After devising a means to block off all but one runner I went through and retested the stock intake manifold this time recording each individual runner’s airflow at 10″ H2O and also the airflow of the entire manifold with all of the runners open.

During the setup and verification that everything was sealed I discovered a possible cause for the low readings on the two runners nearest the throttle body.  There’s a port that connects to the F-hose that is set down in the adapter base, shown at the end of the arrow in the picture below.  The end inside the manifold opens up right behind the IAT sensor, which is right around where the openings for the first two runners are. This was left unplugged during the preliminary measurements.

My suspicion is that when I remeasure the air velocity at those runners, with the port capped off, the readings across all six will be about equal.  I’m holding off on the velocity measurements while I obtain a velocity probe better suited to measuring airflow when the flowbench is configured to exhaust air.

The results of the individual runner measurements, measured in CFM, are shown on the image below.