Even casual observers of things automotive have probably the curious tendency for certain sporty-looking cars to sport prominent, well, holes in their hoods. What are these hood scoops supposed to be for? Let’s find out.
HOOD SCOOP FUNCTIONS
Air scoops of various kinds are a common feature on cars with performance pretensions. Many scoops are purely cosmetic, but those that aren’t typically serve one or more of the following functions:
COLD AIR INTAKE
Burning fuel requires oxygen. Unless an engine carries its own oxygen supply (as with a rocket engine), that oxygen must come from the surrounding air. The amount of oxygen available to burn — and thus the engine’s maximum power output — depends on ambient temperature and local static atmospheric pressure. As a rule, cooler, denser air will yield more power while warmer, thinner air (such as on a hot day or at high altitudes) yields less.
The engine compartment of the average automobile tends to be very warm indeed. The normal operating temperature of the typical water-cooled passenger car engine is well above 160 degrees Fahrenheit (71°C) and an air-cooled engine may be considerably hotter. The heat radiated by a running engine quickly heats the air around it. Since most automotive engine compartments are enclosed and rather cramped, with few opportunities for the heat to escape, the air in the engine compartment is usually significantly hotter than the outside air. If the engine draws its intake air from under the hood, the high temperatures will reduce the density of the intake charge and thus reduce the engine’s net power output.
An obvious solution to this problem is to add a cold air intake channel that allows the engine to draw its intake air from the cooler, denser air outside the engine compartment. An effective cold air system can counteract much of the power loss caused by high under-hood temperatures, potentially improving engine output by 5% or more.
Simply cutting a hole in the hood does not a functional cold air scoop make. To be effective, a cold air intake (a) must be located in a high-pressure area of the hood; (b) must be designed in such a way that it actually allows outside air to pass through the inlet; and (c) must have a tightly sealed connection to the air cleaner and intake manifold so that the engine will breathe through the scoop rather than drawing some of its air from under the hood. The distance from the scoop to the air cleaner must also be as short as possible — the greater the distance the incoming air has to travel, the hotter it will get, both through friction and through absorbing engine compartment heat. A poorly designed or badly placed cold air scoop can be worse than useless, costing power by restricting the flow of engine air.
RAM AIR INTAKE
In a normally aspirated engine, the density of the intake air is dependent on local atmospheric conditions. However, it’s possible to artificially increase the density of the intake charge by compressing the air before it enters the cylinders, an effect generically known as supercharging. There are several ways to achieve supercharging, including the use of a mechanical compressor (a supercharger or turbocharger) or resonance effects within the intake runners (described in greater detail in our article on the Dodge D-500). Another approach is to use the vehicle’s motion to force air into the engine under pressure via a ram scoop.
In any body of moving air (or other compressible fluid), the air’s static pressure is inversely proportional to its velocity. To take advantage of this principle, the cross-sectional area of a ram scoop’s intake plenum typically starts off small and gradually increases. As a result, air enters the plenum at high speed and then slows as the plenum widens. As fast-moving air continues to enter the plenum, air begins to pile up and its pressure increases. If this high-pressure air is admitted to the engine’s intake valves (assuming the pressure is not diffused before that), it can be used to provide a mild supercharging effect. A functional ram scoop generally also serves as a cold air intake, although the reverse is not necessarily true.
The benefits of even a properly designed and well-placed ram scoop can be difficult to quantify because the supercharging only occurs when the vehicle is moving and thus can’t be measured on a stationary dynamometer (unless you also have a wind tunnel). It can also be tricky to determine whether any power gain is due to the ram effect or simply the benefits of admitting cooler air to the engine. However, a good system under ideal conditions might conceivably produce a power gain of up to about 10%.
Supercharging, particularly with a mechanical compressor, increases the pressure of the intake air, but also its temperature. The higher temperature reduces the charge density, which tends to defeat the purpose of supercharging and increases the risk of detonation or preignition within the combustion chamber. To address these problems, many turbocharged or supercharged engines add an intercooler: a heat exchanger that cools the pressurized intake charge before admitting it to the cylinders.
Most intercoolers are of the air-to-air type, which means they need a continuous flow of cooler air to which the heat removed from the intake charge can be transferred. As a result, some vehicles with intercooled forced-induction engines use scoops to channel air over or through the intercooler, carrying away its waste heat.
There are several styles of hood scoop:
Any object moving through the air is soon surrounded by a layer of slower-moving air known as the boundary layer. The boundary layer clings to the surface of the object, interfering with the flow of faster-moving air around or into the object. This can defeat the purpose of a scoop, particularly a ram scoop, by blocking air from entering the scoop inlet (except perhaps a small amount of the boundary layer air itself). To avoid that impediment, many scoops are raised or extended outside the body to place them above the boundary layer. This can increase the scoop’s effectiveness, although the scoop will then increase the vehicle’s aerodynamic drag.
For fast-moving vehicles like jet aircraft, the extra drag caused by a raised or extended scoop is problematic. Recognizing this, back in the forties, the National Advisory Committee for Aeronautics (NACA, the predecessor of NASA) developed a new type of low-drag recessed scoop, now generically known as a NACA duct.
The inlet of a NACA duct is shaped to deflect boundary layer air away from the opening so that the boundary layer won’t block the entrance of faster-moving air. NACA ducts generally can’t admit the same volume of air that a raised scoop can, but the NACA duct generates significantly less aerodynamic drag, a worthwhile tradeoff for race cars or fast jet aircraft. The first use of a NACA duct on a production car was probably the 1969 Shelby Mustang, but they are relatively common on race cars and show up periodically on high-performance street cars like the Ferrari F40.
COWL INDUCTION SCOOPS
Many scoops face forward, in the direction of the oncoming air, but every so often you’ll see a reversed scoop, facing away from the air stream. Why? On most cars, the area at the base of the windshield is a high-pressure area. If a reversed scoop is mounted close enough to the windshield, that high pressure will help to force air into the scoop. For the same reason, many modern cars, even ones with no performance pretensions, take their interior ventilation air from ducts in this region.
A popular muscle car gimmick was to incorporate an integral scoop into the engine air cleaner and extend the entire assembly through a hole in the hood. Since the scoop assembly was rigidly mounted to the engine, you could see the scoop vibrating whenever the engine was running — hence the “shaker” nickname. Shaker hoods fell out of favor for street cars with the end of the muscle car era in the early 1970s, in large part because they make it difficult to meet noise regulations, although they’ve made occasional reappearances since.
MANUALLY OR VACUUM OPERATED SCOOPS
A big problem with hood scoops is rain and snow; internal combustion engines do not, as a general rule, take kindly to ingesting liquid water. Most factory-installed functional scoops have drainage passages to keep water out of the engine, but those drains may not be adequate in heavy rain and they do nothing to keep the scoop from becoming packed with snow, ice, or other debris. In really bad weather or dusty conditions, having a gaping hole in the hood is seldom desirable.
In the late sixties and early seventies, there was a brief vogue for scoops that could be opened and closed remotely. Some were manually controlled by a cockpit lever while others were operated by engine vacuum; the optional scoops on GM cars, for example, typically opened only at full throttle, when the engine most needed the cold air. Sometimes, the intake was opened and closed by an internal door or diaphragm, but the “Air Grabber” offered on B-body Dodges and Plymouths was a retractable, pop-up scoop that sat flush with the hood when closed.
IF YOU CAN’T DO IT, FAKE IT
A functional scoop costs money to develop and engineer and poses certain handicaps in the real world. Moreover, the modest performance gains a working scoop can provide are of more interest to racers than the average Joe or Jane.
Unsurprisingly, then, a fair number of cars with sporty pretensions stick with simulated scoops. Fake scoops often have only the most tangential relationship to the real thing and are usually mounted where the stylists thought they would look cool, not where they would make functional sense. (This is particularly evident in cars that offer functional scoops as options; the working scoops are often in different locations and have very different shapes.)
After all, sometimes it’s more important to look fast than to go fast…
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Great site! This is what I have been looking for all these years. I really wanted to know how rain would have an effect on hood scoops. I am thinking of adding one to my car, a functional one. I didn’t know how car manufacturers kept the rain out. Apparently they cannot. I’ll be reading more articles from this site for sure!
Factory scoops sometimes (though not always) have some provision for drainage, enabling them to cope with at least light rain; how well they would cope with a monsoon is another question.
I haven’t examined hood scoop drainage systems in any detail, but I imagine they would be similar to the setup used on some factory moon roof installations. With the sort of moon roof that retracts into a slot inside the car (rather than popping up outside the cabin), there will usually be drainage channels on either side of the roof opening. Those usually have a drainage tube at each end, running down through the roof pillar to exhaust in the wheel wells. As long as the drainage tubes don’t get plugged with dust and debris — and as long as you don’t have a torrential downpour that overwhelms their drainage capacity — rain water just runs out the drains, rather than into the cabin.
I would think that asking about provisions for drainage (and dealing with rain or snow in general) would be a good question to ask before ordering a car with a hood scoop or having one installed. Even in Southern California, it does rain sometimes…
MOPAR makes an underhood venting/water mgmt system for their T/A scoop hood (dodge challenger) that sounds ideal for dealing with the above issues with cold air intake/supercharging…somebody is at least thinking about the water, venting issues…
The intercooler scoop picture of the Mitsubishi Lancer Evolution is misleading.
The Lancer has what is called a “front mount” intercooler, it is in front of the radiator visible and fed by air from the bumper grille.
The hood opening is not a scoop, but an extractor. The Evo relies on a passing a large volume of air through the front grilles, for the intercooler and the radiator. High pressure underhood is a hinderance to airflow, so the extractor provides and efficient escape path.
A car with an underhood “top mount” intercooler would be a better example of a hood scoop. Any turbocharged Subaru will have a fully functional scoop ducted to an intercooler. Reaching back a little further the Ford Thunderbird Turbocoupe and Mustange SVO used a similar setup with top mounted intercooler. ST185 Celica All-trac, Mazdaspeed3 are more examples.
Top mounted intercoolers are not considered as efficient as a front mount, since they hang out with the hot engine bits. On the other hand they make for a shorter intake tract and reduced turbo lag.
Great site by the way, excellent well researched articles. I have found it a great way to understand the automotive landscape in the 60s-70s, having been born in ’76, more a child of the 80s.
Thanks for the clarification on the Evo; I’ve amended the text. I haven’t looked under the hood of that vintage Evo, and had assumed (incorrectly) that it was similar to the Subaru WRX.
I would like to add one to the bump in my 99 eclipse by simply having a hole cut into it oblong shaped, could someone tell me if this is possible to do without drawing in water and should it face backwards or forwards?
My opinion is it could be done facing forwards with a free flow filter under the hood itself along with a flap to switch between the factory installed and my newly cut hole if you guys think water would be a problem.
I love the bump just wished it had a true function instead of just a rising moon them.
Please search the web for pics and let me know if it can be done. Thanks!
Isn’t the purpose of the bulge to allow clearance for the cam pulleys, which extend higher than the sloping hood would otherwise permit? I’ve never studied the under-hood layout of the DSM cars in any detail, but I thought the air intake was on the opposite side of the engine bay. Even if you could cut a hole in the hood bulge that would admit air, what purpose would it serve?
…Oh and before I forget, This is a very interesting site hope to see it go number one! :)
Oh, for forced-induction cars with intercoolers, add to the above list (d) Help to push air through the intercooler.
In the late 1980’s, my first car was a (rusty) 1976 Trans Am. It was equipped with the 7.6:1 compression 6.6 litre, 180 hp V8. Weighing 3,700 lbs as-measured at a mile-high altitude drag strip,It ran 17.5 second times. Slow for sure, but great fun for a 16-yr old.
After removing the installed block-of plate, it wasn’t terribly obvious if the car actually performed better. Did it work though? You bet. When flooring the throttle, from above 40-45 mph,when snowing, the snow would change direction around the windshield and cowl, literally ramming into the open scoop. The increased “buh-WAAA” sound from the Quadrajet at least made it feel like an extra 50 hp was present
Sure, maybe not the best idea to be driving like this in the snow but again, I was 16 :)
Today, I have a 2012 Mustang GT with the 5.0. The factory airbox has a cute snowflake emblem on it and it reads “cold air”. I doubt it does anything, since Ford had to add a sound-tube, that’s piped up under the dash, to communicate induction roar. I miss the Quadrajet roar.
It should be said that the performance effects of a cold air or ram air intake are at best subtle: The general assumption in the late ’60s and early ’70s was that an effective cold air intake was worth an extra 2-3 mph in quarter mile trap speed, which is significant on a drag strip but not that noticeable on the street unless you really play fast and loose with local “display of speed” laws.
The piped-in induction noise systems don’t indicate anything one way or the other about the effectiveness of a cold-air system — it just means that engine noise is otherwise well-muffled. Effective cold-air or even ram-air systems don’t [i]have[/i] to be accompanied by apocalyptic multi-barrel intake roar, although I realize some people would consider that a definite bonus…
Peter: how’s it going had a question for u and everyone. I got a 2005 grand am gt from dealership has ram air v6 and a unique hood I haven’t seen before with fake hood scoops so I knocked out the block off plates In the hood scoops to the engine, now I’m worried if I made a mistake and have to get a whole new hood, I’m worried about rain. I also live in Idaho any suggestions.
Samuel: I’m somewhat familiar with the Grand Am’s that have the Ram Air V-6 and from memory, it seems like one model year received hood mounted (imitation) scoops and later years had functional scoops ducted into the front grill. A colleague at work has one of these, to this day and she’s not convinced that this “Ram Air” plumbing is meaningfully contributing to increased performance, but did note that “induction roar” is pronounced under hard acceleration. From my experience with the second generation Trans Am’s equipped with the shaker hood scoop, that snow, ice, water, leaves, dirt and small animals could and did make their way into the air cleaner housing. I did occasionally see water in the housing, but I never noticed the air filter element to be excessively damp and I rather assumed that either a minimal amount of water, if any, entered the engine. They were mounted high on the hood and the air cleaner assembly had provisions to drain water. I’m definitely not a mechanic, but my primary concern with your condition, is the relatively low mounting point for your hood scoop and the likelihood of water entering during inclement weather e.g. driving through a large pool of water and/or heavy downpour-I’m just not sure the hood is designed to effectively evacuate water under these conditions and I’d hate to see an excessive amount of water enter your engine under this condition. The aftermarket used to well-support these cars, offering different types of hoods with various induction options, but I don’t know if they’re still available or effectively designed for inclement weather. If it were me, I’d likely re-install the block off plates, just to be safe. May I also recommend that if you’re interested in increased performance, that you search the aftermarket for a well-engineered under hood cold air kit? Some may (also) require an aftermarket (software) tune and I’d let your local emissions rules dictate whether this should or should not be accomplished My antics with the old Trans Am’s was definitely reflective of my young age and questionable maturity. Today, I stick with name brand cold air kits :)
Wondering about airflow dynamics at lower speeds more for off roading, at low speed while off roading the airflow dynamics would not come in to play so much I would suspect. However the purpose of some aftermarket hoods for Jeep JK’s such as the AEV Heat Reduction hood has to side vents on the frontal curve of the hood and one topside vent. At high speeds you can actually see the hood vibrate so I suspect the engine heat and airflow out of this hood would reduced high under hood pressure. However my concern is head reduction at low crawl speeds for hours while off roading.
Can you give me your opinion on the benefits of this hood, actually i was thinking of replicating the same vents on my own hood very similar to the AEV Hood.
I’m afraid I’m really not qualified to comment on the effectiveness of any specific components. I’m not an engineer, and the dynamics of airflow in or out of slots on a moving object are very complex (and obviously the dynamics of moving hot air out of a space are different than the dynamics of forcing cool air in). Sorry!
I wonder what would happen if you turned a shaker so the holes are in front would that be considered a ram air shaker?
The direction of the scoop is not the determining factor here — it’s whether (a) the scoop is positioned in such a way that the inlets are positioned in a high-pressure area, (b) the inlets are designed so that air entering the scoop inlet increases in pressure, and (c) the path between the scoop and the intake ports allows the engine to actually take advantage of the supercharging effect. For instance, if you have a hood scoop that’s open to outside air, but just admits that air to the engine compartment, not into the air cleaner or intake manifold, the scoop probably isn’t really providing any useful ram effect even if the inlet does increase air pressure.
You certainly do have patience when it comes to answering questions regarding how to source/repair/modify/appraise/etc. a vehicle. Especially considering those questions are usually answered many times earlier in the same comment section. I’ll just say that some people learn best by experience and in the scheme of life, one more used Grand Am with a hydro-locked engine is a small price to pay along the road to knowledge.
Possibly so. Mostly, I want to make it emphatically clear — mainly for legal reasons — that I’m not offering repair or appraisal advice. The last thing I want is to get roped into somebody’s dispute over how much some car should cost or how best to fix some problem.
What does the length represent when the catalog says the length of the scoop is 55.3 inches? I am guessing it is not the length of the scoop. I am looking to get a scoop for an old 77 chevy truck I’m restoring and I have no idea what that measurement is unless it is the size of the hood you want to put the scoop on. Do you know?
I don’t know what catalog you’re looking at or what it says, so I’m afraid I really don’t know. That wouldn’t be an entirely implausible length for a snorkel-type air intake hose (of the kind used on modern cars and trucks to connect the intake manifold to an airdam below the front bumper), but it wouldn’t make much sense for a hood scoop. Your suggestion that it may refer to the hood size for which it’s intended seems a little more plausible. I suppose it might also be a typographical error of some kind. I don’t have enough information to suggest a better explanation.
I inherited a 77 c3 corvette. The previous owner modified the engine, boring the cylinders and adding an Edelbrock 4 brl carb. They cut a hole in the hood and added a scoop over the air breather via a mounting plate, and did it badly.
Long story short I removed the scoop and had the hood fiber glassed with a modified reverse scoop but I did it for cosmetic reasons, not thinking about the effect it would have in the engine compartment. Needless to say I am overheating badly now and need to add some sort of cold air induction system.
Yes, I am an inexperienced novice and just want a fun car to drive. Given the limited details I have supplied, how would you suggest I approach this problem?
As I’ve told other people on this thread, I can’t provide any advice on modifications or repairs. I’m not a mechanic and I’m not qualified to give mechanical advice!
Aaron, I was wondering what are those kinds of scoops called where the engine is sticking out of the hood, kind of like men blocks awd mustang, where the engines intake is sticking out the hood.
A scoop that’s attached to or integral with the engine air cleaner and just protrudes through a hole on the hood is typically called a shaker.
my 04 grand am has the ram air intake. i took the plastic air channel off the car and wrapped it in foil covered insulation. also did the air filter box bottom half with the same insulation. made sure not to block the water drain hole.
at idle without insulation, the intake temp with the engine at idle and full operating temp was 90f. after adding the insulation and same engine conditions, the temp is 70f. i forgot what the outside temp was, but i liked the results alot. i plan to wrap the top of the air cleaner box and test the temp again.
I’m thinking about making my ’78TA shaker scoop functional and to work off of the engine vacuum. This car will NOT see any rain except for the unforeseen cloud bursts that the weatherman may have missed. Do you know if there are any air filters out there that could withstand a minimum amount of moisture? I know there is one brand that you can wash then spray with an “oil”. Also, could I keep the original air scoop through the front grill, or will it impede the functional scoop performance?
We in Australia are in the middle of a heatwave (45 degree plus on some of the roads I drive)Overheating is a problem so my question to anyone is . hat would be the effect and ramifications if I raise the rear of bonnet(windshield) with blocks to give say a 25mm (1 inch) gap along the windshield?
Will this have some cooling effect the same as a scoop?
I am not a mechanic and am not qualified to tell you how or whether to modify your car. I also live in a much more temperate climate, so I can’t claim any familiarity with the tricks Australians use to not end up stranded in the scorching sun to become dinner for your colorful native fauna.
I will note, however, that for modern cars, airflow into and through the engine compartment is something that often gets quite a lot of design attention (in part because it’s significant for aerodynamics as well as cooling). So, for modern vehicles, I would be wary of shade-tree modifications not conceived by an engineer with some knowledge of how the specific vehicle’s under-bonnet airflow is supposed to work. With what you suggest, I would also be concerned with the increased risk of the bonnet flying up at some inopportune moment, leading to crashing and being eaten by local wildlife.
My suggestion would be to first consider contacting a dealer service department or technician who deals with your specific type of vehicle and asking if they have any recommendations for avoiding overheating.
To give you an idea of air flow for a rear facing scoop. Friend of mine had a 76 trans am 455. we opened the scope and fiberglassed in some chrome screen to the inlet. One day he came by and picked me up excited saying there was something he had to show me. we went to a car wash and soaked the car. pulled out on the street. when the rear barrels of the carb opened, all the water on the entire windshield went down the scoop. I mean whomp and it was gone, cleaned off the entire width of the windshield. Now that is inhaling air and one of the most impressive things i ever saw in connection with hood scoops.
How does the 65-66 coevette power bulge function? Does it actually get cool air to the engine or just vent the engine compartment?
Many thanks in advance
Given how shallow it is, my assumption has always been that it doesn’t meaningfully do either, and is mostly there to decorate the bulge added to the hood to accommodate the physically larger Turbo-Jet engines. The contemporary brochures and advertising material don’t make any claims for its functionality (although they do claim that the front fender louvers are functional, in the sense of ventilating the engine compartment).