Background:
After taking measurements of “ram air” pressure generated by a few different intakes, of which there was very little, I decided to measure the ram air pressure outside the engine compartment.
The easiest way to do this was to use the existing mount for the averaging flow sensor (AFS), shown below.
Installation:
A GoPro suction cup mount was suitable for attaching the AFS to the GTI hood.
The location of the AFS on the hood probably matters somewhat to the reading it takes, but I chose a spot convenient for running the pressure lines to a differential pressure sensor.
My thought was to place it over the location where the inlet to the airbox is and far enough back along the hood for the airflow to have finished being disturbed by the irregular shape of the front of the car.
I didn’t plan to move the mount around to see if the readings changed appreciably in other spots. The goal was to gauge the magnitude of “ram air” pressure outside the engine compartment to compare with the readings taken at the outlet of the air intake.
Testing:
Testing consisted of driving at various speeds, with a maximum of 70 mph, and logging the air pressure.
Comparison data from the air intake was recorded using the setup below:
Note: The Eventuri picture illustrates the setup; the intake ram air data used for comparison was logged using the stock GTI airbox.
Test Results:
The chart below shows the vehicle speed (mph) and corresponding air pressure (Inches of H2O).
The following chart shows the external ram air pressure (Orange line) compared with the ram air pressure at the outlet of the air intake (Dark blue line).
Conclusions:
The ram air effect outside the GTI engine compartment was measured while driving on the street at speeds up to 70 mph. The pressure readings were compared with the ram air measurement at the outlet of the stock GTI airbox.
The external ram air pressure varies more with vehicle speed than the interior ram air pressure and was measured to be 250-450% greater than the interior ram air pressure.