The question in the title prompted this post after seeing it regularly being phrased as a statement; “a modified stock airbox is as good as an aftermarket intake“.
After having flow tested a number of aftermarket intakes and various modifications to the stock airbox the results show that the stock airbox can perform close to the level of an aftermarket intake, but not quite equal.
With a flow rate of 383 CFM the modified stock airbox comes up short against the aftermarket intakes that are flowing around 400 CFM. This is a relatively small difference and is unlikely to be detectable except on a well controlled test, the type the flow bench allows.
What is important to note is that each of the intakes tested includes a TIP, turbo inlet pipe / elbow, that joins the airbox to the stock compressor housing.
The opening in the stock compressor housing has a diameter of approximately 47 mm.
A question this raises, what happens with a turbocharger that doesn’t have an inlet that is the same size as the stock IS20 or IS38? Is the stock airbox still “almost as good as an aftermarket intake?“
The BorgWarner EFR7163 is an example of a turbocharger that is available for use on the Mk7 GTI and Golf R that doesn’t have a compressor inlet of the same size as the IS20/38. The EFR7163 has an inlet that is approximately 59mm in diameter.
While this specific size inlet has not been tested, a number of the intakes have been tested using an adapter that is a couple millimeters larger than the EFR turbo inlet and also eliminates one of the bends in the stock intake.
Flow results using the larger adapter are shown on the next chart:
With the larger-straighter adapter the stock airbox modified with an aFe drop in panel air filter and with the removal of the “snow” grate flows 527 CFM. The top performing aftermarket air intakes flow approximately 630 CFM. The roughly 100 additional CFM of airflow with the aftermarket intakes is an approximate 20% gain.
Comparing the flow performance of the modified stock airbox with aftermarket intakes shows that the results depend on the flow performance of the adapter that the airbox is joined to the turbocharger with. These results show there is not a one-size-fits-all characterization of how the modified stock airbox performs compared to aftermarket intakes.
If the vehicle is equipped with a turbocharger that has a larger than stock size turbo compressor inlet there can be substantial flow differences between air intakes.