Hood scoops are not necessary technology for automobiles to have but they can enhance a car’s performance if designed properly. Many performance cars use hood scoops for just that reason.
The way that hood scoops work is actually pretty simple, but not widely known. An engine gets oxygen from the outside air. Cool air is denser than hot air and thus has more oxygen in it, which produces more power when it burns. Since the air in the engine compartment is usually considerably hotter than the outside air, if the engine takes its intake air from under the hood, its power output will be reduced. The solution is allow the engine to breathe cooler, denser outside air, such as from a hood scoop.
Over the years various hood scoops designs have occurred.
Raised Scoops – These are the ones that stick up in the air. Some are often relative high off the hood so as to escape the slower “boundary layer” level air near the hood surface. This allows them to not only funnel cool air down into the engine but enjoy a ram-effect whereby air is pushed into the engine.
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 backwards. The way this works is that the area at the base of the windshield on most cars is actually a high-pressure zone. If a reversed scoop is mounted close enough to the windshield, that high pressure will force nice cool air into the scoop.
Duct Scoops – You’ve probably seen them on racing cars. They are a sort of reversed scoop buried below the surface of a car’s exterior with a gently sloped ramp and curved walls. These are often called called “NACA ducts”, at least in the US, as that was the Federal agency that developed them in 1940s for aircraft. NACA ducts are common on serious race cars. They don’t allow a high volume of airflow but they produce much less drag than other protruding scoops do.
Are you wondering why hood scoops aren’t more popular on today’s cars? Well, they have disadvantages too. First, functional scoops often don’t appeal to the design guys. Unless you are designing a car to have a race car look, it is unlikely the design department is going to slap hood scoops on a vehicle anywhere. It just wouldn’t look right. Plus, there are times you don’t want a scoop operating, such as during rainstorms and heavy snow.
Source: Ziegler Chrysler