Originally Posted by Juan_servomedia
What are you using to measure pressure? I know there is a gentleman who makes USB barometers that he has used to measure pressure on a vehicle in different areas.
So essentially the vents are using the low air pressure to pretty much suck the air coming through the nose out of the top? I haven't gotten around to taking the cover off from underneath the hood so I don't know what she looks like structurally. But I am guessing two smaller vents, rather then long ones, positioned towards the front of those two rounded shapes on the hood would be most efficient. I think that puts them right over the large area over where the intake and battery are? That would create a ton of sucking force right above the radiator and intercooler wouldn't it?
We use an electronic digital manometer for this purpose. However you can use a simple home made water level type manometer. Just tape a hose end to the location where you want to measure pressure, and drive the car. The more the water moves, the higher the pressure.
In regards to vent location, the high pressure air under the hood will escape through the vents. The lower the pressure on top the hood the greater the differential, and the faster air will escape. Of course if the vents are located where the pressure is higher than it is under the hood, then air will go into the engine bay through the vents, and not out! That's why some vents draw air in, and some draw air out.
When I get my new 124 I'll take some measurements with the digital unit and let you guys know what I find. Here is a picture of a home made type. As you can see this is cheap and simple, so there is no reason not to take measurements.