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Old 07-21-2003, 02:28 PM   #1
Scooby Newbie
Member#: 24269
Join Date: Sep 2002
Location: ATL GA
2006 LGT

Question TMIC airflow improvement

I have heard of people using splitters in the hood scoop to make the air entering turn the corner and better distribute airflow over the intercooler. Has anybody heard of using vortex generators to help the air make the downward turn into the intercooler?

I have seen vortex generators used on NASCAR cars in front of NACA ducts to promote better airflow. Of course this is also the reason why NACA ducts have sharp corners at the surface.

Turbulent air is less likely to separate from the surface and therefore would make the turn down into any air inlet easier. So, has anyone tried using vortex generators for their hood scoop?
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Old 07-21-2003, 03:25 PM   #2
Scooby Specialist
Member#: 37731
Join Date: Jun 2003

just so you know, the STi scoop already has an air splitter.
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Old 07-21-2003, 05:41 PM   #3
Scooby Newbie
Member#: 34699
Join Date: Mar 2003

In my opinion, it's a mistake to talk about aerodynamic flow through geometry like an intercooler. The path is so tortured that really all you care about is pressure gradients. If you were to measure the pressure at the top and bottom of the I/C, this is the only thing that matters. The flow through this system is highly turbulent and separated as it is. The hood scoop is probably already operating in the turbulent boundry layer anyway--note how leaves sometimes stay on your windscreen? There are wierd recirculating flow patterns there in cars with upright windscreens like the STi.

Now, if the pressure at the top-front of the I/C is less than the top-rear, then you might have an argument to put some control vanes to try and redirect the flow. But remember, every time you put something else in the flow, you increase pressure losses. I would first look to see some very sound empirical data on this subject before throwing a buch of wings, fins, and flow disruptors in your scoop or on your hood. I have a hard time believing this phenomena actually exists or has any appreciable effect while driving the car on the street/track.

Also, measure the area of the I/C versus the scoop. The peak efficiency for intercoolers is between 15-30 MPH. I would expect the choke area of the scoop to be about two or three times smaller, meaning that the car works best (from a intake charge cooling perspective) at 45-90 MPH.

Remember, air doesn't act like a bunch of tiny racketballs. Kinetic theories of fluid have been wrong since Bernoulli.

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Old 07-21-2003, 07:40 PM   #4
Scooby Newbie
Member#: 31822
Join Date: Jan 2003
Location: Dallas - Boston

Are you suggesting that a smaller hood scoop would actually work better at higher speeds because the inverted funnel effect would slow the speed of the air to a more efficient rate? ... this is groundbreaking stuff ... if so ... please clarify
Beldrueger is offline  
Old 07-22-2003, 07:24 AM   #5
Scooby Newbie
Member#: 24269
Join Date: Sep 2002
Location: ATL GA
2006 LGT


It is true that if an air inlet is too big, it will become inefficient. Depending on the spacing and density of vanes in the intercooler, it will only allow a certain amount of air to flow past it and at a certain speed. If the air inlet(or hood scoop in this case) is trying to feed too much air, a portion of that will not be able to go through the intercooler. The extra air has nowhere to go but to recirculate and spill back out through the inlet or seep out through the seems in the ducting to the inlet. This will cause inefficiency and choked flow.

The big "however" is...the science/math behind this is unpredictable to say the least and will only get you in the ball park. Testing is required to fine tune the size of your inlet. There is no way for any of us to know how well subaru sized the hood scoop or for what range of speeds.

So, it is slightly conceivable that if you ONLY run at high speeds, a smaller hood scoop may be better. However, the more likely solution would be to get a more efficient, higher flowing intercooler to handle the larger flow of air.
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