This post is a continuation of an investigation into an airbox modification on the stock Mk7 GTI airbox by cutting slots into the side of the lower half of the airbox.
In a previous post, the effect on airflow through the air intake was measured. The slots were shown to provide a small increase in airflow compared to the unaltered GTI airbox.
This post will address the effect on the air temperature exiting the airbox with the addition of the openings.
The GTI airbox has a temperature probe inserted at the outlet of the airbox to measure the temperature of the air being fed to the turbocharger compressor.
Of interest is the air temperature leaving the airbox during a full-throttle pull. It is at this time that the air temperature can affect the operating efficiency of the turbocharger.
The GTI is driven for several minutes and then a series of full-throttle pulls are made in third gear. The pull begins with no boost pressure being created and engine speed around 2000-2500 rpm. The accelerator is fully depressed and the car accelerates until the engine rpm is around 6300 rpm.
The air temperature leaving the airbox and the outside air temperature is recorded during this pull.
The airbox with slots is tested first. The air temperature throughout the test is shown on the chart:
The stock airbox without the slots is then tested. During the drive, the GTI was refueled, and only the portion of the drive after the refueling is shown on the chart.
Because the engine had been operating for several minutes, and then the car sat while refueling, the starting temperature is elevated compared to the slotted airbox, where the GTI had just started with a cold engine.
The next chart shows the airbox temperature for each configuration during the pulls and the ambient air temperature.
The airbox temperature during the pulls is shown with the solid lines, blue for the slotted airbox and red for the closed airbox. The dashed line shows the ambient air temperature, blue during the slotted airbox test and red during the closed airbox test.
Also shown at the bottom of the chart is the boost pressure during each pull.
To account for the slight difference in ambient air temperature between the two test sessions the airbox air temperature is compared with the ambient air temperature to calculate a temperature delta.
The next chart shows the median delta for the two test cases.
Again the slotted airbox is shown with the blue line and the closed airbox with the red line.
The stock airbox modification of making additional openings in the lower half of the airbox appears to increase the air temperature leaving the airbox slightly during normal operation. During the drives that were recorded this modification was correlated with approximately five degrees higher air temperature out of the airbox.
During the full-throttle period of the test, the air temperature exiting the airbox is approximately four degrees greater than the modified stock airbox.
The result during the full-throttle pull may be a function of the initial condition at the start of the pull. Because the unmodified airbox had a lower delta during routine driving, this could be factoring in the full-throttle pull. An additional test with a similar starting temperature delta might provide information on this theory.
The stock airbox modification with additional openings had predictable outcomes. The airflow through the airbox increased slightly, and the air temperature exiting the airbox increased slightly.