The velocity stack increases airflow velocity to aid power production, and it also measures the airflow going in. The ventilation hood overhead provides cool air, which also contributes to power. Of course these aren't real-world conditions. A dragstrip on a hot summer day is very different than a dyno cell. You may also need to change jet sizing and ignition timing when you go to the 'strip because conditions (temperature, humidity, and barometric pressure) can change dramatically.