you feel a lot of puffing coming out of the hole, then the cat is blocked.

I bought an 05 for "parts" because it had many issues and was undriveable (long story). My 05 had a completely stuffed up bottom converter, so bad the car would barely run (and it sounded like a Hoover running beneath the car). I removed it and bashed it out hollow. Other than a slight change of exhaust tone, it runs just fine. Whether it runs better is something I can't tell you because I never hot-footed this car, but it most certainly did not run worse.

But, after I hollowed out the lower cat, I got an upper cat efficiency code. Seems like the upper cat's innards blocked the lower cat. I was lucky in that the upper cat didn't melt down and plug up, but it did break up and shed some material downstream.

John