Functional bonnet scoop on a Subaru Impreza WRX

A bonnet/hood scoop is an air vent on the bonnet 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.

Bonnet scoop functions

Cool air

One possible use of a bonnet 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-bonnet 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 28°C (50°F) or more warmer than the ambient temperature, and consequently less dense. A bonnet scoop can provide the engine with cooler, denser outside air, increasing power.

Ram air

At higher road speeds, a properly designed bonnet scoop can increase the speed and pressure with which air enters the engine's intake, creating a resonance supercharging effect. 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 bonnet. For that reason, some functional scoops are located at the rear of the bonnet, near the vehicle's cowl, where the curvature of the windscreen creates such a high-pressure zone, and may be placed so that their opening faces the windscreen (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 faster moving air outside of 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 bonnet, 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 underbonnet heat.

A scoop may be part of the bonnet, or may be part of the engine's air cleaner assembly, protruding through a hole cut into the bonnet. Such a scoop is called a shaker hood, because the scoop vibrates noticeably when the engine is running, especially under power.

Bonnet scoops and off-road racing

A bonnet 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.

Bonnet 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 breathe 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 bonnet scoops slightly increase the drag coefficient of a car.

References

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External links

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