Gas Saving Front Bumper Air Dam




Introduction: Gas Saving Front Bumper Air Dam

A while back my parents were visiting our babies and my dad was getting a little bored. I told him I had a project for us. The front end of my Honda Civic had been scrapped and busted and was hanging free in a couple of places so I decided to do something about it.  

We needed to work on filling in the gaping turbulence producing hole below the front bumper!! If you didn't know anything that hangs down under your car (or those voids in the bumper that fog lights and things sit in) either catches air like a brake or disrupts air flow creating drag. Both of which make it harder for your car to move through the air thereby wasting gas.

I forgot to take a before shot but needless to say my front plastic had taken a huge beating over its life and has been "repaired" multiple times.

Step 1: Cut Out the Gnarly Bits

We used a skill saw to cut out the gnarly. While under there I noticed that there was a 1/4 inch lip that served no purpose but to catch the wind and slow me down so I cut in front of that line.

You can see the part that we cut out laying on the table.

Step 2: Fill the Gap

The stock bumper has a rather large pass-through. Cut the coroplast (or political yard signs, or luan, etc.) to fit a little larger than the opening. I used Gorilla Tape to hold it well. 

I'm sure most of you are thinking, "NOOOO, that's an air intake and you can't close it. You'll overheat your engine." Well if you don't think you should then don't but my temp gauge hasn't moved a bit even in this past 113 degree Oklahoma summer. This lower port doesn't have a radiator behind it so all its doing is creating drag. The upper intake is still open to the radiator. If your temp gauge does go higher then simply open it back up.

Step 3: Paint and Mount

I only taped the sides and top so I used spray paint to cover the white of the coroplast. Mount the bumper back on the car and bob's your uncle.

My best tank before this mod was 35mpg. I have since gotten over 36 and twice up to 44mpg using hypermiling techniques I've picked up. I'd say this mod was only good for 1 - 2 mpg but I needed to clean up the damage anyway so it definitely fixed and improved.



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    23 Discussions

    i only get 7MPG!......... oh wait i forgot i have a supercharged v8 lol.... this is a nice project i wonder what you could do with fiberglass or carbon fiber... might look a little cleaner/smoother?

    1 reply

    "Function over fashion" I always say.
    I have a real problem beating these skirts on parking barriers and curbs so I'm not going to invest the money in beauty.

    I could have sworn those holes were to allow air to flow through the bumper. so there's less resistance than a flat face. This seems like it would give you more drag. But as far as people talking about overheating, it shouldn't. The grill will feed the stock intake and radiator fine. The radiators on civics are high up and quite tiny actually. but your ac compressor is a bit lower, might have an effect on it, might not =\ but if you're going for a gas mileage record remove it. It'll save you a decent amount of weight and energy. ;-]

    2 replies

    From wikipedia: ( "often called air dams, because in addition to directing air flow they also reduce the amount of air flowing underneath the vehicle which generally reduces aerodynamic lift and drag."

    Its the reason that race cars are built low to the ground. Race cars also need downforce which is why below the fron airdam there is sometimes fitted a plinth or splitter to catch wind and point the nose of the car down for better grip on the tires. In this case I want very little downforce. (I actually want lift to reduce friction with the road.)

    "Those holes" act as downforce and sometimes to a transmision cooler if needed, but I don't so this is much better than the gnarled old bumper I had when I bought the car.

    See the reply to the other comment on drag under the car.

    A great resource for all aerodynamic modifications is I suggest spending time over there to learn more than you ever wanted to know about vehicle aerodynamics.

    Thanks for the comment!

    Those holes reduce the drag more than a flat surface, Thats why almost all street cars have plastic flaps under the car and in the wheel wells to direct air flow so it doesn't get caught in between suspension and the back bumper. Many race cars have diffusers going all the way under the car to get the air moving under the car faster than above the car, it creates a bit of down force and holds the cars down in the turns. if you want lift. you're gonna lose grip, and if you wan't to sacrifice that bit of safety you could put super thin tires on as well.

    hi 13 blue, that's great that it worked for you, that's something that I have not tried.
    here is a couple more mods that I've have used, Fully synthetic oil in the engine and trans if is a manual, I recommend Mobil one. Low rolling resistance tyres, Michelin make them. New thermostat will give quicker engine warm up times, as they get lazy after a couple of years. The O2 sensor in the exhaust also gets lazy after about 100 000km and is easy to replace. and these 2 may not be legal where you are but are well worth doing, remove the catalytic converter and smash out all that ceramic crap inside it, and refit. they are very restrictive, and can block up, Also block up the EGR valve while you are at it. The last 2 are the cheapest and have the best results.
    Ive always wanted to fit my car with those "full moon racing disc hub caps" to see if they help, but mostly because they look cool....

    2 replies

    Thanks for the comment liquidhandwash. You sound like you're a real hypermiler already! Great going and great for our environment. Here's a resource I'd bet you'd love. and

    Keep up the good work!!!!

    How do you prove that this works for: "... the gaping turbulence producing hole below the front bumper .... make it harder for your car to move through the air thereby wasting gas......." ?

    Why do you think that a solid almost vertical sheet closing the gap would reduce the air resistance?

    How do you swim? With your fingers apart or with them together forming a no-gap-surface?

    1 reply

    To prove that an aerodynamic modification works you a. take it to an air tunnel b. Do tuft testing (small strings or "tufts" are attached all over the car to visually check for "attached" airflow. c. perform "AB" testing and record differences in several tests (A= without mod. B= with mod. Then compare.

    As far as the gaping hole that creates turbulence goes... the difference is that an Air Dam routes air to the sides of the car where it will then follow the shape of the car and be released behind the car instead of going under the car to "catch" on every piece of suspension, oil pan, exhaust, and gas tank, not to mention the inside concave of the rear bumper. All of that air touching those parts creates drag like driving with your window down creates an "air brake" by catching on the door pillar and the inside of the back window.

    I would recommend against doing this without precaution, especially during summer. If you choose to do this, you should add an inlet for air intake (or go directly for a cold air intake), else you'll reduce the airflow going through your air intake and increase its temperature, reducing the pressure, thus reducing your engine efficiency. You'll also reduce the air going through the radiator, triggering more often the radiator fan, thus reducing its service life.

    Watch closely your engine temperature. An engine is conceived with an optimal running temperature. Running it too high will damage it and may require some adjustment in order to tolerate it. You should use a higher temperature rated oil.

    2 replies

    Neither myself or other Civic owners who have done this mod have had any rise in temp. Like I said in the text: the grill is still completely open, this is only the lower opening in the bumper mostly created to add downforce to the front of the car. The opening to the radiator and engine is still completely open. The cold air intake is near the top of the engine and this is 8 inches from the pavement.

    if its near the top of the engine its not a cold air intake. Cold air intakes usually involve leading a pipe to the bottom front of the car, close to the ground and away from the heat.

    instead of making the whole thing fiberglass just use metal mesh rigged up to cover the dams, then fiberglass the openings and blend it into the bumper. it's more fun than hard but it takes a long time (had to do this for that bottom lip you threw out.

    2 replies

    Will the FG "stick" to the plastic?

    Actually the Gorilla tape has done so well I don't really need to think about fiberglass. I did this last winter and the tape is still stuck after a record breaking heat wave/drought this summer!

    Here's a response to your question about fiberglass adhering to plastic, as well as an explanation of various methods and materials that you might use while modifying plastic bodywork. I hope that this is helpful to someone!

    As you likely know, the process of fiberglassing typically involves the fiberglass fabric as well as a resin of some kind to saturate the fabric. The adhesion is determined by whatever that resin is, and how well it interacts with the particular plastic that your bumper is made of. Some plastics, like polyethylene (PE), are practically immune to the effects of any adhesive. Even good epoxy will eventually come away from PE. Other plastics, such as ABS (a common bumper material) will be bonded by some epoxies. If bonding ABS to itself, it is best to use an epoxy specifically designed for plastics containing methyl methacrylate, which will partially dissolve the plastic on both sides of the bond, allowing it to co-mingle before the solvent evaporates and the plastic re-solidifies.

    I should note that I have a lot of fiberglass boat-repair experience, so my solutions and suggestions are coming from that angle.

    In boats, the resin will typically be either a polyester resin or an epoxy resin. While a majority of fiberglass boats are polyester resin, I would recommend epoxy resin for home projects and repairs. It has better mechanical properties, bonds stronger, and adheres to a wider range of materials (including cured polyester resin, so don't worry about whatever your object was originally made of). Polyester resin's primary attractiveness to the industry is that it's cheap.

    Body-filler (e.g. "Bondo") is a polyester resin combined with some fillers to make it thicker, easier to paint, and easier to sand. Being polyester, it does not have fantastic chemical adhesion properties, which is why you'll often see it being used on very rough surfaces that provide some degree of mechanical holding-power. It also lacks mechanical strength and flexibility, so it tends to crack eventually.

    For a permanent modification, I would use some combination of epoxy resin, fiberglass cloth and whatever other structure you can to provide physical strength (e.g. stainless steel mesh, plates of plastic, etc.) note that many epoxy resins are transparent when they cure, so adding a slight amount of opaque pigment will make painting a lot easier. Also realize that epoxy is going to be a pain to sand compared to body-filler, so try to get darn close to the shape that you want! You can also use a skim-coat of body-filler over a much stronger structural repair made of epoxy to get things ready for paint. (As always, never sand anything without adequate dust-control and respiratory protection.)

    Lastly, if you find your epoxy to be too runny, there are plenty of filler materials that you can use. They will slightly reduce mechanical strength and bond strength, but compared to body-filler, you're still doing pretty darn well!

    I hope that this small treatise is helpful. If anyone has questions about this type of modification or repair work, shoot me a message! (Or, if you're near Boston, hire me!)

    Love it.
    Could have done a bit cleaner but hey whatcha gonna do in an afternoon on the spot?
    Heck I would not have taken the bumper off to do this work.
    Anyway I really like the smooth look to it now.
    Wal mart sells flame patterned duct tape... eh?

    2 replies

    Thanks. My ultimate goal is to replace the whole bumper with fiberglass- but that's a whole learning curve I don't have time to tackle right now.

    Yeah fiberglass is awful stuff if you aren't ready for it.
    Dont do anything important your first time.
    Watch a lot of youtube about it...?