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Thread: Effectiveness of Vented / Extractor Hoods?

  1. #1
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    Effectiveness of Vented / Extractor Hoods?

    Logically it seems like some well placed vents would help out greatly for a cars aero. I was wondering if anyone has any data or tests showing the actual effects of vented hoods on things like downforce, drag, and even gas mileage. Hoping someone can chime in that has raced with and without one to provide some insight on the differences it made. Since there is such a build up of pressure in front of the radiator/bumper I would imagine the aero benefits should be noticeable even at highway speeds (65-80mph).

    I searched but mostly found hear say and nothing quantitative. Is there an e36 vented hood that stands out as the best in terms of functionality? Has anyone ever measured the pressure differentials across different areas of the e36 hood to find out where the best place is to put a vent?

    I can't think of any cons to a properly vented hood either, which makes it seem to be a very worthwhile upgrade.

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    Quote Originally Posted by 95StreetDoc View Post
    Has anyone ever measured the pressure differentials across different areas of the e36 hood
    me.







    That's at 75mph. Theory says that the lowest pressure is at the point of maximum curvature and lo, there it is. The higher pressure at the rear is due to the partial stagnation area at the windshield.

    Note that the air pressure is measured relative to the car interior. With one window cracked 1/4". I suspect that the cabin is at a slight -ve pressure relative to ambient (due to the low-pressure area outside the windows). So the absolute value of the pressures in that graph aren't very interesting.

    to find out where the best place is to put a vent?
    It depends on what your objectives are. If you want to decrease the under-hood pressure then put the vents as far forward as possible. If you want to increase the under-hood pressure then put the vents as far rearward as possible.

    I can't imagine why one would want to increase the under-hood pressure, so put the vents at the front and seal the gap between the rear of the hood and the firewall.

    Lower under-hood pressures will increase the pressure drop across the radiator and will improve engine cooling. But this isn't necessarily a good thing! More cooling equals more drag. If you already have enough cooling then don't go adding more cooling&drag.

    There's a theory that injecting the radiator exhaust air into the low-pressure area over the hood will decrease lift. I suppose that's right, but the airflow through the radiator is low, so my seat-of-the-pants guess is that the effect is small.

    There's a theory that lowering the under-hood pressure will reduce lift by reducing upward pressure on the hood. I suspect that's bunk, because the high under-hood pressure presses downwards too.

    There are probably aero advantages to venting the cooling area through the hood: if you don't do this then more of it will be vented under the car. Low speed air hits high-speed air and reduces downforce, adds turbulence and drag.

    A downside to reducing under-hood pressure is that there's necessarily less air flow through the engine bay. My headers are wrapped. My power steering reservoir has had three heat-related failures which wrecked three race weekends, and I'm suspecting that the lowered flow across the p/s cooler might have contributed. Last time the reservoir actually melted so I now have an aluminum tank and an extra finned cooler.

    I don't recall ever seeing a quantitative treatment of the effects of hood venting, sorry. Having fiddled with it for a while and thought about it, I don't think it makes much difference to anything really, apart from improved flow through the radiator. Not much difference to lift/downforce and not much difference to drag.

  3. #3
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    thanks so much for posting this info.

    you have a twin car on the east coast btw...


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    Quote Originally Posted by M Dizzle View Post
    thanks so much for posting this info.
    I enjoy fiddling with aero. This week's project:



    I wonder how I'll get on with 55mm ground clearance. Probably everyone will be racing around while I'm back home working out how to get it on the trailer.

    I want a wind tunnel.

    you have a twin car on the east coast btw...
    A pale imitation!

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    with the lager pressure drop/more cooling should be able to reduce radiator frontal area/inlet area that would reduce drag.

    btcc with tiny radiator opening

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    i did not measure it, but i will confirm that you will feel the difference. when i switched years ago, the car went from understeer at speed to be rather neutral. that was the effect if canceling front end lift. its worth doing if your class allows.
    Peter Carroll - http://www.driversmeeting.com/pcarroll
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    Quote Originally Posted by sstecker View Post
    with the lager pressure drop/more cooling should be able to reduce radiator frontal area/inlet area that would reduce drag.
    Yup. Reducing the intake area will take the grams/sec intake airflow back to what it was before you vented the hood. As drag is the energy loss caused by accelerating air's mass, you're back to where you started.

    btcc with tiny radiator opening
    Wow, good photo.

    In NASCAR the upper grill is fake and if you look closely they just have a 2"-3" high letterbox slot lower on the bumper. I've read that they put blanking plates behind that slot to tune the opening area to the track and the weather conditions.

    Deliberately reducing your engine cooling to reduce drag is a bit scary. Obviously the best approach to minimize cooling drag would be a thermostatically-controlled shutter. Some modern street cars are being designed with them for fuel economy reasons.

    Drag doesn't seem to matter a lot. I figure that an e36 at 100mph loses 45-50 hp to drag. So if you can reduce that by 10%, you gained 5hp in the faster straights. You'd be hard-pressed to notice that.

    And a 10% overall drag reduction isn't unreasonable. Around 30% of drag on a street car is said to be due to cooling air, and street cars are designed to suck in waaaay more air than a race car needs. Unless you drive your race car in stop/go traffic with the a/c on flat out

    The #1 thing you should do if you're concerned about drag is to seal the radiator to the bumper openings. Sucking in (and accelerating) air and then not using it for anything is pretty dumb!

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    Quote Originally Posted by flink View Post
    Deliberately reducing your engine cooling to reduce drag is a bit scary. Obviously the best approach to minimize cooling drag would be a thermostatically-controlled shutter. Some modern street cars are being designed with them for fuel economy reasons.

    Drag doesn't seem to matter a lot. I figure that an e36 at 100mph loses 45-50 hp to drag. So if you can reduce that by 10%, you gained 5hp in the faster straights. You'd be hard-pressed to notice that.

    And a 10% overall drag reduction isn't unreasonable. Around 30% of drag on a street car is said to be due to cooling air, and street cars are designed to suck in waaaay more air than a race car needs. Unless you drive your race car in stop/go traffic with the a/c on flat out

    The #1 thing you should do if you're concerned about drag is to seal the radiator to the bumper openings. Sucking in (and accelerating) air and then not using it for anything is pretty dumb!
    With as close as some people race, I think you would notice 5hp, people cheat to get 2-3hp. That siad, most people like me just suck enough to not notice anything .

    The pictures you posted above on your car show vents directly in airflow with no lip. That concerns me that your actual forcing air into those vents. Have you done a tuft test? The lip helps provide evacuation, and every vent ive seen without a lip or louver has actually shoved or stagnated air in that area. People cite "evos have hoods cut out like that", but then fail to notice the small lips to help create evacuation.

    Notice the designs of professionally done vented hoods, they all use either large evacuation areas with smoothly radiant outflows, or use lips.

    If it wasnt for me hmmm and haaaing on which vent to get, I would have one, in fact I would be putting a Carbontrix hood vent on my car right now.

    PS- Im no engineer, but have played with hood vents before on a previous "race" actually HPDE car lol.
    Last edited by Spec3HOR; 06-30-2011 at 02:28 PM.

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    Quote Originally Posted by Spec3HOR View Post
    The pictures you posted above on your car show vents directly in airflow with no lip.
    I spent unbelievable amounts of time trying to find some louvers. I once found a very nice one on the side of a Volvo D70 prime mover but alas, it didn't have a mirror-imaged one on the other side. So I gave up.

    I don't think it matters a lot - there should be high pressure in the engine bay and low pressure over the hood, so the air should travel upwards. If it doesn't then the theories are all wrong so you have bigger problems than not having louvers!

    Have you done a tuft test?
    Yep. [ame]http://www.youtube.com/watch?v=a4KOja5rnXg[/ame]

    That's what I'd expect to see. The flow through the radiator is low - people have measured it at 15mph max. A low flow through the radiator combined with a high flow over the hood results in the tufts standing up at a shallow angle.

    The lip helps provide evacuation, and every vent ive seen without a lip or louver has actually shoved or stagnated air in that area. People cite "evos have hoods cut out like that", but then fail to notice the small lips to help create evacuation.
    1/15th of a PSI may not sound like much of a pressure difference, but over an area like that it will move a hell of a lot of air. I expect that louvers would help, but the existing pressure differences are enough to move the air.

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    I'd be interested in seeing a comparison tuft test on some louvers in the same area (any mashaw hood guys got anything?)

    Or even a single louver-ish vent like so :

    http://www.allfordmustangs.com/forum...-today-054.jpg

    http://www.allfordmustangs.com/forum...-today-055.jpg
    Last edited by M Dizzle; 06-30-2011 at 05:52 PM.


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    Decent test results are hard to come by. Which is unfortunate, because aero effects are often unintuitive and confusing. It's also unsurprising because testing is expensive, and a universal product on car model A can have quite different behavior on car model B.

    CFD is progressing. Perhaps one day you'll be able to easily buy an e36 model, add parts to it on your PC using an aero model editor then "test" the results.


    One other thought on all of this: decreasing the air pressure in the engine bay will decrease the air pressure around the engine intake, if the two are connected. That will decrease engine power slightly. Build a box around the intake which seals it to the front stagnation area, which is also where the cold air is.

    The ram air effect is very small - I figure it buys me 2.9hp at 100mph, relative to what the dyno said. And that's assuming no ill effects - Karl at activeautowerke doesn't like ram intakes - he says he's seen them cause turbulence which screws with the MAF reading.

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    Quote Originally Posted by flink View Post
    The ram air effect is very small - I figure it buys me 2.9hp at 100mph, relative to what the dyno said. And that's assuming no ill effects - Karl at activeautowerke doesn't like ram intakes - he says he's seen them cause turbulence which screws with the MAF reading.
    Ive also been playing with the idea of two small velocity stacks direct air into the airbox instead of just a rectangular hole. It would decrease turbulence, hopefully increasing air ingestion and reducing drag slightly.

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    I placed an oil cooler behind the drivers side headlight. Air comes into headlight (sealed inlet duct), and flows out at a low angle through the hood through another sealed duct. On a turbo m52 its keeping temps during a 25min session under 190.

    I really wanted to do a large rear radiator extraction duct, but with the intercooler I could not get the radiator angled forward enough to get enough clearance.






  14. #14
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    e30 windtunnel footage from a long time ago...
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    Quote Originally Posted by Steve J. View Post
    I placed an oil cooler behind the drivers side headlight. Air comes into headlight (sealed inlet duct), and flows out at a low angle through the hood through another sealed duct. On a turbo m52 its keeping temps during a 25min session under 190.

    I really wanted to do a large rear radiator extraction duct, but with the intercooler I could not get the radiator angled forward enough to get enough clearance.





    are you trying to accelerate air back to free stream air speed?

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    Quote Originally Posted by sstecker View Post
    are you trying to accelerate air back to free stream air speed?
    Air is simply being directed through a duct and out from the hood. Its an easy in/out and has proved very affective. Same principal behind extraction ducts behind the radiator out through the hood. The air is being slowed down when it hits the front of the oil cooler, and the air flowing over the hood can help create a low pressure area and offer a little bit of pull, but it won't be matching the air speed going over the hood, not immediately anyways.

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    Quote Originally Posted by sstecker View Post
    are you trying to accelerate air back to free stream air speed?
    A duct like that won't appreciably change air stream velocity (it's not sealed up for any large section). It does increase the delta P across the heat exchanger though by letting the exit be exposed to a low pressure region compared to the underhood ambient pressure.

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    Quote Originally Posted by Def View Post
    A duct like that won't appreciably change air stream velocity (it's not sealed up for any large section). It does increase the delta P across the heat exchanger though by letting the exit be exposed to a low pressure region compared to the underhood ambient pressure.
    yeah was interested if there was a top half to the duct that sealed and narrowed it down

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    The duct I have pictures above IS fully sealed. The duct seals against the hood, and it creates a closed duct that gets slightly wider towards the hood opening with a very low angle. Its not worth it to sensor it up - I know it works well, so thats that

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    How do you go about determining the ideal size of a vent on the hood? Is there some basic starting formula that has to do with how much air is moving through that location at a given speed? Is bigger better up to a certain point?

    My train of thought would assume the vent should be a little smaller than say the radiator if everything was completely ducted/closed. As the area gets smaller the air has to move quicker, which seems beneficial as it meets with the fast moving air above the hood. Whch leads me to ask if there are any well documented threads showing the best way to seal the radiator with the bumper so extraneous air does not escape?

    -----Another point I'd like to add

    Quote Originally Posted by flink View Post
    Lower under-hood pressures will increase the pressure drop across the radiator and will improve engine cooling. But this isn't necessarily a good thing! More cooling equals more drag. If you already have enough cooling then don't go adding more cooling&drag.
    Can you explain this more, I can't quite make sense of that concept.
    Last edited by 95StreetDoc; 07-05-2011 at 11:52 PM.

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    Quote Originally Posted by 95StreetDoc View Post
    How do you go about determining the ideal size of a vent on the hood? Is there some basic starting formula that has to do with how much air is moving through that location at a given speed? Is bigger better up to a certain point?
    I've never seen such an analysis. Surely it has been done, but the results are probably proprietary and will vary quite a lot with engine power, radiator efficiency, average speed, etc.

    I made mine enormous (http://i240.photobucket.com/albums/f...d/dsc00126.jpg) because it's a heck of a lot easier to cover them up than it is to cut new ones. I haven't yet seen a reason to cover any of them up.

    I don't see much downside to having them larger. There will be a little more flow, but the returns will be diminishing. More area = lower flow velocities.

    In engineering this, the first question should be "what are you trying to achieve?". Then work out a way of measuring it.

    If it's engine cooling then that's fairly easy to measure with a good gauge and back-to-back sessions with various sizes of blanking plates.

    If it's drag reduction or downforce then that's much harder - needs a wind tunnel or sensitive ride-height measurement/logging. Or, of course, driver feedback.

    There's a wonderful web page where some guys took a Datsun (240z?) into a wind tunnel, did all sorts of things to it and recorded the effects. Data here: http://forums.hybridz.org/index.php?...pic=55944kcalc, discussion here: http://forums.hybridz.org/index.php/...datsun-s-30-z/. Alas, one thing they did not play with was hood venting. The test which really stands out there is step 14, "block off lower half of front opening", which reduced total drag by 10% and front lift by 15%. But what they don't say (I think) is where that opening went to originally. Was it sealed to the radiator, or was it uselessly blowing air all over the engine compartment? That matters a lot. I suspect that if the lower opening was sealed to the radiator then blocking it wouldn't help much, because it's already (almost) blocked!

    Whch leads me to ask if there are any well documented threads showing the best way to seal the radiator with the bumper so extraneous air does not escape?
    I grabbed some 0.1" lexan and went hack hack. Pretty straightforward.



    The car ran cool as a cucumber lat time when it was 80F out. It'll be 100 this weekend so I'll be watching that needle! If it's good then I might try taping up the front grille, see how much area I *really* need. But I probably won't get around to it - it's a lot more comfortable doing these things on real test days.

    Quote Originally Posted by 95StreetDoc View Post
    -----Another point I'd like to add



    Can you explain this more, I can't quite make sense of that concept.
    Well, cooling air adds drag. Lots of it. You're taking air into the car and accelerating it up to car speed. Air has mass and e=half-em-vee-squared. If you prevent that air from entering the car and instead push it up over the top of the car, it gets accelerated to a lower speed, so the car loses less e. The way to minimize drag from cooling air is to minimize the mass of air which the car is accelerating - choose the intake area which admits enough air to cool the engine, and no more.
    Last edited by flink; 07-06-2011 at 12:09 AM. Reason: Automerged Doublepost

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    Great post, that helps a lot! Test #14 was very interesting, definitely reduced front end lift, drag, and CD. However, it messed with the rear lift quite a bit.

    I just recently bought my first e36 m3 so I'm relatively new to it. Is there a real purpose of the inlets next to the fog lights in the bumper? Would it be beneficial to block these off?


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    ^in stock configuration, they are brake duct inlets
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    Quote Originally Posted by 95StreetDoc View Post
    Is there a real purpose of the inlets next to the fog lights in the bumper? Would it be beneficial to block these off?


    Quote Originally Posted by ThreeD View Post
    ^in stock configuration, they are brake duct inlets
    ...and they can be blocked-off if you don't need the brake cooling (e.g., cooler day; track with braking zones that are well-spaced and/or requiring moderate braking at most). I use painters tape to block them off. Aero-wise, real block-off plates would be better.
    Last edited by dradernh; 07-06-2011 at 08:21 AM.

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    Any updates on this? a 4x24 opening is usually all that is needed for cooling even on professional race cars. Blocking it off before it even enters the bumper cover would help direct air around instead of under.

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