Real or simulated air scoops are a common feature on cars with performance pretensions. There are four principal permutations:

1. Cold air intake.

Engines burn oxygen as well as fuel. Unless the engine carries its own oxygen supply (like a rocket engine), it gets that oxygen by burning the surrounding air. Cool air is denser than hot air, and thus has more oxygen, producing more power when it burns. Since automobile engines have normal operating temperatures well above 160°F (71°C), and the space under the hood is usually quite cramped, the air in the engine compartment is always considerably hotter than the outside air, typically by 50° or more. If the engine takes its intake air from under the hood, therefore, its power output will be reduced noticeably, by as much as 10%.

The obvious solution is allow the engine to breathe cooler, denser outside air. A cold air intake channels outside air into the engine's intake. An effective cold air system counteracts much of the power loss caused by breathing hot, under-hood air, potentially improving the engine's output by 5% or more. To be effective, the intake must be located in a high-pressure area of the hood, it must be tightly sealed to the engine intake (so that the engine will breathe only through the scoop), and the engine management system must be capable of handling the increased air volume. (A poorly designed cold air system can actually cost power.)

There are various commons styles of hood scoop:

(A) Raised scoops: Any moving object is surrounded by a thin layer of slower-moving air, known as the boundary layer. The boundary layer can reduce the effectiveness of an air scoop, because its sluggish air blocks faster-moving air from entering the scoop intake. To avoid that impediment, many scoops are raised or extended above the surface of the body, above the boundary layer. This makes the scoop much more effective, but it also significantly increases the car's aerodynamic drag.


The 1969 Hurst/Olds has two massive raised scoops on the hood. They channel air through two holes (rather crudely hacked into the fiberglass of the hood) and into the air cleaner assembly on top of the engine. The air cleaner housing is mated to the hood with a rubber gasket, forcing the engine to breathe only through the scoops. The scoops are definitely functional, though they're anything but subtle, and they create a great deal of drag.

(B) NACA ducts: The extra drag caused by raised or extended scoops can be significant, so back in 1945 the National Advisory Committee for Aeronautics (the predecessor of NASA) developed a type of submerged scoop for fast-jet aircraft, now known as a NACA duct. Rather than protruding above the body, a NACA duct sits below the surface, with a gently sloped ramp and curved walls. (The curvature of the duct's sides creates vortices that deflect the boundary layer away from the faster-moving air entering the scoop.) A NACA duct creates less drag than a protruding intake, although it doesn't allow a high volume of airflow. The first use of functional NACA ducts in a production car was probably the 1969 Shelby Mustang, and they are relatively common on race cars. They show up periodically on fast street cars like the Ferrari F40.


The central scoop on the 2008 Mitsubishi Lancer Evo X is a NACA-style duct.

(C) Cowl-induction scoop: 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 force air into the scoop. Even cars that don't have hood scoops of any kind usually take their interior ventilation air from a duct in this region.


This 1969 Chevrolet Camaro has the optional "cowl induction" hood. The intake is under the raised lip of the scoop. Notice the small grilles just forward of the windshield wipers -- they're the intakes for the Camaro's interior ventilation system, which also takes its air from this high-pressure area.

(D) "Shaker" hood: 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 is rigidly mounted to the engine, it vibrates noticeably while the engine is 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.


Shaker hood on a 1971 Plymouth Barracuda. There are rubber seals around the inside of the hole in the hood and on the outer edges of the air cleaner to seal out dust and debris.

(E) Manually operated or vacuum-controlled scoops: A big problem with hood scoops is rain: engines do not, as a general rule, take kindly to ingesting big gulps of 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 or 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, either controlled by a cockpit lever or 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 Mopars was a retractable, pop-up scoop that sat flush with the hood when closed.

2. Ram air intake

If air can be forced into an engine at higher than the local static atmospheric pressure, more oxygen is available to burn and more power can be produced, an effect generically known as supercharging. A ram-air scoop has an intake plenum that gradually increases in cross-sectional area. The increase in area reduces the velocity of the incoming air, but increases its static pressure. Since the intake air is at higher-than-atmospheric pressure, it produces a mild supercharging effect. A well-designed ram-air intake can produce a power increase of up to 10% when the car is moving quickly through cool air. (Most ram air scoops are also cold air scoops, but not all cold air scoops provide a ram air effect. It can be difficult to quantify the benefit the ram air effect provides, since any supercharging appears only while the car is moving at high speeds; it can't be measured on a dynamometer, where the car is stationary.)

3. Intercooler exposure

The mild supercharging of a ram-air scoop is nice, but it's only effective at higher speeds. A lot of engines use some form of mechanical supercharger to increase their power output. (The most common type of supercharger is the turbocharger, which uses a turbine wheel in the engine's exhaust stream to provide the energy necessary to run the compressor.) The more boost the compressor provides, the greater the power increase it provides, but compressing the intake air significantly increases its temperature, reducing the density of the intake charge. Not only does that decrease the efficiency of the supercharger, it can cause problems with engine detonation. To avoid those problems, most high-boost supercharged engines add an intercooler, a heat exchanger that reduces the temperature of the pressurized intake air before it goes into the engine. An intercooler is essentially a radiator, and some cars have external scoops to channel outside air over the surface of the intercooler, carrying away its excess heat.


The grille on the top of the hood of this Mitsubishi Lancer Evolution IX is an air extractor for the intercooler, which cools the intake charge of the Evo's highly boosted turbocharged engine. It's highly effective: the intercooled turbo allows the 2.0 liter engine produce more than 276 horsepower (280 ps, 206 kW).

4. Just plain decoration

A functional scoop costs money to develop, and poses certain handicaps in the real world (did we mention engines don't like rain water? or snow?). Furthermore, the modest performance gains a working scoop can provide are of more interest to racers than the average Joe or Jane. Unsurprisingly, then, fair number of cars with sporty pretensions, including the Ford Mustang and some rather ordinary Subarus, have simulated scoops. Fake scoops often have only the most tangential relationship to the real thing, and they're 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.)


Although it looks mean (and apes the 1966 Shelby GT-350, which had a similar-looking arrangement), the hood scoop on this Ford Shelby GT is completely closed, so it does nothing but add drag.

After all, sometimes it's more important to look fast than to go fast...

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