Hood intake air duct for air entry or exit?

[speedaddictM3]

What is the purpose of the air duct on the hood? Is it to let air into the air intake or out of it? There is no air scoop on it and it lies parallel to the direction of air flow over the hood. I'm thinking according to the Bernoulli principle at high speeds air flow would lead to a pocket of lower air pressure on top of the hood opening. It seems that this would help suck hot stagnant air out of the airbox and let cold fresh air enter more easily from the intake ducts at the front bumper. When I drive on the highway during light rain I notice tiny droplets flying out of that opening, not into it. So that seems to agree with my hypothesis that air is flowing out of the duct not into it. Any ideas or thoughts?

[kenwelch]

From the BMW 4th Generation Introduction manual:
"New "Powerdome" aluminum hood lid with air apertures. The aperture on the left when viewed in the direction of travel is used for incoming engine air, and the right-hand aperture offers optical symmetry."

[SenorFunkyPants]

Both.

[///Matthew]

Originally Posted by kenwelch

From the BMW 4th Generation Introduction manual:
"New "Powerdome" aluminum hood lid with air apertures. The aperture on the left when viewed in the direction of travel is used for incoming engine air, and the right-hand aperture offers optical symmetry."

nice way of saying it's fake

[Jive]

At WOT it sucks air in. Otherwise, it let's the excess out. Tie a ribbon to the mesh to test.

[rev9k]

Originally Posted by speedaddictM3

What is the purpose of the air duct on the hood? Is it to let air into the air intake or out of it? There is no air scoop on it and it lies parallel to the direction of air flow over the hood. I'm thinking according to the Bernoulli principle at high speeds air flow would lead to a pocket of lower air pressure on top of the hood opening. It seems that this would help suck hot stagnant air out of the airbox and let cold fresh air enter more easily from the intake ducts at the front bumper. When I drive on the highway during light rain I notice tiny droplets flying out of that opening, not into it. So that seems to agree with my hypothesis that air is flowing out of the duct not into it. Any ideas or thoughts?

you're right, it's like an escape for air moving through the box. Anyone who thinks its taking air in with regards to moving and the force of the other intake tracts is wrong.

front under grilles= to intake
left side opening duct under headlamp= to intake

the left side hood vent is a relief for the air..

You could close it up and maybe see some positive intake pressure under a VERY high speed run

[CanAutM3]

Originally Posted by speedaddictM3

What is the purpose of the air duct on the hood? Is it to let air into the air intake or out of it? There is no air scoop on it and it lies parallel to the direction of air flow over the hood. I'm thinking according to the Bernoulli principle at high speeds air flow would lead to a pocket of lower air pressure on top of the hood opening. It seems that this would help suck hot stagnant air out of the airbox and let cold fresh air enter more easily from the intake ducts at the front bumper. When I drive on the highway during light rain I notice tiny droplets flying out of that opening, not into it. So that seems to agree with my hypothesis that air is flowing out of the duct not into it. Any ideas or thoughts?

This has been discussed plenty in other threads. IMO, your assumption is correct. The opening on the hoop is actually to let air out to prevent heat soak in the intake tract and filter plenum. The other two inlets (below the bumper on the left side and behind the kidney grills) are large enough by themeselves to accomodate WOT air flow.

[peeti]

After heavy rain, accelerate - you'll see the functional one spitting water.

[SenorFunkyPants]

If there is low pressure in the air intake it will suck in air from every opening....ie if stationary or low speed. If there is positive pressure due ram air then it will exit through the hood outlet. Its why scoops make no difference.

[CanAutM3]

Originally Posted by SenorFunkyPants

If there is low pressure in the air intake it will suck in air from every opening....ie if stationary or low speed. If there is positive pressure due ram air then it will exit through the hood outlet. Its why scoops make no difference.

You are correct.

However, from a design perspective, I don't think that the primary intent is to allow more air in; even if does it by default when there is no ram. I still believe the primary function of the opening in the hood is to extract hot air out of the filter plenum.

[325rider]

Originally Posted by Dashman

It's not fake. It's a real mesh grill duct that exists in time and space. It's just not functional

[Candide13]

Originally Posted by peeti

After heavy rain, accelerate - you'll see the functional one spitting water.

Ah, now I understand why I could see the stem of a trapped leaf sticking out of that mesh! It wasn't Houdini who put it there..

[Linz3]

Generally, I think the hood duct can act in one of 2 ways:

1) help suck air through the other vents, and keep airbox air cooler -- "pump", based on negative pressure on the hood

2) extra air at WOT -- at some point the flow may reverse, and actually go into the airbox, then engine, from the hood vent

Overall, doesn't do much, but let some of the lovely induction sound out!

The passenger side vent is, of course, the most efficient-- it is perfect in its functional design to do nothing!

There's another good thread here: http://www.m3post.com/forums/showthread.php?t=135075

[W///]

Originally Posted by ///Matthew

nice way of saying it's fake

Mine's not fake haha