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Old 06-06-2018, 04:13 AM   #1 (permalink)
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Reducing back pressure - delete secondary catalytic converter

Hey all!
I'm trying to coax a few more MPGs out of my 2011 Hyundai Santa Fe V6. I know that reducing exhaust back pressure can improve performance, but does it have an effect on economy? My car is set up for California emissions, though since I live in Jersey I don't need to maintain that. Part of the Cali package is a secondary catalytic converter after the two main manifold ones. All of the O2 sensors are upstream of it, so nothing monitors what is coming out of it. Do you think removing it, along with the resonator, would boost my mileage at all?

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Old 06-06-2018, 04:33 AM   #2 (permalink)
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Old 06-06-2018, 07:35 AM   #3 (permalink)
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You might lose economy if you reduce back pressure. About the only time it would potentially increase economy would be at WOT, as the engine theoretically could gain a % or 2 on the BSFC. The rest of the time it would potentially be a liability.

I would think back pressure would behave similar to EGR and reduce pumping losses at anything other than WOT. And besides, too much scavenging and it would start pulling unburnt air/fuel right out the exhaust.

(That's how it works in my head, at least)
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Old 06-06-2018, 10:21 AM   #4 (permalink)
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I'm going to agree with Stubby79. You only need to reduce backpressure when you are trying to flow the maximum amount of exhaust through your exhaust system. When does this happen? Only at peak horsepower output. The rest of the time, it is quite possible that it will hurt engine efficiency.
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Old 06-06-2018, 11:00 AM   #5 (permalink)
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Also agree with Stubby. We drive under part throttle because that is the most efficient. Without tuning your entire exhaust, you will not gain FE simply by reducing back pressure after a primary CAT.

And I would love to add a second CAT to my car, if I could be confident it would improve emissions. Clean these gross polluters up!

Also, I started a thread on this topic a few years ago that was meant to try and capture the consensus of the best informed on this topic, on this site: the muffler and exhaust mod thread

Might be useful for you....
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Old 06-06-2018, 06:46 PM   #6 (permalink)
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You could try a high flow cat. Do some back to back testing.

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