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Old 08-11-2018, 12:57 PM   #1 (permalink)
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Vents

Hi everyone,
I'm a new member and can't find much information here on this topic. I want to add hood vents to my vehicle solely for aerodynamic purposes. I know that by doing so, it will help decrease some lift in the front of my vehicle. Where would the best place to put hood vents be? For styling purposes, I am wanting to place them towards the back. I drive a 97 Keep Cherokee if that helps.

If anyone knows much about the effectiveness of venting the wheel wells as well, that information would be welcome.

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Old 08-11-2018, 01:44 PM   #2 (permalink)
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Cherokee

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Originally Posted by Taylor95 View Post
Hi everyone,
I'm a new member and can't find much information here on this topic. I want to add hood vents to my vehicle solely for aerodynamic purposes. I know that by doing so, it will help decrease some lift in the front of my vehicle. Where would the best place to put hood vents be? For styling purposes, I am wanting to place them towards the back. I drive a 97 Keep Cherokee if that helps.

If anyone knows much about the effectiveness of venting the wheel wells as well, that information would be welcome.
*technically,you'd have to do either a professional CFD,or wind tunnel pressure profile analysis of the JEEP to locate the ideal extractor sites.
*and according to the experts,at normal, legal highway speeds,lift would never be an issue.
*The proximity of the radiator,cooling fan,and shroud to the engine,are going to make it very difficult to come up with enough clearance to create truly low-drag extractor ducting,either below, above,or to the sides of the radiator.
*The hood's underside reinforcement webbing will dictate where you could make a penetration without ruining the structural integrity of the panel.If you cut through the wrong place,the hood will 'fold' on you.
*For a 'guess',attach a bunch of woolen tufts to the hood,drive it above 20-mph,and photograph the pattern they make on a calm day.
*Where the tufts are squeezed the closest together,is where the air is moving the fastest,and pressure the lowest,making for the best extraction.
*Mark that area on top,then open the hood and see if there's a space below there,where there isn't bracing.If so,make your penetration there.
*Bear in mind that 'weather' is going to enter your new opening.You don't want water to flood electronics under there,so check that out.
*If you have clearance,you can attach ducting to the underside of the hood penetrations which will protect under-hood areas from unwanted wetness.
*And the ducting needs to be radius'd such that the air escaping the engine bay blends into the over-hood airflow otherwise you'll create separation and turbulence right there.
I had to do this on my truck.It took over 1-week for the fabrication.
------------------------------------------------------------------------
*You might look at the 1978 Pininfarina CNR 'banana' car for it's cooling outlets.
*Also,the 1957 MG EX 181 land speed record car.
*Also,SAE/Shell Eco-Marathon racers.
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Old 08-11-2018, 03:17 PM   #3 (permalink)
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I should try that test with the cotton tufts. That is probably the only way to find the ideal spot.

Surely putting vents on would increase FE though. At the very least it would divert air bring forced underneath the car to go over. I'm not very concerned about water though. I want vents that came from a stock Chrysler Lebaron or New Yorker Turbo. I don't think they would let much water in as they are OE. If it does become a concern however I can probably make drip pans or something underneath...
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Old 08-11-2018, 05:33 PM   #4 (permalink)
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FE

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Originally Posted by Taylor95 View Post
I should try that test with the cotton tufts. That is probably the only way to find the ideal spot.

Surely putting vents on would increase FE though. At the very least it would divert air bring forced underneath the car to go over. I'm not very concerned about water though. I want vents that came from a stock Chrysler Lebaron or New Yorker Turbo. I don't think they would let much water in as they are OE. If it does become a concern however I can probably make drip pans or something underneath...
The front fender vents on the 1982 Pontiac Trans Am Firebird increased drag,so there is at least one example that we know of where FE would suffer.
Unfortunately for us,is the little published data we have to work from.
Pininfarina spent over $100,000 to perfect the 4-vents on their CNR car.There are no quarantee'd results.
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Old 08-11-2018, 06:07 PM   #5 (permalink)
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Quote:
I'm a new member and can't find much information here on this topic.
You'll find a Search button in the upper left. Using as an example "hood vent location" returns... About 395 results (0.41 seconds). Have you looked at those?

If it's 'solely for aerodynamic purposes' you should establish goals and parameters. Increased engine cooling or blocked, cabin intake vents downstream, etc. How much length can you add in front of the bumper? Would venting into the wheelwells be superior?

While it's a perplexing task, as aerohead says; I think he'd agree that trying something is better than not.

Edit: Search tip: "jeep hood vent location' returns about 166 results (0.45 seconds)
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Old 08-11-2018, 07:27 PM   #6 (permalink)
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That was helpful, thank you. I found someone talking about rift testing on the jeep. Apparently the best location is further towards the front. I may settle for around the middle, but I'm not sure yet.

I don't want to vent air from the engine bay into the wheel wells. I want to vent the rear part of it under the jeep. If I added one at the top, I think air would escape out the hood vents I'm going to put in, so I may do it. I think a lot of gains are available to me in the front wheel wells, since my jeep is lifted and I have the air dam and bumper caps removed. Along with the vent I would like to add a sheet metal covering to direct air towards the vent. Right now it kind of goes to the back of the fender and runs into my front doors.

Any thoughts on these ideas? I'm hoping these mods will earn me a few mpg. I'd like to get to 25 mpg combined if possible. Right now my best tank is 20.8 mpg. I'm doing a tune up later today with quality parts so I'm hoping to get to at least 22 mpg with that. If I can squeeze 3 more with aero mods I'll be pretty happy
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Old 08-11-2018, 09:28 PM   #7 (permalink)
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Quote:
I think a lot of gains are available to me in the front wheel wells, since my jeep is lifted and I have the air dam and bumper caps removed. Along with the vent I would like to add a sheet metal covering to direct air towards the vent. Right now it kind of goes to the back of the fender and runs into my front doors.
Needs pictures.

A tool you might find useful is the differential manometer. It would allow comparing the air pressure on top of and underneath the hood.

AutoSpeed - Undertrays, Spoilers & Bonnet Vents, Part 1
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Old 08-12-2018, 01:50 AM   #8 (permalink)
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Have hood vents ever helped? From what I've seen that's a positive pressure area all around.
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Old 08-12-2018, 12:54 PM   #9 (permalink)
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I'll post some pictures tomorrow. I think I need a certain number of posts to post pictures.

If anyone was wondering, the tune up was great. I got a little more low rpm torque which is awesome.
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Old 08-13-2018, 01:06 PM   #10 (permalink)
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This is a picture of my drivers side wheel well. I'm thinking of improving it by blocking off the area in the back and forcing the air down... And I'll possibly add a vent too

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