[freebeard]

S-okay.

You can make a differential manometer with a long piece of fuel line with a bubble of liquid at the bottom of a loop. The bubble moves toward low pressure.

Your baseline should be what happens underneath with the hood gone completely. A vent in the right place will lower underhood pressure, maybe draw cooling air into the engine compartment. A 'vent' in the wrong place will act like a scoop.

Another example was the MGA, two oval vents halfway back. Whichever way it went, they wanted the opposite.

Proven techniques to manage underbody airflow include bellypans, diverters and wheel spats. Diverters lower the stagnation point, but act counter to approach angle, which may be important in a Jeep.