The "puckering" only occurred when tested at 20" of H20, not at 10". The actual airflow is then converted via a graph to 28". In the case of the 10" tests, there is less air actually flowing through the unit, so the puckering was not apparent. Still, the unit flowed better without the rubber when tested at 10" of H20. The corrected 10" reading were 607cfm with, and 642.5 cfm without the rubber. At 20" the readings were 586.69/632.5. That says to me that the rubber over the bell mouth of the adapter is doing something to the airflow.
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