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Old 08-13-2018, 02:38 PM   #11 (permalink)
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Quote:
Originally Posted by Taylor95 View Post
I'm not very concerned about water though. ..........
Please note that if you live in a northern climate where they salt the roads, the road mist during freeze/thaw cycles, the splashy muck that gets kicked up by the tires is full of salt.

Salt is a corrosive.

Salt eats up electronics even quicker than it does sheet metal.

This is what killed my old Geo Tracker of 20 years.

Just a warning.

I am getting confused about your concept, at one point it sounds like these hood vents are supposed to increase air flow to under the truck.

The only way they will do that is if they are sucking air in.

Then at another point it sounds like the vents are to exhaust air.

You need to post a simple diagram of your proposed concept.

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Old 08-13-2018, 02:58 PM   #12 (permalink)
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[Attached Thumbnail]

Lacks context. Is it upside down?

Putting stainless steel splash guards in the wheelwell might prevent corrosion.

Vents on the hood are not going to suck on the underbody unless you remove the engine.
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Old 08-13-2018, 04:01 PM   #13 (permalink)
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They don't salt the roads where I live so corrosion is not an issue. We get very little rain but we do get snow.

I guess I need to explain myself better. I want hood vents, not a hood scoop. As air comes through the grill, I want it to be vented out the hood vents to go over the smoother top of my vehicle rather than forced underneath.

The picture is sideways for some reason. The tire should give you some context. Basically it's a picture of my un aerodynamic wheel well.
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Old 08-13-2018, 04:08 PM   #14 (permalink)
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My goal with the hood vents is to decrease airflow underneath my jeep. I'm not entirely sure what to do with my wheel wells yet. That's probably where the confusion came from--sorry
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Old 08-13-2018, 04:43 PM   #15 (permalink)
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S-okay.

You can make a differential manometer with a long piece of fuel line with a bubble of liquid at the bottom of a loop. The bubble moves toward low pressure.

Your baseline should be what happens underneath with the hood gone completely. A vent in the right place will lower underhood pressure, maybe draw cooling air into the engine compartment. A 'vent' in the wrong place will act like a scoop.

Another example was the MGA, two oval vents halfway back. Whichever way it went, they wanted the opposite.

Proven techniques to manage underbody airflow include bellypans, diverters and wheel spats. Diverters lower the stagnation point, but act counter to approach angle, which may be important in a Jeep.
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Old 08-14-2018, 05:37 PM   #16 (permalink)
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So how exactly would you measure underhood pressure using that method?

Yeah my jeep is going to be a crawler eventually. I just want to do everything I can to maximize fuel efficiency. I know some people that only get like 175 miles per tank... I do not want my jeep to be like that.

I'll post my ideas on wheel well vents soon since that is a big area for improvement on my vehicle.
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Old 08-14-2018, 08:15 PM   #17 (permalink)
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Quote:
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So how exactly would you measure underhood pressure using that method?
....
Quote:
AutoSpeed - Undertrays, Spoilers & Bonnet Vents, Part 1
Read the link that explains how to apply how to build and how to interpret your results to get your desired effects
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Old 08-14-2018, 08:37 PM   #18 (permalink)
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Obtain a piece of fuel line or surgical tubing or similar twice as long as is needed. Zip-tie off a loop of maybe 12-18" in your line of sight. Introduce a slug of liquid that fills the tube for a [small] length. Mark of above and below the liquid's level in inches. Feed one end to one test location and the other to another. Duct tape everything down.

At speed if there is a pressure differential, the liquid will move up one side or the other. You can do math to figure out at the ambient air pressure what the absolute values are, or you can just do comparisons.

Else you can get a Dwyer Magnehelic on eBay for cheap.

Aerodynamic wheelwells are skirted and spatted and air curtained. With minimal volume around the tire. All this works against a tire that needs to clear debris and climb tree trunks.

I'd accept that it's an open wheel vehicle and work with that. Mainly a three-piece visor/windscreen and a full boat tail with a hinged floor. An example on a Beetle:



Clear plastic with Rain-X, else perforated metal with sightlines cut out/holes enlarged. Here's the shape I'd work toward with a rock crawler:



A curved bottom cage and reverse tumble-home on the upper roll cage for more interior space.

—— took me that long to write it out.
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Old 08-14-2018, 09:55 PM   #19 (permalink)
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Quote:
Originally Posted by Taylor95 View Post
I guess I need to explain myself better. I want hood vents, not a hood scoop. As air comes through the grill, I want it to be vented out the hood vents to go over the smoother top of my vehicle rather than forced underneath.
Yep, a Magnehelic gauge (manometer) is the way to go here--you can measure the pressure differential at various points across the hood. Site the vents where you have the greatest different between high underhood pressure and low overhood pressure.

I think using hood vents can have a positive effect on drag and lift; Hucho indicates in his Aerodynamics of Road Vehicles that this is both due to the venting of high pressure air underhood and the introduction of heated air to the airstream above the vehicle. We're seeing hood vents on more expensive cars--the 2008-2010 Viper introduced rather large hood vents, continued on the 2013-2017 Viper, which used two different designs; the current Corvette Z06, Z07, and ZR1 use them; various other super- and hypercars; the BMW i8 has a very nice radiator duct that vents to the center of the hood (I'll post a picture when I get back to my home computer). If you search this site for the Green Grand Prix Modding Thread, you'll see my setup and results of pressure testing; I'm planning on adding hood vents like you to reduce drag.

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Old 08-15-2018, 09:54 AM   #20 (permalink)
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I apologize for being off topic but is that the book (Aerodynamics of Road Vehicles) that you all speak of when you reference Hucho?

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