Let’s say you have a car running twin Weber DCOE carbs. And let’s also say you’re childish and decide you like the look of NACA ducts, even though there’s probably no real benefit in this case. But your inner 12-year-old is chanting: NACA, NACA, NACA!
How would you approach this? Where would you buy a NACA duct? Would you go with a simple slit or a duct-and-hose type of inlet? Top mount or bottom mount? Rivets or epoxy? How would you cut the hood so it doesn’t look totally embarrassing?
You can buy premade NACA ducts from speed shops, or 3D-print your own. I'd use rivets, and for feeding carbs, I'd use an airbox that seals to the hood where the NACA ducts are. Half-decent cutting with a dremel is good enough because the flange of the NACA duct will cover the cut.
You can also make your own fairly easily with nothing more than tin snips, a drill and a rivet gun. It might actually work better than a molded plastic one, as they tend to lack the sharp lip that drives the vortices that make them so effective. You can download the templates online from NASA.
This one was built by cutting out the plan view other than the nose, bending the "floor" down, and building walls with a spare piece of aluminum.
Don’t forget to face it the right direction! It’s a scoop, NOT a Venturi!
to feed the engine, the NACA duct (or submerged inlet) will need to be large (~6 inches wide like the one Keith posted). Any smaller and the potential for choking the inlet flow at high RPM and low car speed is a concern. The original intended use of these was to get a high pressure recovery without the drag of an external duct face. The commercially available ones are really pretty much crap unless they have sharp edges and recessed mounting flanges at the edges. If all the edges look smooth, don't buy it. You may be able to find some well designed ones thru the second hand NASCAR parts stores. Several of the teams have spent some money in developing good versions, but they are going to be small since they are generally sized for a 3 inch hose connection.
If you make your own, the length to width to depth profile is pretty well documented and easy to find. Keep the edges sharp. Also keep in mind they work best when they are aligned with the flow, so a little bit of oil on the surface you want to mount them will help determine what the local front to back direction is (which may change you mind about how they would look in place). If you want the knowledge behind the duct try searching for teh NACA Technical Note #2323 (NACA-TN-2323) printed in 1951.
A scoop and a tunnel will work better. There's a reason they use that instead of NACA ducts for race car intakes.
We helped a guy who was having fueling problems at Bonneville, and I noticed a splash of fuel on the back of his engine compartment. He had a NACA duct on the top of the hood directly feeding his very large carb, and since there was nothing to stop the high speed air flow, it actually created a low pressure zone right above the carb, so bad that it actually pulled fuel out of it.
Once he went back to a traditional scoop design, he set records.
So, yes, a NACA duct will flow a lot of air, but you then have to treat it right so that it feeds your carbs.
I need a hood scoop on my RAV4. I was thinking of doing like the above home brew sheet metal, only augmented with fiberglass. I have the shape saved from the NACA ducts that are under the engine on an Elise. I was thinking of using that shape for a vent going to the air cleaner and a conventional hood scoop for the charge cooler. This is what I need air on:
I am open for suggestions.
In reply to Dr. Hess :
A simple slotted duct at the bottom leading edge of the hood that feeds thru a duct attached to the bottom surface of the hood and blows down onto the intercooler is your best bet for high pressure air and stealthy appearance. Second choice would be a thin-wide scoop near the front of the hood blowing down over the intercooler. You dont have enough distance to use a proper submerged NACA duct in that location.
I'd run induction from the cowl rather than a naca on the hood
In reply to stafford1500 :
Thanks. I'm going to have to think about that. Not sure there is room to do anything at the front of the hood with the radiator there. So, a slot opening at the front of the hood, then a ductwork tacked under the hood ending with a downward piece to push the air into the fans of the intercooler. I'll have to play with some cardboard mockups.
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