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…