Quote:
Originally Posted by CamaroCracka
The stock 2.5" exhaust is plenty for the LS3. A little bit of welding slag is not going to affect it. I would be looking for an obstruction, like the internal tubing on the muffler broke or a something clogging the cat. I would also look at the header gasket as small leak created by the hit on the exhaust could affect performance.
Are you only noticing the difference when the NPP should be kicking in, i.e. is the dual mode working. IIRC, they operate off of vacuum and should be open when the car is shut off. As was mentioned a cheap borescope might be a good investment.
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Along the lines posted above If you hit something hard enough you can break the ceramic material inside the cat causing a blockage. It has happened to me on a different vehicle and I had to replace the cat