What is this 2nd upstream air filter in the Hyundai Air box intake? Pics.

My BMW had this. Strictly for US market. European version did not have it. Called my brother in Bosnia thinking there has to be European intake somewhere on the junkyard. He found it $50. Mahle filter filter for EU intake is like $16 here. Better sound and throttle response.
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Many years ago I had a 325i with the M56 SULEV motor. Pretty neat to read about but I'm sure a nightmare when parts broke or wore out. It had a hydrocarbon filter, sealed stainless steel fuel system, & a coated radiator to reduce ozone. BMW extended the warranty on the fuel system to something like 15yr/150k miles.

 
I don't have one of those on my 2018 Hyundai Kona AWD 1.6T. It would be long gone if I did though. Nore do I remember one on my wife's 2013 Elantra GT. Is your car a California car?
Look under the top part of the airbox I bet there's one there. Many cars have these and it's probably a form of idiot proofing.
 
The amount of air the engine ingests at idle under the same conditions shouldn't really change going from one filter to another. If the secondary filter was indeed restrictive, that restriction would become increasingly evident as load and engine speed increased.

What caused me to raise an eyebrow was the air mass idle reading, which shouldn't have changed with the TB being the biggest restriction in the system which is why I tossed that out there. This is where vacuum measured in front of the MAF using a restriction gauge or sensitive vacuum gauge would be helpful in determining if restriction did in fact change or if we are just seeing an artifact of the airstream changing due to the elimination of the secondary filter.
Agreed, changes in things like Abs P before and after the throttle body, and the MAF readings with just a filter change may basically be in the noise level and hard to get any confident change data at idle. The MAF data at WOT vs RPM would be more accurate as long as there wasn't some turblance factor. If the intake designer did it right, the MAF sensor won't be in a turbulant area of the stock intake tract regardless if a filter is installed or not.

A simple vacuum gauge or restriction gauge between the filter and TB won't tell much and be pretty insensitive at an idle - unless the filter(s) were basically clogged. If the filters aren't dirty, the vacuum level after the filter will be pretty miniscule at idle when the throttle is barely cracked open. A very sensitive vacuum gauge (not a simple restriction gauge) between the filter and TB could give good data, but that would take some cost and effort to install.

The best test would be to read the intake manifold Abs P at WOT plotted against RPM with different filters, with/without the charcoal filter configuration. That would take away any MAF turbulant flow factors if that was a concern. If the OP's vehicle has a MAF sensor it may not have an Abs P sensor in the intake manifold. Would have to look at what live OBD2 data is available.
 
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Quick question, Ive noticed on both of the Hyundais 2009 Elantra and 2013 Sonata, I see a "DO NOT REMOVE" printed on this air box intake secondary filter upstream of the main filter on the lid.
Is this "charcoal filter" really upstream of the main filter? If so, is the air actually going through it before the main air filter? Or is this just mounted in top of the air box and it just acts as a charcoal absorber (that's my bet). I'd think it would be way more dirty than it is if it was actually acting as an air filter, and not just a gas fume absorber.
 
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