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Old 01-01-2012, 02:53 PM   #1
vicious_fishes
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Default best place for hood vents

i'm thinking it would be behind the bulge in the middle of the hood ala hood scoop delete (due to the bulge creating a lower pressure area), but i've seen other cars have it in different places - the WRC car is actually in front of the scoop and TIC have theirs on the sides.

any suggestions from those in the know?
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Old 01-01-2012, 03:45 PM   #2
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Let's see if this works:



At a stop, with the fans running on the radiator, you can really feel the air moving out of the vents. I've also got a full skidplate under, so this is pretty much essential to keep underhood temps down.
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Old 01-01-2012, 04:32 PM   #3
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the vents on the hood of a GC are in that place for a very good reason.

as for where they should be, it comes down where the coolers are and how they are ducted.

Some WRC cars vent the radiator out in front of the front tires and the intercooler gets vented out the hood. The older WRC cars ducted the intercooler out the two side vents in the hood:



and had a big carbon shroud arond the back of it:



And then on the newer models they started ducting out right in the front of the hood.





I would say the hood scoop is better off just being blocked off. It's too far back to have air flow cleanly out of it. Here's the Kaminari hood, which I like:



The radiator and IC are angled forward to hopefully aid airflow out of the hood:

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Old 01-01-2012, 05:15 PM   #4
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if we assume a standard FMIC (no v mount) without any ducting?
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Old 01-01-2012, 05:33 PM   #5
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From an aero standpoint the Kaminari hood is about as good as you can get for a vent. The hoodscoop location is too far back to get any flow out of it, and if you just reverse the scoop you can actually get flow in at high speeds due to the high pressure area at the base of the windshield.
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Old 01-01-2012, 08:04 PM   #6
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what about a simple scoop delete?

e.g http://www.kstech.biz/servlet/the-25...I-scoop/Detail

having said that, i can cut holes pretty much anywhere. where would you recommend?
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Old 01-02-2012, 10:47 AM   #7
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bumping for answers.
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Old 01-02-2012, 12:43 PM   #8
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Do you still have a TMIC? FMIC? N/A?
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Old 01-02-2012, 01:10 PM   #9
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FMIC with no ducting.
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Old 01-02-2012, 02:29 PM   #10
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power, fuel, use of the car?

if it's a 300hp street car, get the kstech scoop delete and you'll be fine. 500hp on a road course and you'll need to do more than cut a couple holes in the hood if you want it to stay cool for more than 3-4 laps.
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Old 01-02-2012, 04:04 PM   #11
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Quote:
Originally Posted by vicious_fishes View Post
having said that, i can cut holes pretty much anywhere. where would you recommend?
Do you not believe what speedyHAM said? Look at any sedan type racing series in which hood vents are allowed. A vent behind the radiator is pretty much the standard.
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Old 01-02-2012, 05:31 PM   #12
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There is a really good aero document that subaru created a few years ago that I have not ben able to find recently that goes in depth into the best ducting for a mostly stock bodied car. It describes the general airflow through the front bumper and engine bay to back around the transmission. They even go as far as to make changes to the shape of certain areas then record the coolant and oil temp differences. It is a 10+ page PDF that I think I lost only old computer.

That being said, we used much of this for the development of our car in trying to figure out the ideal place for hood vents. At the time we did the vents on the side of the hoods we were still using a TMIC and could not disrupt the airflow going to the scoop. This is why in the first pic above of the wrc the vents are off the side instead of the middle as the air filter is in the hood scoop location and they did not want to suck hot air into the filter area or disrupt the airflow all together. The side vents work very well to dissipate heat from the engine bay. They are even more effective when you duct them down into the engine compartment and to the radiator.

Additionally, if you are having cooling problems on track, make sure you have the coolest heat exchangers in front of the hotter ones. IE, FMIC, radiator, oil cooler from front to back. You don't want super heated air coming out of an oil cooler with 270* oil temp to hit the radiator filled with 215* coolant. That will only serve to increase air temps.

Finally, this sounds stupid, but if you really want some good reference material on cooling check out the stuff that NASCAR has done over the past 10 years. They have done some really interesting stuff that has a direct application to our type cars.

Tony
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Old 01-03-2012, 01:49 AM   #13
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Quote:
Originally Posted by Patrick Olsen View Post
Do you not believe what speedyHAM said? Look at any sedan type racing series in which hood vents are allowed. A vent behind the radiator is pretty much the standard.
he said "reversing the scoop will actually flow inwards" and "here's this hood i really like". neither of which i suggested - i'm not reversing the scoop and i'm not buying another hood. i just intend on slicing the current one up.

interesting TIC have better information/disagree with what you're saying and are saying go for side vents and in fact have a scoop delete.

you had a little chirp at me in my other thread too, had a bad day or something?

Quote:
Originally Posted by Turn in Concepts View Post
There is a really good aero document that subaru created a few years ago that I have not ben able to find recently that goes in depth into the best ducting for a mostly stock bodied car. It describes the general airflow through the front bumper and engine bay to back around the transmission. They even go as far as to make changes to the shape of certain areas then record the coolant and oil temp differences. It is a 10+ page PDF that I think I lost only old computer.

That being said, we used much of this for the development of our car in trying to figure out the ideal place for hood vents. At the time we did the vents on the side of the hoods we were still using a TMIC and could not disrupt the airflow going to the scoop. This is why in the first pic above of the wrc the vents are off the side instead of the middle as the air filter is in the hood scoop location and they did not want to suck hot air into the filter area or disrupt the airflow all together. The side vents work very well to dissipate heat from the engine bay. They are even more effective when you duct them down into the engine compartment and to the radiator.

Additionally, if you are having cooling problems on track, make sure you have the coolest heat exchangers in front of the hotter ones. IE, FMIC, radiator, oil cooler from front to back. You don't want super heated air coming out of an oil cooler with 270* oil temp to hit the radiator filled with 215* coolant. That will only serve to increase air temps.

Finally, this sounds stupid, but if you really want some good reference material on cooling check out the stuff that NASCAR has done over the past 10 years. They have done some really interesting stuff that has a direct application to our type cars.

Tony
thanks a bunch tony

i checked out your car and you're running these side vents along with a scoop delete - have you found the scoop delete to be particularly beneficial along with the side vents now you have a FMIC? i assume you haven't ducted things down the front yet?

my intention after hearing what you've got to say along with another guy that's posted up above RE: side vents is to cut at these points. i guess all i'm torn on now is whether a scoop delete is worth it - i wouldn't imagine you'd get inflow with a delete as this is merely a flat hole after the bonnet bulge (think low pressure right?) rather than a reverse scoop that opens right in the high pressure area where the windscreen meets the body.

for the record i'm not one of those "tell me everything and i'll give you nothing" types, i've bought from you guys before & intend to again
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Old 01-03-2012, 08:19 AM   #14
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FIrst off, as mentioned above, reverse scoops don't help. They will serve as an induction point instead of venting. Just like cowl induction hoods on muscle cars. We initially deleted our scoop by covering it with a piece of lam, then later removed the plate and made a gurney flap in front of it to try to draw air out.

The idea situation for ducting when running a FMIC is to put the duct in the middle and create a ducting down to the radiator to completely seal it off to the outside. This is somewhat difficult to do on a standard radiator setup though.

In the end, if you are having issues with heat ducting will help.

Tony
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Old 01-03-2012, 11:12 PM   #15
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Quote:
Originally Posted by vicious_fishes View Post
he said "reversing the scoop will actually flow inwards" and "here's this hood i really like". neither of which i suggested - i'm not reversing the scoop and i'm not buying another hood. i just intend on slicing the current one up.
I don't think jamal or speedyHAM were implying that you needed to buy the Kaminari hood, just that the vent in the center of the hood behind the radiator is the way to do it.

Quote:
Originally Posted by vicious_fishes View Post
interesting TIC have better information/ disagree with what you're saying and are saying go for side vents and in fact have a scoop delete.
I don't see anything in his post that disagrees with what I said. "At the time we did the vents on the side of the hoods we were still using a TMIC and could not disrupt the airflow going to the scoop." Once they did the FMIC, they put long louvers across the hood between the side vents. In other words, a vent in the middle of the hood behind the radiator.

Edit: The louvers behind the hood are somewhat hard to see, as they made them by slicing the hood and then twisting the resulting strips so they're angled. I thought I had a pic or two of the hood from OLOA 2011 @ Summit Point, but I can't find any.

Last edited by Patrick Olsen; 01-05-2012 at 01:53 PM.
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Old 01-04-2012, 10:23 PM   #16
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Quote:
Originally Posted by speedyHAM View Post
From an aero standpoint the Kaminari hood is about as good as you can get for a vent. The hoodscoop location is too far back to get any flow out of it, and if you just reverse the scoop you can actually get flow in at high speeds due to the high pressure area at the base of the windshield.
Like or should I say agree.

Quote:
Originally Posted by Turn in Concepts View Post
1.) First off, as mentioned above, reverse scoops don't help. They will serve as an induction point instead of venting. Just like cowl induction hoods on muscle cars.

2.) The idea situation for ducting when running a FMIC is to put the duct in the middle and create a ducting down to the radiator to completely seal it off to the outside. This is somewhat difficult to do on a standard radiator setup though.

True and true. To know where is ideal is the area with the highest velocity flow parallel to the surface. It will allow for a lower lower pressure and will draw air out. See the WRC ducts mentioned above. I would however favor ducts that aren't dead center. You can see my reasoning if you tuft the hood and see the directions of the fibers. You'll see they come back and off the sides of the hood (path of least resistance).

Grateful for my internship time in the windtunnel but I don't miss tufting DP, Cup or Indy cars

Last edited by Homemade WRX; 10-25-2016 at 07:19 AM.
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Old 01-05-2012, 01:54 PM   #17
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Quote:
Originally Posted by Homemade WRX View Post
I would however favor ducks that aren't dead center.
Quack.

I don't have the wind tunnel experience, but I would think the center vent, which should be pretty well forward of the windshield, would still be the best option. Obviously as you get farther back on the hood (closer to the base of the windshield) the airflow will start to skew to the sides to go around the greenhouse. I don't have a good feel for how far forward that effect is evident, though.

I've posted this in a previous aero thread or two, but I'll recycle it here because it's the only neato diagram I've got:

Here's a pretty cool CFD of a typical sedan (a Volvo S80 in this case) that was posted in a thread about vented hoods over on corner-carvers.com.
Quote:
To give an idea of where to position extraction vents, look at the following CFD plots of pressure coefficient on a car body:



Areas of low pressure are in blue, increasing color depth reflecting more negative pressure. Areas of high pressure are in yellow/orange/red with increasing "redness" indicating more positive pressure. Green is zero coefficient of pressure.

The sedan body picture is very representative of the majority of road cars and shows that one wants to place the extraction vents as close to the leading edge of the hood as possible since this is the region of lowest pressure.

Last edited by Patrick Olsen; 01-05-2012 at 02:24 PM.
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Old 01-05-2012, 02:15 PM   #18
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Air definitely flows into (not out of) our Hood Vents at speed, but I have received numerous reports or people with improved cooling at the track with them installed vs. a scoop.

The Kaminari hoods are high quality and have a good air extractor design, if that is not too pricey.

On another note... if you have an open hood scoop on an FMIC car, it tends to pressurize the engine compartment, which in turn inhibits flow through the FMIC and radiator, and it causes front-end lift. This seems to be particularly bad on the 08+.
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Old 01-05-2012, 02:56 PM   #19
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Patrik, do you have a larger view of that photo? me and my slight red/green color blindness are having a hard time seeing the local velocity over the nose (blue pressure area) of the hood. The reason I said with velocity is with a duct () it'll cause a pressure drpo (think of a venturi) but still have the flow to move the hot engine bay air along.

The reason I haven't specified any specific area is there are several Impreza front ends and I'm sure they all behave quite differently, hence the comment of tufting your own car


Though these are pretty long tufts and are a little spaced out. I'd keep them closer to 2" of tail length and make the spacing more dense. You'll also note the tufts behind the scoop are moving less slowly and or curling back to the scoop. Thank you high pressure, slow moving air that tries to push back into a reversed hood scoop.
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Old 01-05-2012, 10:55 PM   #20
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So, if i understand everyone, this style of reverse scoop is actually detrimental?

I will be running a PW V-mount IC setup, any thoughts about hood scoops or not. Just reading this, what about actually reversing a regular bugeye scoop, so the opening is facing the windshield? I also have some aluminum louvers I was planning to put on the hood,I think the TIC car has louvers?
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Old 01-06-2012, 09:42 AM   #21
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Okiedude, I believe what was referred to in earlier post was that the reverse hood scoop actually would injest vs vent air into the engine bay since the opening would be in a high pressure area in front of the windshield. The type of vent you posted wouldn't be as bad but a better vent would be one neaer to the front of the hood.
Here's a reference 03 WRX hood with a vent behind the radiator area.
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Old 01-06-2012, 12:01 PM   #22
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Maybe I should put a louver panel over the old hood scoop area also. One on each side and one in the middle? Give it that "hot rod" look-LOL!
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Old 01-06-2012, 12:11 PM   #23
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Like this?


Duncan
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Old 01-06-2012, 12:14 PM   #24
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You used to be able to buy a fiberglass WRC hood vent on E-bay for fairly cheap. I ran one of my old Subaru. If you're just making a track car you can install with rivets.

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Old 01-06-2012, 01:45 PM   #25
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Quote:
Originally Posted by dunk View Post
You used to be able to buy a fiberglass WRC hood vent on E-bay for fairly cheap. I ran one of my old Subaru. If you're just making a track car you can install with rivets.

will this kakumei hood vent fit o4 st?
i am trying to find 04 sti hood vent!
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