Sealing the hood vent?

[monterey]

Has anyone experimented by sealing the hood vent to force more air into the engine? I know when I go on the freeway in the morning the condensation from the morning dew erupts likes Mount St. Helen onto my windshield.

[Mvy]

Originally Posted by monterey

Has anyone experimented by sealing the hood vent to force more air into the engine? I know when I go on the freeway in the morning the condensation from the morning dew erupts likes Mount St. Helen onto my windshield.

Yup. It causes 0 issues to do so. Some people think its how air gets into the engine. Its not. That happens from ports in the bumper that feed into the motor.

[roastbeef]

Leave it open. It keeps intake air temps down by allowing the intake to be constantly flushed with cool air.
The air flows in through the front, the engine takes what it needs, and the rest flows out through the hood vent. This is why you see dew/rain flying out... it's an outlet.

[monterey]

But it cant work as a ram air intake if its an open box. I guess it would be difficult to mimic on a dyno since you need the car to be moving for the ram air effect. But i guess they can strap a leaf blower and cover the hood vent lol

[roastbeef]

Originally Posted by monterey

But it cant work as a ram air intake if its an open box. I guess it would be difficult to mimic on a dyno since you need the car to be moving for the ram air effect. But i guess they can strap a leaf blower and cover the hood vent lol

If it was better, they would have done that. It's more complicated to do it the way it came.

There are a few "ram air" options, but those systems are optimized for that, and are often making room for other accessories.

[M43S7RO]

Thats assuming that closing it wouldn't suffocate the engine for air. I would think that when standing still or slow speed, the car would be more likely to struggle for air with the top vent closed.

Closing it would truly only be beneficial at high speed if a ram effect could produce positive intake pressure. Given the bottleneck in the lower bumper intake and small intakes at the radiator, I think not.

[Mvy]

Originally Posted by M43S7RO

Thats assuming that closing it wouldn't suffocate the engine for air. I would think that when standing still or slow speed, the car would be more likely to struggle for air with the top vent closed.

Nope. Not at all true. Try it. Ive had my vent plugged for years.

Open your hood and look at the bumper. You will see three air intakes feeding massive amounts of air freely into your intake. The hood vent its not an intake for air, its an output for sound/air.

[monterey]

Originally Posted by Mvy

Nope. Not at all true. Try it. Ive had my vent plugged for years.

Open your hood and look at the bumper. You will see three air intakes feeding massive amounts of air freely into your intake. The hood vent its not an intake for air, its an output for sound/air.

Why did you plug your vent out of curiousity.

[Law]

After over a decade, there's still so much misconceptions about the E9x M3's hood vent/inlet.

From time to time, the old wives tale of M3Post (that the vent is there for sound, or as an outlet) resurfaces and I find myself having to chime in.

Here is a diagram from Official BMW technical literature.


Not only is the vent officially designated as an "inlet" (not "outlet") but it is also noted as being one of the three primary sources of air for the intake system on the S65.

The combustion air enters the engine via three flow-optimized air guides. An air inlet is located on the left side of the engine hood when viewed in the direction of travel.

[///M Power-Belgium]

Interesting fact ...
The S65 engine breathes @ top speed *17,000 liters* of air per minute !

[Dr. Dre]

Originally Posted by Law

After over a decade, there's still so much misconceptions about the E9x M3's hood vent/inlet.

From time to time, the old wives tale of M3Post (that the vent is there for sound, or as an outlet) resurfaces and I find myself having to chime in.

Here is a diagram from Official BMW technical literature.


Not only is the vent officially designated as an "inlet" (not "outlet") but it is also noted as being one of the three primary sources of air for the intake system on the S65.

Game over.

[roastbeef]

Originally Posted by Law

After over a decade, there's still so much misconceptions about the E9x M3's hood vent/inlet.

From time to time, the old wives tale of M3Post (that the vent is there for sound, or as an outlet) resurfaces and I find myself having to chime in.

Here is a diagram from Official BMW technical literature.


Not only is the vent officially designated as an "inlet" (not "outlet") but it is also noted as being one of the three primary sources of air for the intake system on the S65.

Well let's talk about it. You think it stays an inlet over 30mph?

[Law]

Originally Posted by roastbeef

Well let's talk about it. You think it stays an inlet over 30mph?

Let's talk, but don't shoot the messenger.

But in a nutshell, yes.

I'm no engineer but an engine's induction is basically a huge vacuum under acceleration.
Anything that is piped into/routed toward the intake system is going to be sucking in air when the throttle is open.
This includes the hood inlet.

You're basically saying that at high-speeds, the hood inlet acts as an outlet, meaning you're saying that air escapes.
My point is that even if your assertions are true, it is a minimal & unintentional side effect at best since BMW designed it as part of the induction system, as one of the 3 air inlets for introducing/piping air towards the air filter element.
Its (the hood inlet) primary (and only) function intended by design is to supplement the S65's induction system.

The diagram is from the people who built the S65 themselves and illustrates this perfectly and even gives detailed descriptions.

The fact that dew/water droplets sometimes splash out from the inlet doesn't mean air itself is being routed out.
And for the record, the hood inlet also contains a "drainage" system for routing water away from the air filter.

If you want to dispute the diagram or what it claims, you can take it up to BMW or run your own independent scientific tests.
But I'd say, the diagram is pretty conclusive.

[jdamore]

If someone wants to test this theory for fun, I have calibrated equipment that can read pressure down to 0.0001"wc.

For a second, i thought I would test it myself but, I'm supercharged!! Lol

My opinion.. I trust in the engineers that they came up with the best solution. You're probably all correct. Highway speeds at low RPM, the s65 is sucking in less air than is being "rammed" in. This is when the air box is positive. I too have seen moisture coming out of the hood vent. At (or near) redline, as Belgium says, is a lot of freaking air! This is when the air box becomes negative pressurized and sucks in from everywhere.

If you supercharge, you can seal up the hood vent!

[pbonsalb]

It would not make sense to have 3 inlets at low speed and 2 inlets at high speed.

[Scharbag]

It would be nice to have some data on this if someone could gather it. Based on my travels behind a grass hauling truck (only the dairy industry...) that filled my filter box, a lot of air (and grass) comes out of the hood vent under WOT at highway speed...

But data does not lie.

Cheers,

[roastbeef]

i've seen that diagram before, and i think they are over simplifying it for public consumption. most people aren't going to obsess over it like we do.

my sole argument is based on this- i'm tall and i can see the driver's side hood vent. we've all seen the water come out and hit the windshield. from time to time, i see leaves, pieces of trash, pine needles, and string pinned up against the hood vent screen. this is frustrating to me because i can see it when i'm driving, but it falls back into the airbox when i slow down.
if there were suction at all, why would anything get pinned up against it? i think it helps act as an "open" air intake at lower speeds, and it transitions around 30-35mph into a flow-through design.

[Richbot]

Wut

[Law]

Originally Posted by roastbeef

i've seen that diagram before, and i think they are over simplifying it for public consumption. most people aren't going to obsess over it like we do.

Normally, I'd agree if this was some sort of promotional marketing material like a product brochure or an advertisement, but it isn't.
This was from an internal training document (112 pages) detailing every last inch of the E9x M3 & S65B40 in particular, including the induction system, VANOS, camshaft drive, cylinder head, piston cutaway diagram, oil supply distribution chart, electrical/terminal system, etc., all labeled with its functions specifically detailed with absolute clarity.
You said you've read it, so you should know there's no marketing bs in there.
It's all very technical and not some marketing press release crap talking about seats and mirrors written for public consumption.
So while it is concise and simplified for training purposes, that doesn't discount any part of it as non-factual.

That's why I said that unless there is some other verified independent scientific experiment/documentation on this, the information from this document from BMW remains the most substantial evidence we have that details the workings of the S65 and the hood vent in question.
I don't mean it in a condescending manner, just that we should keep it as scientific and fact-based as possible instead of drawing conclusions based on observations.

Originally Posted by roastbeef

my sole argument is based on this- i'm tall and i can see the driver's side hood vent. we've all seen the water come out and hit the windshield. from time to time, i see leaves, pieces of trash, pine needles, and string pinned up against the hood vent screen. this is frustrating to me because i can see it when i'm driving, but it falls back into the airbox when i slow down.
if there were suction at all, why would anything get pinned up against it? i think it helps act as an "open" air intake at lower speeds, and it transitions around 30-35mph into a flow-through design.

I don't doubt for one second that the phenomenon you observed exists.
I've personally experienced the same thing happen on various occasions.

Now, about that.
I take it that while we're both E9x M3 and S65 nerds, none of us are engineers, so I'll just put this in layman's terms.

Leaves, debris, and water in its liquid form, have different properties than atmospheric air.
The vacuum effect, that all engines produce primarily deal with air.
Yes, if all the openings of an intake are submerged below water, an engine can hydrolock, but in normal atmospheric pressure, it is designed to suck in air and air only.

Liquid water and other debris that is visible with the naked eye is more dense/heavy than air is, which explains why the vacuum effect does not seem to effect those things visible to the naked eye.
Water vapor, and smaller particles of lighter density, are much more susceptible, and are sometimes sucked in with the atmospheric air through the vacuum.
This can partially explain why engines make less power in high-humidity, and why air filters get clogged with microscopic particles that also hinder engine power/combustion.

What I'm basically trying to say here is that correlation does not imply causation.
The fact that you witness water droplets or leaves/debris flying out of the vent doesn't make the vent an outlet by design.
It is still part of the "vacuum" that is the induction system of the S65.
Simply put, those things fly out because of other variables, vehicle speed, engine speed, and the throttle closing (which closes the vacuum).

Now to answer your dilemma in a very dumbed down manner, let me present you with a metaphorical example.
For all intents and purposes, the vacuum I keep mentioning also exists in other processes.
One of the most instinctual is breathing.
The lungs are basically the induction/intake system of the human body.
Now, there are two openings where air/oxygen enter and are channeled into the lungs, your mouth and nostrils.
Every now and then, you get a runny nose.
You can still breathe through your nostrils, but you have a combination of phlem/nosehairs/boogers flowing out of your nose.
If you happen to be going high-speed with the wind blowing in your face, let's just say, on a roller coaster, it might even find its way to your face somewhere.
How is it, that oxygen is able to enter your lungs, but your snot does not?
Because the vacuum effect created by your lungs was "designed" to inhale air and air only.
Sure occasionally stuff gets trapped by your nose hairs (air filter lol), but otherwise, it's all air.

[CSBM5]

The hood vent is in the front portion of the region of boundary layer separation and static pressure recovery that all cars experience from about that location on toward the windshield. Someone setup probes on an E90 hood and attempted to measure static pressures years ago now and showed the obvious (well, obvious to a fluid dynamics engineer)...they didn't perform an engineering level test of course since that would require drilling the hood and precisely locating the static pressure probe opening level with the surface, etc.

However, you're going to have a very hard time forcing air to flow out of a vent in the high static pressure zone near the back 1/3 portion of any hood. The faster the car goes, the higher the static pressure in that zone. Hence the reason you see almost all cars have cabin air inlets in this area and also things like "cowl induction" hoods, etc. That dew can emit from the vent is not surprising given water is almost 800 times denser than air. You don't do fluid flow design for a fluid with such dramatically different properties -- in other words, a "system" designed around air will not function in the same manner when you replace the air with water. Similarly, a system designed for water flow will not function in the same way if you replaced the water with mercury. Reynolds numbers rule, and viscosity and density play a large role in fluid behavior.

The M3 vent is not perfectly located in that region as I mentioned, but it's clearly in the region of the hood where the surface curvature has flattened out and static pressure is rising rapidly. At the best location right near the hood to windshield junction, static pressure recovery is near 1.0. BTW, pressure recovery is referring to the static pressure at a location versus stagnation point pressure. The stagnation point is the spot on the car's front where air velocity is zero (right near the center point of the front end), and pressure is at a maximum. In aerodynamic analysis, it's common practice to reference the ratio of local static pressure to stagnation point pressure.

[Mvy]

Originally Posted by Law

After over a decade, there's still so much misconceptions about the E9x M3's hood vent/inlet.

From time to time, the old wives tale of M3Post (that the vent is there for sound, or as an outlet) resurfaces and I find myself having to chime in.

Here is a diagram from Official BMW technical literature.


Not only is the vent officially designated as an "inlet" (not "outlet") but it is also noted as being one of the three primary sources of air for the intake system on the S65.

Thanks for this. For a factory airbox, I stand corrected.

I probably should take into account my car set up when answering this question. I have a SC so getting air into the motor is not an issue. I don't have a factory intake. I have a custom fabricated steel intake elbow and cone filter with an AFE stage 2 airbox. The whole things is open on the bottom. I am sure I am getting more than enough air and have never had any issue with plugging the vent. It reduced the cabin noise level of the supercharger whine. Hence no issues with plugging the vent.

[Law]

Originally Posted by CSBM5

The hood vent is in the front portion of the region of boundary layer separation and static pressure recovery that all cars experience from about that location on toward the windshield. Someone setup probes on an E90 hood and attempted to measure static pressures years ago now and showed the obvious (well, obvious to a fluid dynamics engineer)...they didn't perform an engineering level test of course since that would require drilling the hood and precisely locating the static pressure probe opening level with the surface, etc.

However, you're going to have a very hard time forcing air to flow out of a vent in the high static pressure zone near the back 1/3 portion of any hood. The faster the car goes, the higher the static pressure in that zone. Hence the reason you see almost all cars have cabin air inlets in this area and also things like "cowl induction" hoods, etc. That dew can emit from the vent is not surprising given water is almost 800 times denser than air. You don't do fluid flow design for a fluid with such dramatically different properties -- in other words, a "system" designed around air will not function in the same manner when you replace the air with water. Similarly, a system designed for water flow will not function in the same way if you replaced the water with mercury. Reynolds numbers rule, and viscosity and density play a large role in fluid behavior.

The M3 vent is not perfectly located in that region as I mentioned, but it's clearly in the region of the hood where the surface curvature has flattened out and static pressure is rising rapidly. At the best location right near the hood to windshield junction, static pressure recovery is near 1.0. BTW, pressure recovery is referring to the static pressure at a location versus stagnation point pressure. The stagnation point is the spot on the car's front where air velocity is zero (right near the center point of the front end), and pressure is at a maximum. In aerodynamic analysis, it's common practice to reference the ratio of local static pressure to stagnation point pressure.

Excellent post.

This is what exactly what I was trying to explain and you've done so very eloquently.