Yeah, so I bought an electronic cut-out for my exhaust (mainly b/c I didn't want to spend a ridiculous amount of money on a full cat-back!) only to realize that another cut-out I saw done on here was installed by cutting out the cat from the down pipe, which will never pass smog here. However, the guy at the exhaust shop told me if I can get a flanges he can make a straight clean test pipe for me, so I can just reinstall the stock set-up when I need to smog. Any ideas on this? If I were to do this how could I get the flanges? Is there a test pipe already available somewhere? Pros/cons? He said it should make a huge difference b/c of all the restriction there with the cat and resonators.

 

Modern cats offer very little restiction to exhaust flow. A few HP on a 300+ HP naturally aspirated motor, a bit more on a turbocharged motor. This needs to be weighed against the rather obnoxious "splatty" sound of most cat deletes, risk of getting caught / fined in strict states, hassle of switching back and forth, and the odor of uncatalyzed exhaust. The smell doesn't bother geezers like me, but seems to really upset gen X'ers and millenials, who weren't sucking up much exhaust before 1975. Resonators offer virtually no restriction. They are acoustic resonators tuned to a specific "drone" frequency, not resistive dissipators, as are most mufflers.

 

Hmm, the exhaust guy told me that removing the cat and resonator would be "like removing a trailer from the back of the car." I assume from what you have stated this is not true? If sound was the only importance to me, how would removing the exhaust at the flange from the resonator back affect the sound? How would it affect performance? You stated as far as removing the cat goes "A few HP on a 300+ HP naturally aspirated motor, a bit more on a turbocharged motor." Most aftermarket cat-back exhaust systems make similar claims yet cost several hundreds of dollars. Same with the downpipes with "high-flow" cats , they claim performance gains and cost hundreds of dollars themselves. I am just thinking why would people spend $300 on a downpipe, $600 on a cat-back exhaust, and get the same performance results as just removing everything and putting in straight pipe all the way back or spending $100 and putting in a cut-out with the ability to be stock whenever you want? I understand sound and legality in some states (thanks CA!), but I am referring to the performance aspect.

 

Removing the cat will make your hhr ss sound like a turbo prop airplane in warm up mode (and I have the factory exhaust). But after it warms up it sounds nice and at wot it sounded nice. I could only imagine what a catless exhaust system with the resonator removed would sound like or a straight dump in warm up mode.

From my understanding a catless dp is good for 15-20 whp with tune. A new cat back system yields 5 whp gain over factory. So to me a cat back isn't worth the money for the performance you gain, unless your going for the sound. Only way I would do the dump is if I was planning to track a car often. I bet that thing is loud as hell.

Our factory cat back is very efficient for a factory system.

 

I tend to avoid people who speak in exaggeration and hyperbole, particularly when they're trying to sell me something. I used to work at Stainless Works, who make high end header and exhaust for performance cars. We also sold Random Technology high flow cats. With a turbo car, you could expect modest gains from deleting both cats, but certainly nothing like un hitching a trailer.