Sheet Metal SMIC Duct

This article will cover the planning and fabricating of a sheet metal duct for a side mount intercooler (smic), but the techniques can be used in any simple ducting you may want to make for your car.

A front mount intercooler (fmic) is one of the first things many owners swap into a DSM to improve the performance of the car, as the stock side mount intercooler (smic) is fairly small and will heat soak after only a couple of hard back-to-back pulls. However, with proper ducting, and by sealing the duct to the core, you can improve the performance of the stock smic and spend your hard earned money on other fun bits until you absolutely have to go larger. A new duct will also be in order if you upgrade the smic. In this example, I have made a duct for my Dejon Big SMIC, as the stock piece is nowhere close to fitting the larger core on this IC.

As an aside to this install, I recently did a 2gb Eclipse front bumper cover swap on my '96 Talon, which gives me the added benefit of increased area on the fascia where air can enter the duct, as opposed to the relatively tiny opening for the stock 2ga bumper. I had previously made a custom duct for my 2ga bumper, removing the passenger side foglight/bracket and running the duct over to fill that area, and it was a definite improvement over the stock setup. So don't think that the cover swap is necessary, I just think it's a better starting point for more airflow to a SMIC on a 2ga DSM.

So, first things first: Read up on how to remove the front bumper/cover from your car, as you'll want to do that. It's not impossible to do the duct with the bumper/cover on the car, but for all the time and frustration it adds, you are better off just pulling it. It really makes this a lot easier overall. You can leave your cover attached to the bumper core if you are starting fresh, so that replacing it temporarily during the duct fitting process is easier. If your cover is not attached to the actual bumper core due to past work, then you are 4 bolts ahead of everyone else, and you can just remove the cover. If you google '2g front bumper removal' you'll find a great visual step-by-step writeup for this, and while I've never pulled a bumper off a 1g, I'd imagine it's not too different from a 2g. There are articles here on Tuners for bumper removal too, but none I could find with photos.

Here's a materials list:
Hand tools required to R&R the bumper/cover and install the finished duct
About 9 sq feet of heavy cardboard (a 3'x3' piece was plenty for this project)
Roll of masking or duct tape for the mockup
Roll of 'real' flue/duct tape to seal the final duct (it's stronger than regular duct tape)
Box cutter
Leather gloves
Sharpie pen
Tin snips (mine are offset snips...makes cutting deeper into the sheet metal easier)
Rubber mallet or similar
Workbench or sturdy straight edged table/countertop for light hammering
Drill and appropriate size bit for the rivets you'll use
Aluminum or other sheet metal (I used 0.032" aluminum because it cuts relatively easily and is rustproof)
Rivet gun
Aluminum rivets of appropriate length/width. (Double your metal thickness and add a little for good measure. I used 1/8" diameter x 1/8" grip range.)
Random nuts/bolts for hanging the duct

Be Careful! Use some common sense when using the box cutter, drill, and tin snips! Don't draw the knife towards you, cut away from your body. Sheet metal can be quite sharp when cut with the snips, so be aware of all the edges, and the little scraps and bits that are littering the ground around you. Wear the gloves! When you are drilling the holes for the rivets, support the hole from inside the duct with a piece of wood...don't put your fingers under there!


Here's the car up on jackstands, front bumper/cover off, Dejon Big SMIC mounted under the front fender. Use the cardboard to make a template for the duct, one panel at a time, starting a little larger than actual size and trimming back each panel to fit. I started with the upper and lower panels, as they are essentially identical here, cutting them to fit the inside curve of the bumper cover. I then added the sides. I had to trim the upper and lower panels a bit once the sides were attached, so I could curve around the frame a little better, so don't expect to have it all go together the first time. You'll be taping, cutting, re-taping...there is much trimming and tweaking in this step, and the more you do here the better. It's much harder to trim and refit in metal than in cardboard. While fitting, make sure that all curved surfaces are indeed curved, and flat ones are flat, as minor differences there can be frustrating later. Scoring the cardboard on the outside of the curve helps to get it to cooperate if it's stiff.

 

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Once the mockup is completed and fits the way you want it to, you are going to cut it apart one last time. These individual pieces are now going to be patterns for your final sheet metal panels. I was able to get all my pieces out of a sheet of aluminum 24x30”, so lay them out on a table or the ground and figure out the best way to arrange them, so that you can buy the smallest (cheapest) piece of metal to accommodate all of them. Make sure to leave 1” around each piece for the tabs you’ll have to add. So that means at least 1” from any edge of the sheet metal to any pattern edge, and at least 2” between any two panels laid out on the metal. Here’s a photo of my aluminum after tracing the pattern…note the space around each piece. I had a good idea as to how it would go together, so I pushed the patterns right to the edge in some places so I could keep some edges perfectly straight. You can also see the beginning of some of the tabs through which I riveted for final construction…adding these is why you need the extra space around each piece.

 

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Here are the individual pieces as cut from the aluminum sheet, with approximately ½” extra added to each long edge of the side panels, marked with a sharpie, to allow for tabs. I’ve cut at a diagonal back to the ink line at 5 places along those edges where I wanted the tabs to be. I later changed that as I decided to retain all the metal that would have been cut away between the tabs.

 

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You can see below how I changed the tab configuration. I ended up keeping all the edge material, cutting every inch or so along the edge and making a new tab there. I eventually went back and cut a vee between each tab so that when I bent this panel into a curve, there would be no overlap and binding at the edges of the tabs. Most of the tabs were bent over a little past 90*, and will end up inside the duct. Those that are sticking out, as on the left, will stay outside the duct, and will be riveted down. I ultimately folded them over, too, but not quite to 90* at this time. Keeping all the edge material in place as multiple tabs not only strengthens the duct, but also helps keep it sealed.

I scored the line lightly with the box cutter where it will bend, and used a soft (leather) hammer to bend the tabs over the edge of an anvil so that I can keep everything as straight and crisp as possible. You could use a rubber mallet and the edge of a workbench or countertop, too...it just needs to be straight and sturdy enough to hold up to the light hammering. If you have to use a metal hammer, tap lightly, as you can easily stretch the metal out of shape if you pound on it, leaving you with panels that don't fit properly.

 

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I’ve now finished cutting and bending the tabs, and have put the duct together ‘dry’, with no rivets, to make sure that it will fit together. I’ve made gentle bends in the side panels to follow the curves in the top and bottom panels, and everything seems to be in order. Here you can see the outer tabs through which I‘ll rivet, and the inner tabs folded inside the duct. At this point, I worked the tabs by hand to lay down against the top and bottom panels, which helps seat the panel as far into the crease as possible to help with sealing.

 

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The next step was taping it together, and a quick test on the car again to make sure it fit like the cardboard mockup did. Everything was good, so I began drilling holes and riveting it up. I started at one end of each edge, and drilled and riveted each hole individually, so that I could pull against the last rivet to drill the next hole and keep everything aligned. It takes a little longer that way, but keeps the panels all lined up and it kept the seams tight. I'd recommend using a bit that's about 1/32" larger than the rivet, so that you can better deal with any small amount of slippage between the sheets as you drill through them.

In this photo you can also see the extensions that flank the IC core, and where I cut one to get around the LICP and a mounting bracket on the IC. These extensions are important in helping to seal the duct to the core. You could also extend a flap on the top and bottom panels, but I didn't feel like it was necessary.

 

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Here is the completed duct resting on top of the bumper cover. You can see how closely the shape of the duct mimics the curve of the bumper cover. After riveting, and by retaining all the tabs both inside and out, there are perhaps only a couple dozen holes in total along the edges, no more than a couple millimeters in size, so I didn’t bother sealing the outside edges with tape. I also didn’t worry about smoothing over the inner tabs with tape, as it’s not going to make a big difference regarding airflow. If this was a much smaller cross-section duct, and longer in length, then it may help to do that. But this is pretty much a gaping hole where pressure is going to be more important than flow characteristics through the duct itself.

 

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After final test fitting, I cut this hanger out of some scrap sheet so that I could mount the duct securely in front of the IC. It bolts up to the same place the IC attaches to the car. This, in addition to the tape used to seal it, will be plenty of support for the duct.

 

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After mounting, I still had a small section of the core outside the duct, but once it's sealed up that won't matter so much. Also note the extension on this side of the core...it extends about halfway across the side of the core and provides a good anchor for the sealing tape, and helps keep the duct centered. The wiring harness you see at the right of the photo really presented the biggest challenge with respect to getting this fitted perfectly. In earlier photos you can see that it's taped as far out of the way as possible because I could not find a way to reroute it without major cutting.

You can also see the inner fender liner I made to promote good airflow out the back of the IC, as the Dejon smic sits flush against the stock liner. This one is made of two layers of ¼" hardware cloth, overlaid at a 45* angle. This not only minimizes the size of the holes, to keep out larger stones, but also strengthens the liner.

 

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Here it is from the front, bolted up, prior to sealing. The last step is to seal the duct to the core using at a minimum ‘common’ duct tape, or better yet, actual ductwork/flue tape. Duct/flue tape is like double-thick aluminum foil with an incredibly sticky adhesive on the back, and is more heat resistant than standard duct tape. I used the Ace Hardware house brand, but 3M makes a version of this tape that’s rated to 600*F if you are worried about it coming loose on the hotside of the core. Apply the tape all around the perimeter of the core and duct, making as air-tight a connection between the two as possible. This will ensure that all the air that enters the duct will be forced through the IC. It may not seem like it will make a big difference, but in informal tests before and after sealing, the outlet side of the IC was much cooler after making similar runs when sealed up.

 

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Here it is with the 2gb Eclipse bumper/cover back on. With the foglight removed the inlet is essentially doubled in size, and with a little trimming it’ll be about 30% larger yet.

Have fun!