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Old 05-26-2011, 03:01 PM   #1
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Default Hood scoop vs extraction vent?

the car and setup: 1990 1.6 t25 - 6lbs (FMIC)

After reading through a large bit of Curly's post (https://www.miataturbo.net/showthrea...ght=hood+scoop) , and doing some googling, I'm in need of some help, from the physics standpoint.

What are the benefits of having a hood scoop over an extraction vent?

I have multiple friends with raised hood studs in the back to let hot air escape, but, why do so many oem factory cars prefer hood scoop over that of the extraction vent? Is it purely looks?

I'm not refering to unfunctional hood scoops, I'm refering to both functional, but it seems scoops are more popular than vents?

I'm looking to cool down the engine bay a little more. Not that it is a 'huge' problem now but if I'm aggressive driving for an extended period of time it does rise [both fans on, I have the AC fan on a toggle switch inside my cabin- even though it has practically no effect unless you are going below ~20mph]

Also, I haven't a clue on what high/low pressure does and why that would matter? Placement of the scoop/vent would matter, common sense in my opinion, but how does the pressure effect which scoop and/or vent you would use, and the location of it?

Thanks for your help!
Brendan
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Old 05-26-2011, 03:05 PM   #2
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The scoops are for looks. The last functional scoop I've seen was from the old WS6's. With a few simple mods, they were truly a ram air.

The hoods raised in back is CR.net stuff.
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Old 05-26-2011, 03:08 PM   #3
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most competent WS6 owners will tell you that the hood scoops are no where close to being "ram air"
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Old 05-26-2011, 03:12 PM   #4
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Back of the hood is a high pressure area, it's not letting air out, it's forcing it in, to the detriment of efficient flow through the radiator. Why do you think Mazda put a gasket across the back of the hood? Hint: it ain't just for grins.
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Old 05-26-2011, 03:12 PM   #5
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Quote:
Originally Posted by chpmnsws6 View Post
The scoops are for looks. The last functional scoop I've seen was from the old WS6's. With a few simple mods, they were truly a ram air.

The hoods raised in back is CR.net stuff.
well I don't want to raise my hood in the back, and personally, I don't feel like spending more than 5 minutes on it because my hood (oem studs?) will require more time than the expected few minutes it took to put them on my friend's 240.

Okay, well, extraction seems to be alot more common looking among more pictures, but why? and would it have any negative effect if you put it in the wrong location?
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Old 05-26-2011, 03:22 PM   #6
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If you have a tube of toothpaste and you squeeze it with the cap off, thereby increasing the pressure inside the tube where does the paste go and why?

Why do you think this might be important to how air moves in and around your car?
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Old 05-26-2011, 03:30 PM   #7
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Quote:
Originally Posted by BenR View Post
If you have a tube of toothpaste and you squeeze it with the cap off, thereby increasing the pressure inside the tube where does the paste go and why?

Why do you think this might be important to how air moves in and around your car?
Are you a teacher? Lets see how good his critical thinking skills are Mr. R.
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Old 05-26-2011, 03:43 PM   #8
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Old 05-26-2011, 05:42 PM   #9
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Quote:
Originally Posted by Braineack View Post
most competent WS6 owners will tell you that the hood scoops are no where close to being "ram air"
Odd. We always saw 3-5mph gains from the BGRA's before they quit making them. It was by no means FI, but it did show gains
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Old 05-26-2011, 06:38 PM   #10
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OP - you might want to narrow what information you're actually looking for. If you are looking to expand your physics knowledge on this topic as you indicated, you're going to want to google something like "air pressure differentials on heat exchangers" (i.e. how does a radiator work). If you are looking for the simple answer that's easy, people don't run hood scoops because there's no benefit on a miata. The intercooler is mounted in front, not on top of the engine (which is significantly better anyway due to heat soak from the engine).

I think I read an article in last month's grassroots motorsports where they relocated a Mazdaspeed 3's intercooler from the top of the engine to in front of the radiator and saw significant gains. The Subaru Impreza WRX lineup (specifically the STI) has the hood scoop feeding their intercooler on top of the engine also, but that may just be due to packaging reasons or just another way to differentiate from their Mitsubishi Evo competitor.

Summary, as you drive faster high pressure builds up in front of the nose of the car (in front of the radiator/intercooler), decreases just as the nose flattens out (think 6 inches after your radiator), then builds back up as it approaches the windshield. In order for any heat exchanger to work you want high pressure in front of it, lower pressure behind it, for the directional movement of the air across said heat exchanger. A hood scoop will dump air into your engine bay, increasing it's pressure, which in turn will decrease the pressure difference across your radiator (albeit small unless you seal the bottom of your engine bay) reducing the effectiveness of your radiator. What anyone would gain from that, I don't know, you'd have to vent that extra pressure somehow and for most of us it's directly under the car, just like before your hood scoop except now you've increased drag, decreased the effectiveness of your radiator, all for a small reduction in under-hood temps.
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Old 05-27-2011, 12:36 AM   #11
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If your right, why do you think so many people are using the mini scoop. Do you think its also for looks rather than function?

Quote:
Originally Posted by samtheturtle View Post
OP - you might want to narrow what information you're actually looking for. If you are looking to expand your physics knowledge on this topic as you indicated, you're going to want to google something like "air pressure differentials on heat exchangers" (i.e. how does a radiator work). If you are looking for the simple answer that's easy, people don't run hood scoops because there's no benefit on a miata. The intercooler is mounted in front, not on top of the engine (which is significantly better anyway due to heat soak from the engine).

I think I read an article in last month's grassroots motorsports where they relocated a Mazdaspeed 3's intercooler from the top of the engine to in front of the radiator and saw significant gains. The Subaru Impreza WRX lineup (specifically the STI) has the hood scoop feeding their intercooler on top of the engine also, but that may just be due to packaging reasons or just another way to differentiate from their Mitsubishi Evo competitor.

Summary, as you drive faster high pressure builds up in front of the nose of the car (in front of the radiator/intercooler), decreases just as the nose flattens out (think 6 inches after your radiator), then builds back up as it approaches the windshield. In order for any heat exchanger to work you want high pressure in front of it, lower pressure behind it, for the directional movement of the air across said heat exchanger. A hood scoop will dump air into your engine bay, increasing it's pressure, which in turn will decrease the pressure difference across your radiator (albeit small unless you seal the bottom of your engine bay) reducing the effectiveness of your radiator. What anyone would gain from that, I don't know, you'd have to vent that extra pressure somehow and for most of us it's directly under the car, just like before your hood scoop except now you've increased drag, decreased the effectiveness of your radiator, all for a small reduction in under-hood temps.
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Old 05-27-2011, 12:48 AM   #12
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Quote:
Originally Posted by rleete View Post
Back of the hood is a high pressure area, it's not letting air out, it's forcing it in, to the detriment of efficient flow through the radiator. Why do you think Mazda put a gasket across the back of the hood? Hint: it ain't just for grins.
It's also detrimental to aerodynamics.
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If your right, why do you think so many people are using the mini scoop. Do you think its also for looks rather than function?
The people I've seen use it are using it backwards as an extraction vent, not as a hood scoop.
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Old 05-27-2011, 01:03 AM   #13
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Ok, so because its being used backwards the negative air flow effects are negated?

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It's also detrimental to aerodynamics.

The people I've seen use it are using it backwards as an extraction vent, not as a hood scoop.
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Old 05-27-2011, 01:06 AM   #14
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Quote:
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Ok, so because its being used backwards the air flow effects are negated?
It works like an extractor. Probably will make more sense after seeing these pictures:

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Old 05-27-2011, 01:39 AM   #15
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Most people are saying the right things, although I'd like to clarify a few things.

Don't think of the hood as one object. Your friend's raised studs and extraction scoops like the one posted above are physically doing the same thing, but obviously they're in very different locations, and that's what's important. Raised studs lets air in because it is a high pressure zone due to the aerodynamics of the Miata. The extraction scoops like above are in a low pressure zone, letting hot air from directly behind the radiator escape. That's the plan at least.

Kudos to you for searching and reading though, that is much appreciated.
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Old 05-27-2011, 01:40 AM   #16
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Ok I dont often bash noobs but your an idiot.

View this topis.
https://www.miataturbo.net/race-prep-75/serious-cooling-ducts-hood-vents-think-gt40-now-pics-57909/

Then think. You cant just say I dont know how airflow and pressure differential work but its clear that scoops are better cos its logic. you think the pressure differential on a fighter jets wing is designed based on airflow and pressure differential or because 'its got 2 wings like a bird.. hurr durr its logic'.

There is LOW pressure over the hood, this means its sucking, which is why you get lift at high speeds and the steering goes light.
There is HIGH pressure in the engine bay due to all the air coming through the radiator with not many places to get out easily.
When you cut a hole in the bonnet the low pressure sucks air out and the high pressure pushes it out, just because a little scoop is there wont change that without very serious engineering and design.

Think about how much the low pressure must be sucking to make the whole car light in the front end, then think how hard the wind blows into a small scoop by putting your hand out the window. Who wins?

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Old 05-27-2011, 01:41 AM   #17
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Quote:
Originally Posted by samtheturtle View Post
OP - you might want to narrow what information you're actually looking for. If you are looking to expand your physics knowledge on this topic as you indicated, you're going to want to google something like "air pressure differentials on heat exchangers" (i.e. how does a radiator work). If you are looking for the simple answer that's easy, people don't run hood scoops because there's no benefit on a miata. The intercooler is mounted in front, not on top of the engine (which is significantly better anyway due to heat soak from the engine).

I think I read an article in last month's grassroots motorsports where they relocated a Mazdaspeed 3's intercooler from the top of the engine to in front of the radiator and saw significant gains. The Subaru Impreza WRX lineup (specifically the STI) has the hood scoop feeding their intercooler on top of the engine also, but that may just be due to packaging reasons or just another way to differentiate from their Mitsubishi Evo competitor.

Summary, as you drive faster high pressure builds up in front of the nose of the car (in front of the radiator/intercooler), decreases just as the nose flattens out (think 6 inches after your radiator), then builds back up as it approaches the windshield. In order for any heat exchanger to work you want high pressure in front of it, lower pressure behind it, for the directional movement of the air across said heat exchanger. A hood scoop will dump air into your engine bay, increasing it's pressure, which in turn will decrease the pressure difference across your radiator (albeit small unless you seal the bottom of your engine bay) reducing the effectiveness of your radiator. What anyone would gain from that, I don't know, you'd have to vent that extra pressure somehow and for most of us it's directly under the car, just like before your hood scoop except now you've increased drag, decreased the effectiveness of your radiator, all for a small reduction in under-hood temps.
thank you that was exactly what I was looking for.

now is there a downside as to where you can put a vent or a place that will reduce that pressure more significantly than other places?

(and yes, to the poster who asked about minihood scoops, i did imply using those as an extraction vent, not as a functional hood scoop)
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Old 05-27-2011, 01:49 AM   #18
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Put it where the vent on the silver car is.

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Old 05-27-2011, 03:07 AM   #19
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I vote mini hood scoop and plagiarizing nitrodann's radiator director panel thingamabob from the above linked thread. That ought to do it, no?
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Old 05-27-2011, 03:28 AM   #20
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scoop for sure would increase drag.
im guessing properly placed hood vent will reduce drag besides reducing lift.
venting the rear of the hood absolutely does let air out. only about 12 inches of the very center of the rear of the hood has air going in. i know because i put strips of yarn on my hood and verified it. thats why cowl induction hoods on hot rods are raised in the center and if that section is too wide it's not as efficient. i can also tell you that living in south texas, that lifted hood made a difference in my under hood temps. stuck in traffic, cruising, or on the track, heat was coming out. i know everyone wants to feel smart and say it's bs, it's a c.r. thing, but you don't know the whole story, and you never tried it yourself. only thing about that is, i hate how it looks, and if 1 more person told me my hood was open, i might have stabbed someone in the face. so i removed the hood spacers when i finally got my headlight delete/vented hood. i also had a front splitter, don't know if that makes a difference or not, i never tried it without it.

http://www.youtube.com/watch?v=ejPk2eBnr7o
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