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A completed NACA duct air inlet, installed on my CSP Miata
Looking for instructions on making your own duct?
Check out my NACA duct install page. I took plenty of
pictures and tried to detail each step of the process of creating your own NACA duct headlight lid. On this page
(the one you're looking at right now), you won't find specific slot-a to tab-b instructions, but you
will learn a little about NACA ducts in general.
About NACA ducts
A NACA duct is a common form of low-drag intake design, originally developed by the
National Advisory Committee for Aeronautics in 1945. When properly implemented, it allows fluid (usually air) to
be drawn into an internal duct, with a minimal disturbance to the flow or increase in drag.
The design was originally called a "submerged inlet," since it consists of a shallow ramp with curved
walls recessed into the exposed surface of a streamlined body, such as an airplane or race car. (thanks Wikipedia!)
A few words on implementing your own NACA duct as a means of induction to your race car. 1: design is very important.
The duct is designed to be efficient with the correct wall angles (sharp), base slope, and width-to-depth ratio. 2: The duct
needs to be installed in an area of high pressure. A leading edge of a car is a great place. The Miata headlight is almost
too far back. Placing a NACA-style duct on a car hood would result in a poorly flowing inlet, in my opinion. 3:
most headlight inlet ducts on the market (at least for Miatas) are not of the correct design, so don't look at
them as examples. At the time of this writing, you could download the original NACA spec from
their archive. Just incase
it moves and I don't notice, I've included a few key pieces from the report.
A width-to-depth ratio of about 3-5 and a ramp angle of about 7 degrees is optimal.
Take note of the airfoil at the back of the duct. It enhances the duct's performance.
Airfoil lip profile: a nice, rounded profile provides best suction.
Where to get one
Life is already hard enough. Building a NACA duct won't be the easiest thing you've ever tried to do
with your dremel and some sheet metal, so why not make it a little less painful? There are places on the
web from which you can buy a pre-fabricated NACA duct. All you've got to do is somehow install it on your car.
The one you see here came from McKinney Corp,
a business that usually caters to drag racers. They sell 3 sizes of NACA duct that will easily fit into most popup
headlights (at least, they fit Miata headlights). The one I have is the largest, their 4" duct, which fits between
my dual Moss low-pro headlights perfectly. There are other sources which can be found with a quick Google search,
but I was so happy with McKinney, that they're the only one I'll link to on this page. The duct was very inexpensive at
about $30 shipped.
NACA duct photos
The installed duct. Note the 90 degree edges.
This is an example of a correctly designed NACA inlet, in my opinion. It's installed in a high pressure zone and maintains
each and every attribute that contributes to a high efficiency, low drag inlet. The only room for improvement is a curved
floor, but, since the duct comes from a 3rd party, that's not really a possibility. The duct is installed with Bondo
to make a sharp wall, then reinforced with stainless button head cap screws. The cap screws create a little drag, but
I like the look.
What the duct is feeding.
Here is the duct delivering cold air to my filter (a few airbox incarnations ago). The airbox assures that the cold air isn't
simply zooming past the filter. The box also doesn't seal completely. There's a small void in the back section to keep the
airflow from stalling.
My 2nd attempt at a headlight lid intake.
This pic shows my coldside intake. You can see the filter directly being blown upon by the duct. This picture
shows an absence of an airfoil at the back of the duct. This will soon be fixed.
A new definition for "Cold air intake."
Some folks seem to think that air will not flow into a NACA headlight lid intake, but rather flows outward. This
photo is my attempt to answer that question visually. In reality, I was caught in a blizzard in my Miata, but afterward
I found this nice little snow drift and thought to take a picture of it. Air indeed flows in the duct, even at 10 mph (the
speed I got to creep home at that day).
Just a pretty shot of my engine bay.
Finally, since I'm sure someone will email asking me, yes, this is an illegal SCCA Street Prepared modification.
If I went to Nationals, I'd probably be protested. As it is, my car (or at least the driver) is barely competitive on the regional
level and I'm not the only one with such a duct, so I'm not too worried about protests.
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