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Old 03-28-2015, 05:05 PM   #1 (permalink)
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Cold air feed via naca duct in flat floor

Hi guys, im currently trying to find the best way to get cold car to the air filter and intercooler on my kit car project.
im still learning and find 'racecar aerodynamics - Joseph Katz' to be a very good read!

Number one problem is the engine is in the rear.. i.e mid engined and getting cold air there is quite difficult.

I plan on making a flat floor and use a couple of naca ducts to force air to the rear, one will feed the air filter and one to feed the intercooler. I'm unsure on which route to go... i have a couple ideas in my head.

I was thinking of having the naca duct go to an airbox with an air filter in obviously! i dont know wether to put a lid on the airbox, which may increase lift? as the air will have no where to continue flowing


To have an exit vent out of the engine cover to give a path for the air to flow, the only trouble is induction air may get sucked in via the exit vent and cause some turbulence and this air wont be filtered... Or will the exit location determine wether this will happen or not?

OR just use the naca duct as a cold air feed and have a cone filter placed in the genearal area.

The same principle will apply for the intercooler, although having an exit duct will be useful as there will be no induction problems.

I hope that makes sense to someone, i am by no means an expert, just thinking logically from the little i have learnt.

Can anyone offer any advice on this on what will work? any help will be greatly received


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Old 03-28-2015, 06:38 PM   #2 (permalink)
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NACA duct gets the job done nicely...small or large:

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Old 03-29-2015, 02:55 AM   #3 (permalink)
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Not on the floor! The airflow is messy there and you'll clog your filter up with dirt in no time. Use a side duct, that's what race cars do for cooling. You can open up the rear of the engine bay to a hole through the bumper and that will help suck air through side vents that you can duct to intercoolers and radiators.

I would cut NACA ducts into the side of the car, run a shroud that expands in area slowing the flow for when it hits the intercooler, and pipe the engine intakes into a hole cut somewhere ahead of the intercooler in the shroud. Ultimately, the engine doesn't ingest nearly as much air as is flowing around the car, so this should be okay. Then preferably have ducting to exhaust the intercoolers into the wake.

What chassis are you using?
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Old 03-29-2015, 05:56 AM   #4 (permalink)
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What are yer current intake temps above ambient? If the goal is horsepower, how much more horsepower will you gain by dropping the air inttake temps close to ambient? Have you measured the actual gains? How much more flow are you looking to increase? Do you have a way to measure it?

You could build a dedicated ram system off the nose and straight line the tunnel through the passenger compartment to the engine bay.

The best I ever got my intake temps was just a mere 4 degrees above ambient (measured with OBDII on the air intake temp sensor) using a ram air setup off the lower air dam. Horsepower wasn't measured before or after. Chances of sucking in water into the intake system was greatly increased if I were to drive on any flooded street. I ran with this setup fer over a year and decided it wasn't worth the risk...so I returned my air intake back to stock which pulls air from the front fender. Air intake temps are only 10 degrees above ambient from this location...so a mere 6 degrees difference while at traveling speed. Would 6 degrees make a significant difference to yer goal?

I'd be curious to find out how much of a measurable difference a properly designed NACA air duct would make fer yer application.


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.
-3: Don't buy garbage. Edges on the slopes have to be sharp, otherwise the flow would separate, reducing the duct's efficiency.
-4: NACA duct is only useful in applications where you really don't want much air, at least not as much as you hoped, and certainly less than you expect.
-5: If you want any air at all, make sure the NACA duct is placed in a region with a positive pressure gradient; i.e., put it where the air sees the body as increasing in size, not constant or decreasing.
-6: They don't work well, or at all, when placed on the negative pressure area.
Some flow comparisons on a RAM vs NACA ducts...
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Last edited by BamZipPow; 03-29-2015 at 07:01 AM..
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Old 03-29-2015, 07:12 PM   #5 (permalink)
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VW XL-1 takes it from the top:


Edison2, from the side:


How do you handle the cooling air? Best practice seems to be one radiator in front of each front wheel.

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