Best way to run a cat but still make power?

[Apex1.0]

I have experience running other cars catless, with hfc and factory cats. I know exhaust flow out of the turbine is critical for making power. However I want to run a cat, for a few reasons. 1, I actually like most exhaust sound where there is a cat. 2, I hate stinky raw exhaust. 3, I am not into spewing emissions for a few hp.

I am not judging, I have run a tesp pipe before, but now I don’t mind paying for a hfc pipe.

So the question is, how do I balance running a cat and making power on a fbo n54? Go with Catted down pipes? Or run catless dp and keep the mid pipe cat. Will the mid pipe cat clean up the exhaust enough? Or run both cats?

Thoughts?

[BimmerAg]

What's your definition of "enough" when it comes to cleaning up the exhaust? Catalytic converters operate most efficiently when they are at a specific temperature. Typically, the upstream catalysts in most vehicles are placed there so that they can heat up quickly during a cold start and begin to control emissions. Sometimes the upstream catalysts are a little too undersized (due to packaging constraints) to completely control emissions so downstream catalysts may be added to clean up whatever was missed. Each set could be responsible for anywhere from 10-90% of the emissions reduction depending on hundreds of factors. The only real way to know would be to perform testing in each configuration and measure the emissions. In the end, the emissions from enthusiast cars with or without catalysts isn't gonna make a smidge of difference on a global scale so just do whatever you think sounds/performs/makes you feel the best

[ShocknAwe]

I left my stock secondary cats in place. Smells fine and lets me use catless dp which are certifiably better. I'd only bother with primary cats if required by state inspections.

Hated the whole setup without all of them though.

[Apex1.0]

Originally Posted by BimmerAg

What's your definition of "enough" when it comes to cleaning up the exhaust? Catalytic converters operate most efficiently when they are at a specific temperature. Typically, the upstream catalysts in most vehicles are placed there so that they can heat up quickly during a cold start and begin to control emissions. Sometimes the upstream catalysts are a little too undersized (due to packaging constraints) to completely control emissions so downstream catalysts may be added to clean up whatever was missed. Each set could be responsible for anywhere from 10-90% of the emissions reduction depending on hundreds of factors. The only real way to know would be to perform testing in each configuration and measure the emissions. In the end, the emissions from enthusiast cars with or without catalysts isn't gonna make a smidge of difference on a global scale so just do whatever you think sounds/performs/makes you feel the best

Well, enough where it doesn't smell like an open exhaust. Maybe enough to pass an old sniffer test??? Enough where you clothes don't smell like raw exhaust because you took 30 seconds to load something into the trunk.

[Apex1.0]

Originally Posted by ShocknAwe

I left my stock secondary cats in place. Smells fine and lets me use catless dp which are certifiably better. I'd only bother with primary cats if required by state inspections.

Hated the whole setup without all of them though.

This was my thought too. The dp will be the most important for power and then let the mid pipe cats do the clean up work. Two questions:

How well does the stock n54 mid pipe flow?

How do you pass obd2 emissions with catless dps?

[bluebeemer]

Been wanting to free up my exhaust for a year but no easy answer. I would love to go with high flow cats but seems no one can guarantee no CEL. So you have to deal with it somehow. DP fix is supposed to only be used to pass inspection or AFR eventually goes wonky per the mfr. Don't know that sensor spacers will do long term (?). Don't know if i can delete CEL and use my dinan stage 3 tune. Mine is a daily driver so reliability and drivability are top priority. There is a very expensive cat mfr mfr (european?) that makes the best high flow cat but not for N54. Wish for a better answer for those of us with similar goals/priorities......

[c1pher]

I think this is car dependent. I was able to get hiflow cats for my Mustang and only lost 1-2hp compared to no cats. The car was still loud as hell and I still passed emissions.

[Apex1.0]

HFC are not a big penalty for a NA car. The trade off of having reasonable sound and less stinky exhaust is usually worth it. With a turbo setup however, any cat close to the turbine will reduce efficiency. I am sure HFC down pipes work better than stock. But they will never flow like the cat-less version will. Moving the cats downs stream should help flow but also reduce effectiveness. Its just a balance.

However because of the extra cats on the N54 midpipe, it appears the best compromise between power, price and functionality may be cat-less down pipes and OEM mid/cat pipe.

Not sure how well the factory mid pipe flows, or how you can tune your way around emissions.