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I ran my car with Lts and stock cats and mid-section for about a year and then switched to Hi-flow cats and x-pipe. Big difference! Should have done it from the beginning and saved some money.
I ran an 11.74 quarter last week with high flow cats. I bought a Dynomaxx header system that came with the high flows and x-pipe that bolts right on to the Corsa's. With the 04's it takes an extension on the front (I believe) CO sensor off of the Cats.
I was really worried about passing Missouri emission tests with the headers and 233/239 cam that I put in. After I got all the "stuff" put on I had traveled about 50 miles and got a code for emisssions. Long story short I had the tuner come to my house and block out the CO sensor (but shows it is working when performing an emission check through the OBD port). As one of the posters said the exhaust is pretty strong until the high flows get heated up. My wife complains about it and I tell her that is why Mustangs are afraid of her when she drives it!
put a good set of hi-flows unless you like the smell of poop but seriosly leave them the gains aren't worth the risk of not passing inspection and it will smell like farts.