Custom Cold Air Intake (CAI) for the CA18DET


Replace the ca18det’s flexible intake with a CAI (cold air intake)

This tutorial covers the steps necessary to replace the factory flexible intake with a solid, metal pipe unit that will always draw ambient air from outside of the engine bay. This will ensure that the turbo is supplied with the coolest air possible and will provide a little less thermal load on the motor and its supporting systems. Additionally, running a custom hard pipe instead of the flexible factory unit with the internal coil will result in less turbulence along the intake track. Since everyone’s car is different, this tutorial will highlight the general guidelines, supplies and principles involved in creating the Cold Air Intake (CAI). (Adjust the tutorial to fit your car’s layout)

Supplies and Tools

There are many way to go about the fabrication of a new CAI for the CA18DET but for simplicity, I will only talk about the methods and tools I have access too.


  1. Access to a welder. Whether you own one or go to a shop and have them assemble the pieces is up to you. I have a 110v Lincoln Electric wire feed welder converted to mig. It worked just fine.
  2. Some way to cut the metal piping. I used a standard chop saw with a metal cutting wheel.
  3. A dremel or drill with multiple sanding, grinding and or finishing attachments. This will be used to prepare the cut metal for joining.
  4. Several Different sized drill bits or a drill press.
  5. Rivet tool with rivets.
  6. Metal Sheers


  1. Depending on the length of your system, 1 or 2 x 2.5 in aluminized exhaust steel from Autozone.
  2. 2 x silicone couplings to connect the new CAI to the turbo and the MAF.
  3. 1 sheet A/C duct sheetmetal from the hardware store.
  4. 3/4 ” x 4″ galvanized steel gas pipe from the hardware store.
  5. An assortment of clamps.
  6. 1″ heater hose (O’reilly’s)
  7. Proper eye, ear and nose protection.

Getting Started

If you note on the pictures of my engine, I am no longer running an intercooler. I am installing a water injection only system that will chemically cool my intake charge. Because of this, I have a large hole in the sheetmetal in front of the driver’s side wheel well (where the liquid intercooler reservoir was located). This is where I will pass my intake filter through. It just so happens that this will place the air intake filter just behind the front bumper where the SMIC (side mount intercoolers) are typically located. There is an inlet vent on the front of the s13 just below the turn signal that will supply fresh air to this area. Additionally, the splash guards in the wheel well will shield the filter from water so water ingestion is really not an issue. With that in mind, most of you probably aren’t intercooling the way I will be so you have to make some adjustments. If you are running a FMIC (front mount intercoooler) then you could easily run the filter to this chamber using the path along the pass though of the hot pipe. If you are using the SMIC, you will just have to get a little more creative.

This is the opening I will use to route the CAI to the outside of the ca18det’s engine bay.
The hole I am using was originally used for the water reservoir of my liquid intercooler setup. The hole that you use can be much, much smaller.

Cutting the Metal

Before cutting the metal, make sure you have all of your safety equipment on and have a good idea of what lengths and angles you need to cut. You can measure if necessary but this is a pretty easy build to adjust if something doesn’t turn out right as the CAI will be test fit as pieces are cut. So the first two pieces I cut were the beginning (at the compressor) and the first angled piece. The beginning piece is a little bit longer, I cut it to clear the power steering lines, and includes the flat edge and then a 10 degree cut on the other end. The rest of the pipe is then spun 180 degrees and cut again at a 10 degree angle about 2 inches in. (the pictures to come will make all of this clear) Now take the two pieces and hold them in their locations to determine the size needed for the next piece. In the end, I made five pieces. Two longer ones for the ends and then 3 short ones for the bend. All cuts were made at 10 degrees to allow a smooth transition in the pipe.

Pictured are the 5 pieces I cut for my CAI on the ca18det.  You may need more or less pieces.
Pictured are the five pieces I used plus a piece of scrap. Notice the rough edges left by the chop saw. These will have to be sanded down to a smooth finish (inside and out) before welding.

Preparing the Metal

Now the metal needs to sanded and cleaned before the welding begins. First, using a grinding stone and a finishing wheel (I used a 2.5in brass wheel) sand the grind the rough edges from the inside of each end of each pipe piece and then smooth the inside of the piece with the finishing wheel. (The main focus is the edge but I like to give the whole inside a brushing with the wheel to ensure the surface is completely smooth) The surface is smooth enough when you can run you bare finger tip along the inside edge and feel no snags or even the possibility of being cut. (be gentle here because if it isn’t smooth enough you could get cut) Now perform the same procedure to the outside edge of each piece. Smooth the ends and take the top layer of metal off of the edge to ensure a fresh welding surface.These two pieces have been smoothed out on both the inside and outside.
Note that unlike the previous image, both of these pieces have had the rough edge removed from the inside and outside. They are now ready for welding.


Welding the Pieces Together

For my car, I simply tacked the pieces together and created a very gradual bend that led the CAI from the turbo to the sheetmetal opening. If you do not have access to a welder, duct tape the pieces in the appropriate alignment and then take it up to a local welding shop. They should be able to put it together for you. I then test fit the main pipe one last time and then completed the welds.

This is the CAI (cold air intake) tacked together before a final test fit into the ca18det.
This image shows the pieces after tack welding. Notice the flat ends at each side of the CAI. The rest of the cuts are at 10 degrees and are aligned to create a very gradual bend.

The tacked together CAI test fit on the ca18det 240sx
One final test fit before finishing the welds.

The CAI for the ca18det with finished welds
The finished welds should go all the way around each joint to ensure no air leaks.

The BOV Nipple

Unless you are running a MAP system, you more than likely need to recirculate your BOV (blow off valve). I have the SQVV with the recirculation attachment. This attachment will accept 1″ heater hose perfectly so I use the heater hose as a flexible bypass line back to the intake. The inlet on the intake is constructed of a cut piece of 3/4″ galvanized gas pipe.

This image shows where the recirculated BOV will connect to the new CAI on the ca18det 240sx s13
This image shows the recirculation line joining the CAI upstream of the MAF.

To construct the recirculation system, first cut the 3/4″ x 4″ steel gas pipe in half using a 45 degree angle. Just like before the cut side needs to smoothed by removing any jagged edges with the grinding tools. Now, with the main section of the CAI installed, fund the perfect location for the inlet and mark its center point with a sharpie. I located mine just under the power steering line.

This is the 45 degree cut in the galvanized gas line that will function as the recirculation nipple for the new ca18det CAI
The galvanized gas line after being prepped for nipple duty.

This is the test fit location for the recirculated bov nipple
When satisfied with the nipple location, mark the area with a sharpie and remove the main pipe.

After marking the center point for the nipple, remove the CAI and using consecutively larger drill bits, drill a hole where the new nipple will be located. I recommend a diameter slightly smaller than the 45 degree angle opening on the nipple. Also, I did not have a drill bit large enough so I used a small grinding stone to increase the diameter of the opening once I ran out of bits. To finish the hole, both its inside and outside edge need to be smoothed to prevent any metal fragments from making their way into the turbo. The outside of the hole is easy to smooth but the inside will require a Dremel and a small grinding stone, preferably a tapered one. Using the small stone, sand the inside edge and then the underside by tilting the Dremel around the hole while maintaining upward pressure. Again, use the finger test on the inside to make sure all of the metal is smooth.

This is the outside of the hole that will be used for recirculating the BOV on the custom CAI for the ca18det s13 240sx
The outside edge of the recirculation hole.

This is the inside of the hole that will be used for recirculating the BOV on the custom CAI for the ca18det s13 240sx
Notice that the inside of the hole should be smooth and free of any fragmentation.

Next, weld the nipple in place over the hole in the same position that was determined during the test fit. Before the final install of the finished CAI, make sure that you clean the inside of the intake and nipple thoroughly. I usually start with an air compressor to blow out any dust or metal fragments and then use a cleaning solution and snake a rag through the pipe. Finally, give the CAI one more spray with the compressor and then install it using the two silicone couplings to connect the CAI to the turbo and then to the MAF. Use the clamps to secure everything and then, using smaller clamps, install the flexible recirculation line from the BOV to the nipple.

The BOV recirculation nipple attached to the new CAI on the ca18det swapped 240sx s13
The recirculation nipple attached to the new CAI.

Once installed the CAI will look like this in the 240sx’s engine bay.
Image of the finished CAI installed.

Constructing the Heat Shielding

At this point the intake is now finished. The air filter now pics up air from outside of the engine bay and the path the air must travel to the turbo is smooth and direct. But to take things a step further, we can add heat shielding around the pass through to fill in any gaps and completely seal off the intake. This will prevent the intake from ever seeing hot engine bay air. To add the heat shielding, I used one sheet of A/C duct sheetmetal, a pair or metal shears, rivets and rivet tool and a drill with a small bit. Just cut the sheetmetal with the sheers into a few small pieces that will slide around the intake. I made one C shaped piece and then one rectangular piece. The C shaped piece sat around the intake while the rectangular piece covered the 4th exposed side just opposite the opening on the C. With the two pieces cut and in place, drill a small hole through the A/C sheetmetal and then through car’s sheetmetal. Take a rivet and place it flat side up through the hole. The long end should now be pointing straight up. Thread the long end of the rivet through the rivet tool and then give the tool two quick squeezes. The long piece will snap off and in the process the bottom of the rivet will expand, securing the sheetmetal to the car. Repeat this process several more times until the heat shielding is no longer free to move.

Image of a rivet and rivet gun used to make the CAI heat shielding for the ca18det swapped 240sx s13
The rivet works by spanning the distance between two holes. With the long, nail protrusion pointing upward, the rivet tool grabs onto this part and pulls it through the body of the rivet. In doing so, the larger, bottom side of the rivet is pushed through its hollow body where it enlarges the bottom end. The top part then breaks off and the rivet now holds the two pieces together with the enlarged body.

The heat shielding installed around the CAI on the ca18det swapped 240sx s13.
Notice how much smaller the opening is around the new CAI with the sheetmetal in place. Now the filter is almost completely isolated from rising engine bay temps.

By 240am

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