Sealing the hood vent?

[CSBM5]

Oh well. It's not worth going into car body aero again....

[Richbot]

Originally Posted by CSBM5

Oh well. It's not worth going into car body aero again....

Well you see Bernoulli’s 2nd Law of Thermaerodynamics states that the more holes in an aerodynamic surface, the faster it goes, commonly known as the Speed Hole Effect

[Scharbag]

1.21 Gigawatts!!!!!!

[LaLaRay]

lol if i open the other vent up, rain wont destroy anything right ?

( Nobody shoot ! nobody shoot ! )

[Mvy]

Originally Posted by LaLaRay

lol if i open the other vent up, rain wont destroy anything right ?

( Nobody shoot ! nobody shoot ! )

Its not well set up to drain on that side like it is on the other side. It would dump onto the plenum and run off to the sides. Since everything seems to have some purpose, Id worry it could put water into electrical bits if you pull the passenger side rubber gasket.

[Redd]

If you all want to know what the hood vent does, tie a string to it and go for a drive. If the string goes into the vent, it's an inlet. If it goes out, it's an outlet.

No need for aerodynamics and boundary layer effects and pressure zones. Lol

[Law]

Originally Posted by Redd

If you all want to know what the hood vent does, tie a string to it and go for a drive. If the string goes into the vent, it's an inlet. If it goes out, it's an outlet.

No need for aerodynamics and boundary layer effects and pressure zones. Lol

This was already addressed in the first two pages.
String, being a solid object, and water, being a liquid, do not contain the same properties as air, which is what aerodynamics and pressure zones are designed around.

It may sound like irrelevant fancy-talk to you, but that's the cost of absorbing and understanding the details about the true function of something like this.

[Redd]

Strings have been, and still are, used to examine airflow around aerodynamic bodies. Boundary layer theory applies. If the string is sitting in a boundary layer, it would simply not move.

Bear in mind, you're not really examining the property of air, rather the flow of air. You just want to know what the air is doing at a particular location - is it moving in a certain direction, or sitting still (boundary layer effect).

I'm no automotive expert but I did spend 4 years studying aerodynamics as a part of my aerospace engineering course. My best guess is that the hood vent is an outlet. Why?

1. Assuming normal hood without vent, airflows quickly over the surface. Bernoulli tells us where there is high velocity airflow, pressure is lower. So there should be a nett negative pressure zone (or a slight vacuum/suction) above the hood.

2. You have a vent/hole with air movement into/out of the vent. So boundary layer effect does not matter as there is air movement at the particular spot of the hole.

3. So the real question is whether the airbox is pressurised or not. Assuming the two frontal air intakes do what they're supposed to do, then I think there is nett positive pressure inside the airbox. If not, this whole ram air effect discussion is moot anyway.

4. If there is nett positive pressure in the airbox, Bernoulli also tells us that air will look for an outlet at a lower pressure location. Which, so happens to be the hood vent.

5. The above does not take into consideration of the suction/vacuum produced by the engine at max rpm though. It's entirely possible that the suction is strong enough to turn the hood vent into an inlet. In either case, a simple string will tell you where the air flow is moving.

My 2 cents.

[Law]

Originally Posted by Redd

Strings have been, and still are, used to examine airflow around aerodynamic bodies. Boundary layer theory applies. If the string is sitting in a boundary layer, it would simply not move.

Bear in mind, you're not really examining the property of air, rather the flow of air. You just want to know what the air is doing at a particular location - is it moving in a certain direction, or sitting still (boundary layer effect).

I'm no automotive expert but I did spend 4 years studying aerodynamics as a part of my aerospace engineering course. My best guess is that the hood vent is an outlet. Why?

1. Assuming normal hood without vent, airflows quickly over the surface. Bernoulli tells us where there is high velocity airflow, pressure is lower. So there should be a nett negative pressure zone (or a slight vacuum/suction) above the hood.

2. You have a vent/hole with air movement into/out of the vent. So boundary layer effect does not matter as there is air movement at the particular spot of the hole.

3. So the real question is whether the airbox is pressurised or not. Assuming the two frontal air intakes do what they're supposed to do, then I think there is nett positive pressure inside the airbox. If not, this whole ram air effect discussion is moot anyway.

4. If there is nett positive pressure in the airbox, Bernoulli also tells us that air will look for an outlet at a lower pressure location. Which, so happens to be the hood vent.

5. The above does not take into consideration of the suction/vacuum produced by the engine at max rpm though. It's entirely possible that the suction is strong enough to turn the hood vent into an inlet. In either case, a simple string will tell you where the air flow is moving.

My 2 cents.

That's more like it and admittedly completely unexpected!

That's an interesting hypothesis you have.
Still, it bears the question if there might be a concrete reason BMW in its official documentation classifies the opening as an inlet.
I'm seeing that it is indeed possible based on your analysis.
Perhaps one way to find out (besides your proposed string experiment) would be, like you mentioned, to determine whether or not the airbox is pressurized.
The other possibility where I agree with you is also that the engine generates enough vacuum to render the opening an inlet...in that case perhaps that's where your string experiment would come in handy.

Hopefully more people can share their knowledge here.

Thanks for sharing yours.

[roastbeef]

i'm just a nobody with a keyboard- no studying of anything related to this. but to rebut the claim that leaves, trash, and water blow out of there isn't the same as airflowing out of there... it just doesn't make sense to my simple brain. the air is what is causing those items to blow out. those items are heavier than air. the air is what is carrying them out. and somehow the air is carrying them out and turning around to go back inside the intake?

[LaLaRay]

Originally Posted by Mvy

Its not well set up to drain on that side like it is on the other side. It would dump onto the plenum and run off to the sides. Since everything seems to have some purpose, Id worry it could put water into electrical bits if you pull the passenger side rubber gasket.

Exactly ok just making sure. Cause somebody else said they did and im like uhhh idk i dont got time to be f***ing anything up lol

[roastbeef]

I've had my secondary hole [giggidy giggidy] wide open for the last three years and I park my car outside. Nothing has happened. I drive in the rain, too.

[Mvy]

Originally Posted by roastbeef

I've had my secondary hole [giggidy giggidy] wide open for the last three years and I park my car outside. Nothing has happened. I drive in the rain, too.

OMG thats so bad.......

[Mvy]

Originally Posted by M3Post

Here's what worked for me -> from another thread..



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_

I think the point of this thread is what it does when you mess with the ports on an NA car with a factory airbox vs the supercharged set up.

I did a version of this to reduce whine with my Harrop supercharger set up which prompted me to answer no covering the vents doesnt hurt anything. As you could see from the replies of some posters much smarter than me, we see thats not the same situation when you are NA. When you have a giant air pump pushing mass quantities of o2 into the motor, risk of stalling or other problems covering the hole are moot. Seems to be some point to the ports when you have not modded the car. On a side note to your comment, I dont have a factory airbox so it didnt do anything close to a 30% (maybe 10%)reduction in whine which does sometimes makes me want to go back to the factory airbox. Sometimes. . Honestly, cold start delete was far more impactful in making my morning commute out of the neighborhood more quiet. Ive discovered after two years of ownership on the Harrop, the sound is what makes the experience. I actually miss that noise when not driving my car.

[roastbeef]

Race m3's had intake restrictors far smaller than the oem intake outlet on the hood and made more power. "Choking" it is not an issue.

[roastbeef]

460hp is made with the air going through these restrictors.

[Dr. Dre]

What happens if water goes into that inlet?

[Redd]

Originally Posted by Dr. Dre

What happens if water goes into that inlet?

Nothing. At best it flows out the bottom. At worst, it wets the air filter a little.

[G80indy]

Dumb

[E90beemerfan]

I cannot see this being very complicated. The air vent on the hood is a vent to let air out when the car is moving forward and may let some air in when stationary or reversing. When driving forward, the air box is fed with air from the front left fender opening and a small shoot in the kidney grill.

The only place the air can escape is through the engine and through the air vent on top of the hood.

The air the engine is not consuming will escape through the hood vent. If the engine needs more air than what is fed through the air inlets in the front of the car, this would clearly be a design fault. If the engine would have to depend on air coming through the very small opening on top of the hood, I think the engine would choke easily.

Please open the airbox, look at the relatively big air shoot entering the airbox and the small opening in top of the box (hood vent). Realistically, which direction do you think the air is flowing when the car is moving forward in 60 miles/h?

By the way, the pressure inside the airbox will be higher than on top of the hood as long as the car is moving forward at normal cruising speeds (higher velocity of the air passing over the hood than the air passing through the air box, as long as the engine is not choking with lack of air).

During a rainy day, water droplets stuck on top of the air vent on the hood will be taken by the air flowing through the hood vent. Some of the water will naturally end up on the wind shield. Even though we cannot see the air, we can see the water coming up from the air vent, which is proving the direction of the air flow.

Water entering the air vent is drained in the bottom of the air box and will not be in touch with the air filter.

Please believe me

[Scharbag]

It also goes to simple physics.

Why would air "want" to do a hard 90 degree turn, enter that vent and then go in the opposite direction of travel to enter the engine? Significant force would be required for this to happen. Physics rarely gives you anything for free.

One reason air may "want" to enter that hood vent is negative pressure relative to the external air. The only way that the vent relative pressure can be negative is if the engine is causing a vacuum in the air box. Only way that is possible is if the other 2 air-paths are insufficient for flow (unlikely given their size).

The hood vent is not a required source of air for the engine. Best case, it allows a continuous stream of airflow through the airbox to lower temperatures and ensure the engine is always eating the coolest possible air. Like a CAI, without the worry when puddles are near.

Cheers,

[burtonsbs5114]

I'm sure the engineers at BMW M GmbH, who spent years designing the engine/car, know nothing about physics and aerodynamics so when they placed a hood vent to act as an air inlet it actually works as an outlet. Makes perfect sense.