A cowling is a device used on a vehicle to allow air to escape an engine. Propeller-driven aircraft use an adjustable cowling to regulate engine temperature. Motorcycles use a cowling to cover the engine, not only protecting the rider from heat and burns, but also using the flow of air to aid in engine cooling. Many high-performance automobiles use an opening at the rear of the hood or cowling to direct cool air into the carburetor, thereby increasing the performance and horsepower of the engine.
In the early years of airplane flight, it was discovered that the same engine that needed to run cool on the ground needed to operate at a higher temperature at altitude to prevent freezing up. The solution to the problem was the adjustable cowling. When idling on the ground, the pilot could open the cowling, allowing air to flow past the engine, thereby cooling it. When the plane had reached its cruising altitude, the cowling could be closed, thereby retaining heat and preventing freeze-up.
Motorcycle builders soon realized that the use of a cowling would protect the riders of their machines from being burned on the engine components protruding from the frame. Changes in design soon implemented a cooling benefit to the added component and the design of the protective covering soon became the styling signature on many manufacturer's designs. The engine covers soon gave aerodynamic advantages to the machines as well as more surface to paint and chrome, adding style to the motorcycle.
In the search for performance and power, engine builders realized that allowing an engine to ingest cool air equaled an increase in power. By examining the airflow characteristics of automobiles in wind tunnels, designers found that a pocket of low-pressure cool air was situated at the base of the vehicle's windshield. Soon performance-oriented automobiles found new cowl induction hoods being installed by the factory.
While some of the hoods featured an open cowl, many offered opening cowls controlled by the engine's vacuum system. When the operator stomped the throttle to the floor, the drop in vacuum triggered the cowl flap to open. This allowed the engine to breathe the horsepower-enhancing cool air charge directly off of the vehicle's cowling. While drag racing vehicles can benefit from a forward-mounted air scoop to funnel cold air in as the vehicle charges in a straight line down the track, circle track racers still take the cool air charge from the rear of the hood via an air box.