Functional hood scoop on a Subaru Impreza WRX
A hood scoop is an air vent on the hood of an automobile that either allows a flow of air to directly enter the engine compartment, or appears to do so. It may be closed, and thus purely decorative, or serve to enhance performance in several possible ways.
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Contents
- 1 Hood scoop functions
- 1.1 Cool air
- 1.2 Ram air
- 1.3 Intercooler scoops
- 2 Scoop design
- 2.1 Hood scoops and off-road racing
- 3 Hood scoop problems
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Hood scoop functions
Cool air
One possible use of a hood scoop is to admit outside air into the engine's intake ahead of the air cleaner and carburetor or fuel injection manifold. In most modern automobiles, internal combustion engines "breathe" under-hood air or air ducted from under the front bumper through plastic and rubber tubing. The high operating temperatures in the engine compartment result in intake air that is 50°F or more warmer than the ambient temperature, and consequently less dense. A hood scoop can provide the engine with cooler, denser outside air, increasing power. A properly designed cold air intake can add 10-20 horsepower (7.5 to 15 kW) to the engine.
Ram air
At higher road speeds, a properly designed hood scoop can increase the speed and pressure with which air enters the engine's intake, creating a resonance supercharging effect that can add an additional 10-15 horsepower (7.5 to 12 kW). Such effects are typically only felt at very high speeds, making ram air primarily useful for racing, not street performance.
Pontiac used the trade name Ram Air to describe its engines equipped with functional scoops. Despite the name, most of these systems only provided cool air, with little or no supercharging effect.
Intercooler scoops
Some engines with turbochargers or superchargers are also equipped with top mounted intercoolers to reduce the temperature and increase the density of the high-pressure air produced by the compressor. Channeling outside air to the intercooler (which is a heat exchanger similar to a radiator) increases its effectiveness, providing a slight improvement in power.
Scoop design
To be effective, a functional scoop must be located at a high-pressure area on the hood. For that reason, some functional scoops are located at the rear of the hood, near the vehicle's cowl, where the curvature of the windshield creates such a high-pressure zone, and may be placed so that their opening faces the windshield (a reversed scoop).
The scoop will be most effective if it is either mounted high enough to clear the boundary layer (the slow-moving air that clings to the surface of a moving object) or if it is a "NACA duct," mounted below the surface and designed to draw the boundary layer into the duct. A shallow scoop that is not a NACA duct may not admit a useful amount of air even if it is open.
Under the hood, an effective scoop must funnel air into the engine's intake in as short and direct a path as possible, preferably through a tub or channel that is insulated against underhood heat.
A scoop may be part of the hood, or may be part of the engine's air cleaner assembly, protruding through a hole cut into the hood. Such a scoop is called a shaker hood, because the scoop vibrates noticeably when the engine is running, especially under power.
Hood scoops and off-road racing
A hood scoop/top mounted intercooler can be beneficial, especially during an off-road rally race. Rocks and debris can be kicked up by a car in front, and those objects can damage a front mounted intercooler. However, rock guards can be installed to prevent this problem.
Hood scoop problems
A functional scoop presents several possible problems in addition to its benefits:
- The scoop opening increases audible engine noise. This is of concern in areas where local law regulates the maximum permissible noise levels of vehicles.
- An open scoop may admit debris or water directly into the engine, which is a hazard. If the air cleaner element is in place, it will generally prevent debris from entering the engine, although such detritus can quickly clog the air filter. Many scoops for vehicles intended for street use have drainage channels to prevent water from entering the engine, although the channels may be overwhelmed in heavy rain.
- The cooling effect of the scoop's intake air may complicate engine warm-up and pollution control. During the early days of automotive emissions controls, it was customary for stock air-intake systems to have flaps that would only allow the engine to breath cool, outside air at or near full throttle.
Because of these limitations, some scoops are designed so that they can be closed by the driver (using a cabin-mounted lever) or so that they remain shut until opened by engine vacuum.
Both functional and non-functional hood scoops slightly increase the drag coefficient of a car.
