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.

Cherokee
 

*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.:o
*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.

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...

FE
 

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.:o

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)

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 :)

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

Have hood vents ever helped? From what I've seen that's a positive pressure area all around.

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.

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

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.

[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.

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.

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

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.

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.

Read the link that explains how to apply how to build and how to interpret your results to get your desired effects

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:

https://ecomodder.com/forum/member-f...windshield.jpg

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:

https://ecomodder.com/forum/member-f...po-d5b2ct2.jpg

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.

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.

https://ecomodder.com/forum/member-v...101-161848.jpg

I apologize for being off topic but is that the book (Aerodynamics of Road Vehicles) that you all speak of when you reference Hucho?

ever helped?
 

Yes,Hucho spoke of reducing the cooling system drag,from say 12% of overall drag,down to 2% with 'good racing design.'
2% would require a fully-ducted,airtight, inlet and outlet design.
Since the 1978 Pininfarina CNR car is the only vehicle designed for low drag it might serve as a benchmark for low drag in a passenger car.
The inlet would be sited exactly at the forward stagnation point,and the extractors would be designed to merge the engine bay air,out into the surrounding flow at exactly the same velocity and direction.Something that cost them a $100,000 to figure out.

Aerodynamics of Road Vehicles?
 

Yes!
Or the original: Aerodynamiks des Kraftfahrzeugs,by Reinhard von Koenig-Fachsenfeld,1951.

That's all very interesting. The idea of a ducted cooling system intrigues me. I will have to do a lot of thinking about all this.

Yeah I get your point about the wheel wells. Perhaps I'll just cover the ugly parts with sheet metal to make it less un aerodynamic :)

I may end up doing a lot more body work than I originally intended... I could angle the back a bit by chopping the back part off and using steel tubing as framework for all that (doubles as a roll cage).

Fyi I am in the process of moving now and it may be a while before I can implement these ideas.

Thanks Aerohead. I found a used copy online an hour or so ago and ordered it. Not that I'm going to understand much if it, but the price was right. I'm always willing to try and learn.

Now, back to your regularly scheduled program (thread).

understand
 

You'll get it!
I've read mine at least a dozen times and I keep 'seeing' things.And it's not the Kool-Aid!

Not to diverge too far from the topic but since this doesn't warrant it's own thread... What do you guys think about stinger bumpers? Conventional wisdom says it would hurt FE, but one person here claimed it helped by "breaking up the air before it reaches the vehicle".

Also would this book be worth getting? It sounds interesting.

stinger bumper
 

Scam!

Haha I'll make the assumption you're not talking about the book

Not even it it were boxed in solid. The most it will do is add turbulence to the air entering the radiator.

The book would increase your knowledge, but it's kind of the long way around to a solution to your specific situation. Do some aero mods and work on 'the nut behind the wheel'.

I've already done some work on my own driving. I'm sure there's some hypermiling techniques I haven't tried yet. Do you think coasting in neutral would cause extra wear on automatic transmissions?

My whole interest in gas mileage started awhile back. My own driving habits caused me to get a lovely 14 mpg mostly highway on that jeep. That was back when gas was super expensive too! But after I modified my driving habits I started to get more reasonably numbers than 14 mpg.

On a side note, I have a Jeep friend that would be very happy to get 14 mpg. He gets 10 now... So I suppose I'm lucky.

book
 

All I saw was the off road bumper component.:confused:

Yeah that is what I'm talking about. Like a regular steel bumper vs. one that has that stinger on it. It's nice for a little extra protection but I think it will add a lot more drag, as one of the previous posters said.

It also looks cool, but I can live without it haha.

Here's an example

example
 

Okay,so it's a catalog.Now I get it.
I suppose we'd have to look at each product.
If you're still gonna go off road,the only thing you'd need to respect is your approach,breakover,and departure clearances,so nothing got ripped off,hitting some obstruction out in the boonies.
Other than that,you could do anything an car did.
The JEEP LIBERTY has many 'car' features and is light years ahead,aerodynamically than say the Wrangler.
An extreme 'version' of an SUV like the Cherokee would be the TESLA X.

A deflector to break up the air, actually works quite well during high speed ejections with a Russian K36 ejection seat ...

Same for aerodynamic spikes

But they won't work at the kind of speed we're driving :turtle:

That's true I do like some parts of the Jeep liberty. I've always wanted to make flush headlight covers but I'm not sure about the inside of it getting dirty...

That is interesting but yeah I think whatever benefit I would get from a stinger would be negated by the extra drag created.

You could use the stinger as the opening of a diverging converging ducting system and flash in to the rest of the raidator support. it would basically change the grill opening from a hillbilly smile to a boxfish eating a minnow.

Doing that with a stinger would definitely be unique!

So I realized now that my options for hood vents are actually pretty limited because of these braces in my hood. The only realistic spot would be by the windshield or a little towards the middle.

So I found the best place for hood vents on my jeep today. I parked under a tree and when I went to leave, there was a bunch of leaves on my hood. As I drove away, there was a single leaf that was there almost my entire ride home! I think that was amazing because my top speed was about 68 mph. It did eventually fall off--at a stoplight.

So the lowest pressure area on my hood is about a foot to 18 inches away from the windshield, in the center of both halves of my hood.

This is well overdue, but I found some nice hood vents that I like! I'll buy them soon and post the results here. Since I have a scangauge now, I'll do an A-B test and share the results.