bumper cover

For the NACA duct to flow efficiently, you need to shape the cut in the bumper cover in a specific manor.

You'll see that the plastic from which the cover is made is rather thick. The biggest problem this causes is a large "step" at the leading edge of the duct from the outside of the cover to the floor of the duct. That step will keep a lot of air from entering the duct. You need to sand this step down, basically making the front cut in the bumper at an angle that closely matches the angle of the duct floor. Due to the extreme angle and the thickness of the plastic, the leading edge of your cut should be more than 1/2" wide, like shown below:

You can kind of see what I'm talking about here. After I installed the duct, I realized I needed to sand the leading edge of the cut quite a bit more than is shown here. I used the Dremel with a sanding bit for this even when the duct was installed. Aside from that, you want to make the aft edge of the cut rounded off as much as possible. A sharp edge here will cause aerodynamic drag.

The sides (or top and bottom when installed on the car) need to be as sharp as possible. This will cause vortices in the airflow around the edges that will "suck in" the thin boundary layer of air that would otherwise pass by the duct. You'll probably want to round them off a little so that the paint doesn't chip easily, but keep them fairly sharp.

------------------------------------------------------ Then, simply glue the ducts into place. I used a two-part "5 minute" glue sold at Pep Boys (the 60 second stuff sets up much to quickly). It was 2500 psi tensile strength and said it stuck well to plastic (they have lots of different types from which to choose). Be sure and clean the inside of your fender pretty well so the glue will stick. You will then notice a gap between the duct and the bumper cover as shown below. There will also be some glue squeeze out to varying degrees in this gap. That's what Bondo is for!

Say what you might about Bondo, for this application it worked quite well. Simply fill the gap (use a little extra so you don't have any low spots) and sand it down. Be careful when sanding not to scratch your bumper (it's harder than it sounds). I found that some clear coat touch-up paint worked well to "erase" scratches in the bumper and the duct. After everything is smooth, simply get out your touch-up paint and go to work! (If you really know what you're doing, this is where you'd spray the thing with whatever people that know what they're doing spray stuff with.)

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Now, for the important part. You can take air in any way you like but if you don't get it into the rotor it won't do you much good.

After hearing of a number of people that had their "Home Depot Plastic Pipe" systems melt, I wanted to use better materials--like I said, I wanted to build them and forget about them.

To end the ducting at the rotor, I used 2 1/2" 304 Stainless Steel exhaust pipe. You can get a three foot chunk from JC Whitney for about $30. It won't rust, it won't melt and it won't break.

You can see the basic shape of the end pipes in the pic below:

This takes a lot of trial and error. I think the pipes ended up around 4" long or so, but start them out longer than that. I hammered the pipes progressively to an oval shape at one end with a rubber hammer (you'll see the air intake of the rotor is fairly thin and tall where they will be installed).

Then, test fit them onto the spindle. You'll see they kind of naturally want to sit at an angle. This puts them at an angle with the rotor--mark the angle and cut off the oval end at that angle. Then cut the excess length from the round end so that it will clear the swaybar endlink at full steering lock.