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.