Stock Airbox Chamber Test - Foam Insert

Stock Airbox Chamber Flow Test


A question was asked if the small chamber at the back part of the lower half of the stock airbox has an effect on the airflow through the airbox.

Stock Airbox Chamber Test
Stock Airbox Chamber Flow Test

This was not something that I’ve tested so when an opportunity arose I ran an experiment to see if a difference in airflow would result from blocking off the airbox chamber.

Test Procedure:

The stock airbox was assembled with an air filter and accordion hose and attached to the flow bench. The bench was operated at 28″ of H2O and the airflow through the airbox was recorded.

The airbox was then opened and a piece of foam was inserted into the chamber at the back of the airbox to block it off. The airbox was re-assembled and attached to the flow bench for a repeat of the test.

Stock Airbox Chamber Test - Foam Insert
Stock Airbox Chamber Test – Foam Insert

Test Results:

There was no measurable difference in airflow between the two airbox configurations within the margin of error of +/- 2 CFM.


Airflow measurement made with a flow bench did not provide any evidence to indicate that the chamber opening at the back of the stock GTI airbox has an effect on airflow through the airbox.

6 thoughts on “Stock Airbox Chamber Flow Test”

  1. Hey Jeff,

    I already did this mod for my Cupra.

    Why I did it? I was expecting to reduce the air turbulence and increase sound.

    What are you thoughts on that?

    Paul C.

    1. I suppose it could make a change to the sound.

      I doubt it would change the amount of turbulence in any appreciable amount since there was no change to the amount of airflow through the box.

    2. Thank you for doing this test! I’ve also experimented blocking this off, recording audio samples before and after in both the engine compartment and interior cabin. With my ears I could hear no difference. However, analyzing the recordings showed increased output of centered at roughly 220 hz with octave harmonics on both sides. Additionally, there was no change in pitch across engine RPM, but output level coincided with intake air volume, such as under boost. Im curious where the noise originates. So it turns out this is indeed a Helmholtz resonator designed to reduce this tape of intake noise.

  2. Hi Jeff,

    Love your analysis of the stock airbox. Based on all the various combinations you have measured, it looks like the narrowing duct and opening into the lower half of the airbox is the restriction.

    I would love to see a test of the total mod stock airbox without the lower part of the duct and ultimately if a larger single opening in the airbox with an oversized duct would make a difference? (versus the additional holes you have tried).



    1. Thanks Glen!

      Based on the test that I did where I removed the lower half of the airbox, and saw modest gains, I think making those changes would at best net a few more CFM.
      Half Airbox Test

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