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Building a Cowl Induction Hood Scoop

A cowl induction hood scoop draws cool, pressurized air from the base of the windshield into the engine compartment where it can be used to cool the air fed into the carburetor. The faster you go, the more pressurized air is fed to the engine. At lower speeds, or when the car isn’t moving, the cowl gives the warm air under the hood another avenue of escape to help dissipate heat. Cooler temps = greater performance. This type of hood scoop has been around since the 60's and has been a popular modification ever since because of it's simple styling and classic look. I've done this modification myself, the pictures on this page are of my own truck. I was surprised how easy it was once I figured out a few tricks. This hood is all steel and the scoop is functional.

Building a functional hood scoop for your vehicle isn't as hard as it seems. Anyone with some basic knowledge, the right tools, and a bit of ambition, can build it in a day or so (the bodywork may take longer). Once you understand how this process works, you can incorporate some of the techniques into other projects on your vehicle. This procedure uses the existing sheet metal on your hood for the top surface of the scoop, and additional sheet metal is welded to the sides and rear. Since many of the design factors will be up to you, I’ll try to include as much info as I can about each design so you can choose what is best for your hood. If you take your time and do it right, it will look as good as any aftermarket hood scoop costing several hundred dollars, and will not add a substantial amount of weight to the front of your car or truck.

Tools and Materials:
You’ll need a few tools to get this project started. Since this is an all-steel hood scoop mod, you'll need a basic welder; either MIG or TIG will work fine, but it doesn't need to be a high end unit. TIG welders work better for the thin sheet metal you’ll be working with, but a MIG will work too. I use a 110v MIG welder I bought new for $200. You’ll also need an angle grinder (with a ⅛” wide cut-off wheel), a metal file, putty knife, body hammer, tin snips and a sander. You can use fiberglass filler or bondo to smooth out the corners when you’re finished. Cut two strips of sheet metal to the correct width before you begin. If you want the scoop to rise 3” off the hoods surface, cut 3” wide strips with either tin snips or a cut-off wheel. Don’t worry about getting the length exactly right, you’ll be trimming it to size later, cut them a few inches longer than necessary. You’ll need another pair of strips a bit wider for the splayed sides, but that will depend on the angle of the splay you decide to use.

Design & Layout:
You’ll need to figure out the dimensions of your hood scoop first. Determine how high you want the scoop to rise off the hood, how wide you want the top to be, and whether the scoop will be full or partial length of the hood. Aftermarket Cowl Induction hood scoops are usually ¼ to ⅓ of the width of the hood, 2” – 3” high, and can run the full length of the hood or stop short of the leading edge (front). You can also choose either vertical or splayed (flared) sides depending on the look you want to achieve. You can tweak the dimensions and other design features however it looks best for your hood. I have found that hoods that curve down in front (like my S10) look better with the scoop stretching all the way to the front. The natural curve of the hoods leading edge helps the scoop blend back into the hood. Flat hoods (like my El Camino) tend to look better with the scoop stopping short of the leading edge since it's easier to define the rise with some flat area in front of the scoop.

If your hood does not have a visible centerline, measure the width of the hood at the front (near the grill) and the rear (near the windshield), mark the centerline at each location, and connect the two points down the length of the hood using a sharpie. On a dark colored hood, it may help to lay a line of tape down and make your marks on that. Determine how wide you want the scoop and measure out from the centerline half the width of the scoop on each side at the front and rear of the hood, and then connect those points. Be sure to keep it even on both sides. This is where you’ll be cutting your hood, and these lines will be the width of the top of your new scoop. You can also choose to have the scoop narrow in front and wider at the rear, but be careful not to make the difference too drastic, once you have made your cuts, there is a lot of work to repair the damage. Follow the hoods natural form for the best results.

Once you’re happy with the width, it’s time to work on the length of the scoop. Typically the scoop will hang over the back of the hood by 2”- 3”. It’s very important to take this measurement with the hood closed AND open while it is still mounted on the car. Make sure your wiper arms clear the scoop when the hood is open (you’ll only make this mistake once). If your hood is curved at the back (near the windshield) you can have the rear overhang of your scoop follow this curve, or make it straight. Either way, make a template using a piece of cardboard and trace the rear curve of the hood and mark the center point, then cut out the shape of the overhang you want to use (curved or squared), and the length and width you want it to be. Now determine how close to the front of the hood you want the scoop to blend back into the stock hood. Using construction paper / cardboard and masking tape here to build a rough pattern may help you see it better. You can have the scoop come almost all the way to the front of the hood, but it may look better to have it start 8” – 10” back, depending on how your hood is designed. I have made them both ways, and they both look good. Be sure to measure and mark your lines very clearly, including where you want the scoop to end at the front (fade back into the original hood).

Next, determine if you want the sides of the scoop vertical |  | or splayed (flared)  /   \  Personally, I like the splayed look for anything under a 4” rise. To help with rigidity of the hood, I usually make the scoop with vertical  |  | sides to hold the scoop up, then add the spayed sides  /   \  for a more finished look.

Now determine how high you want the scoop to rise above the stock hood. 2½” – 3” is common for a street car, 4” or larger may not be legal in some areas (if it reduces visibility) but looks great on a drag car. If you need the extra hood clearance, make the scoop as tall or as short as you need to clear your engine.

Warnings:
Read through these instructions carefully several times and make sure you understand exactly what needs to be done. You may want to print the instructions and keep them handy while you’re working. The procedure is pretty straight forward, but it assumes you're comfortable with metal /welding /fabricating work. If not, this may not be the best “first time” fabrication project for you. It will be real easy to mess up a perfectly good hood, and I can’t take responsibility for any damage that comes from misinterpreting the instructions.

You’ll want to build this scoop with the hood off the car. The sparks that will shoot up from the grinder are actually tiny bits of molten metal. When they hit a glass surface it melts and they become tiny bits of metal imbedded in your windshield. It’s very important that you DO NOT CUT THROUGH THE SUPPORTS on the underside of the hood. I’ve done this, it’s not good. The hood will lose its structural integrity and repairing it will cause you to throw tools across the garage and use bad language. This is why it’s not a good idea to use a jigsaw to cut your hood. Use a cut-off wheel on an angle grinder and be very careful to only cut through the top surface of the hoods sheet metal.

The Work:
Now the fun begins. First, remove the insulation from the underside of the hood if it has any, and make sure the hood is well supported. As you cut the hood, it may want to sag in the middle at the back. This would be bad, you need to keep the hood straight after you make your cuts. The supports underneath will help, but it’s better to be safe than sorry. You’ll also want to be sure you can get at the underside of the hood as your working on it. You’ll need to separate the top from the support structure underneath. A little planning now may save you more work later.

Step 1: Carefully cut on the lines you drew on either side of the hood with a cut-off wheel (the first cut in a perfectly good hood is the hardest). Depending on how your hood is constructed, you may need to cut all the way through at the rear (near the windshield), and then cut along the underside to allow the sheet metal to be lifted up. Make the cut at the front only as long as you want the scoop to be. In other words, where your cut ends towards the front of the hood is where the scoop will start to rise off the hood. Now, working from underneath the hood, carefully separate the hoods top from the supports underneath using a putty knife. Start at the rear and work your way forward, lifting the metal slightly as you go. The last thing you want to do here is pull too hard on the top layer and put a crease in the metal.

Step 2: Once you have the top and bottom pieces separated, you can lay the strips of sheet metal you cut earlier and get an idea of what the hood will look like when you’re finished. You’ll need to determine how far the strips can go forward before they need to start tapering in the front. This is another judgment call, I usually have the taper as close to the front of the hood scoop as I can get it without causing the hoods original sheet metal to buckle or crease. That way it rises off the hood at a steep angle and levels out evenly over the rest of the hood. Cut, fit and trim the side pieces as often as it takes to get it right. Also, be sure to leave excess material at the back of the cowl to accommodate the overhang. You’ll trim this to your design later.

Step 3: After trimming the vertical strips to fit in front you can weld them to the hood. Sand off the old paint about 1” on each side of your cut, be sure to sand down to bare metal. This will give you enough room to spot weld the hood to the strips without burning the old paint.  Stand the strips on the supports underneath and spot weld them to the hood on the bottom first leaving a minimum of 1” between welds, start at the front of the hood and work your way back. It will be easier to weld the top seam after the bottom is tacked in place securely. The thin sheet metal WILL warp if you get it too hot too quick. Leaving a gap between spot welds allows the metal to cool naturally and will reduce the amount of warping. An experienced TIG welder may be able to do this with very little or no warping at all, but I have never been that lucky with my MIG welder. That’s what body filler is for. Check the height on each side to make sure they are even as you go.

Step 4: Once you have the vertical pieces completely welded up, you can weld on the splayed pieces. This is optional, as I’ve said, but I like the look. Just be sure to spray a good coat of primer and paint on the exposed surfaces of the metal that will be hidden. You will want to protect as much of the exposed metal buried under the splayed sides as you can, you won’t be able to paint them after the splayed sides are welded up. When you’re welding in the splayed pieces, you should start at the front and work your way back again. Cut, grind and hammer the pieces in until you get the look and fit you want. Again, leave some excess at the rear.

Step 5: Now you can start working on the overhang at the rear of the scoop. Remember that template you made earlier? Use it to trace your sheet metal, then cut it out and test the fit. You’ll need to start trimming the back ends you left long on the vertical supports earlier (hopefully you left them long enough). Once everything is trimmed nice and neat and has the look you want, you can weld the extension onto the original hood and the vertical pieces. You can grind the vertical pieces with a curve that will follow the top down to the bottom of the scoop on the rear. Now you can add a filler piece in the back of the hood scoop opening to conceal the support piece you welded in, and attach it to the underside support you had to remove earlier to help tie the back of the hood scoop to the underside of the hood. How you finish the back is up to you. I have seen them completely filled in, then a design of some kind cut into the filler panel like a Bow Tie, Oval Holes, Wire Mesh, etc. Use your imagination.

Grind and sand the seams you welded as needed to get the edges finished. Just be careful not to grind your welds off. If you were careful, there should be very little filling you will need to do on the seams and the flat areas of the hood and scoop. I use the fiberglass jelly-type resin to smooth the edges, then bondo as a final coat if needed. All that’s left is to finish the bodywork, repaint and remount the hood. You can also mount something under the hood that will direct the cool air being drawn in directly over the air cleaner, so it gets where it is needed most. Or just cut the hoods insulation around the air cleaner to help direct the cool air.

 

When you’re finished you’ll have a truly custom hood scoop that is as good as anything you can buy, and you’ll have the satisfaction of knowing you did it yourself.
Good luck with your hood.

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