Air Fuel Ratio and throttle response on modified 1.6t


Elantra GT 1.6t
Hello all,

I am new to turbo tuning and looking for some advice/insight into the recent symptoms I have experienced with my 2020 Elantra GT N Line 1.6 turbo. I bought the car with 40k miles on it and it came with an aFe Takeda Momentum Intake and Velossa Big Mouth ram/snorkle.

Lately, when accelerating from a stop light there has been a strong hesitation before the car starts moving quickly. The power/boost seemed to be getting worse as well as time went on. I attributed to something with the stock throttle delay from the pedal so I bought a Pedal Commander thinking it might help. There was a noticeable difference but it was still jerky from stopped to moving and power was still dropping. The more I reduced the throttle delay the jerkier it got, so that wasn't the problem. One day I noticed the Takeda intake box had a large hole in the side I am assuming for sound/extra airflow so I decided to look for the cap in the glovebox and found it, so I capped it off to see if there was any change. (I know, LESS airflow?!) To my amazement the car almost immediately cleared up all symptoms and the power/acceleration went through the roof. Acceleration improved so much I removed the Pedal Commander completely as it was accelerating too fast from a stop even at the 'city' driving setting.

At this point I was very confused so I decide to check some data.
- With the cap out, the AFR is consistently staying between 14.6- 14.9 regardless of throttle % or speed, RPM, etc. Boost maxing at 12-13 psi. noticeable slower 0-60.
- With the cap installed, I am seeing around 13.5 AFR at idle and cruise and around 13.2-13.4 under moderate load. Boost maxing at 17-18 psi. Quicker and more powerful all around.

So my real question is, why would this happen? My understanding of AFR in modern engines is the ECU should control the parameters as long as they are within a range. Example, If the airbox lets in more air, the ECU should be adjusting the fuel to match for whatever predetermined AFR it is aiming for? In this case, the 14.7 AFR makes sense as it is the "ideal" ratio you would expect from the factory. But if so, then why would the AFR drop to 13-13.5 by reducing the airflow? Shouldn't the ECU adjust the fuel pressure/flow to maintain the desired 14.7 AFR? It's not making a whole lot of sense to me at the moment and I am hoping an experienced tech can chime in here. I considered a boost/vacuum leak but I would think the increased boost reading with cap installed would compromise the same leak and let the extra boost escape just the same? Side note - the car also seems to run smoother and with a bit better acceleration on regular vs 93.. makes 0 sense to me.

Also, I had checked and replaced several parts with little effect:
air filter was cleaned and re-oiled,
motor oil changed,
new NGK Iridium plugs and MSD coils,
computer reset via battery disconnect method
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