Spend some time watching racing (or, alternately, ogling Lamborghini Countachs or Bugatti Veyron Super Sports), and odds are good you’ll notice, among all the spoilers and protuberances and other aerodynamic advancements, some roughly Y-shaped indentations in the cars’ slippery skins. They’re called NACA ducts, and like the best aerodynamics, they improve performance and make the car look cooler at the same time.
That kid actually doesn't love Ferraris, he just loves NACA ducts. (Ferrari F40 in IMS praking lot; photo courtesy Wikipedia)
Because NACA ducts are close to the surface of a vehicle, the air they consume is being jostled, pushed around, and otherwise disrupted by the vehicle’s moving body. Thus, the air they eat tends to be more turbulent—“messier,” if you will. This zone of messy air is known as the “boundary layer” in aerodynamic terms. In addition, the design of the NACA duct itself—gentle angle leading into the duct, along with the curvature of the duct itself—disrupts the airflow even further by generating vortices that pull in some of the air above the boundary layer, which is actually going faster than the car. (Science!)
Old-school schematics always make us happy.
Now, “messy air” probably sounds like a bad thing—but it’s not. Messy air actually helps to atomize and emulsify the gasoline in a naturally aspirated engine (it doesn’t work so well for turbocharged and supercharged engines, since the air doesn’t flow directly in), allowing it to burn more efficiently. But what messy air is even better for is cooling—which is why you’re likely to see NACA ducts leading into wheel wells and radiators. (Race cars also like to use them to suck cool air in for the driver to breathe, which is why you'll often see them on race car windows.)
Yo dawg, we heard you like NACA ducts.
Of course, there are other ways to catch passing air and reroute it where needed inside a car—and many of them, like scoops, actually provide far better air flow than the NACAs. But the NACA duct’s primary advantage is the same reason you’re less like to notice it than you are a big hood scoop: it’s very low profile. Which means adding one to a design adds very little in the way of drag. Which is good. Drag is a drag. They’re also relatively easy to add to a car, which is why both automotive designers and aftermarket modders have an affinity for them.
You can even find a NACA duct inside the new Mercedes-AMG GT S. Admittedly, it doesn't do much but look cool, but hey, that's all a lot of people use them for.
But if you’re thinking about carving up your car’s bodywork in order to install a NACA duct or two, there are a couple of things you need to know. The design of the duct is important; the edges need to be sharp, and it needs to be appropriately sized for the application (physics forums are great places to ask questions if you don’t know what size you should use). But most of all, the duct has to be placed somewhere that always experiences positive pressure. On a car, that basically means “point it into the wind.”
It's often awkward for young cars when they realize their NACA ducts may not be the same size or shape as their friends'. But that's okay.
Follow those tips, and you can have your Miata looking like a Ferrari F40 in no time. (Well, except for...everything but the NACA ducts.)
If you really squint...that could kinda look like a Viper RT/10, though.
*Oh, in case you’re wondering what NACA is, it’s an acronym for “National Advisory Committee on Aeronautics,” which was NASA’s predecessor. If you don’t know what NASA is, though, you’re beyond our help.