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My bellypan consists of 3 pieces of coroplast, one each running from the front bumper down the driver's and passenger's side, covering the space between the central tunnel and the edge (underside of rocker panels, up inside of the wheel wells). The 3rd piece continues on back down the driver's side of the central tunnel and widens out to cover the underside of the gas tank going back to the rear bumper with the underside of the muffler exposed with a 1" space between the muffler and coroplast. 24" wide aluminum flashing runs from the front bumper down the length of the central tunnel and a 2nd piece follows the jog of the exhaust tunnel toward the right rear wheel well. The coroplast is secured with 6-32 screws/washers going into holes drilled and tapped into the underside of the car along the front/rear bumpers, the plastic rocker panel, and along the edge of the central tunnel. I used either tapped holes or clearance holes with screw/washer/nut on the parts of the coroplast that went up into the wheel wells depending on whether I was going into either metal or rubber sheet inside the wheel well. I used 1/8"X1" aluminum bar to provide attachment points for the coroplast spanning the space underneath the engine compartment and under the gas tank. The flashing is attached by screws into tapped holes into the underside of the car along the edge of the central tunnel. I positioned and cut the coroplast so the parts of the coroplast sitting under the suspension arms can flex as the suspension moves. Its been in place for over 150,000 miles without any fatique problems on the part of the coroplast.
I didn't have a SuperMID installed until I had already installed most of my aero mods, so I had to use at the pump calculations and changes in coasting performance to determine the effects of the aero mods, but the boattail improved mileage by about 4 mpg.
I only ran the partial boattail for less than one tank before completing it. I was working on the complete boattail, but ran out of time and had to drive with the partial boattail for a couple of days before I had time to complete it. I got a slight coasting improvement going from the partial to the complete boattail, so I figure about a 1 mpg improvement from completing the boattail. Assuming attached airflow along the entire boattail, the F.E. lost by a partial boattail would be proportional to the cross sectional area (sq. ft.) at the back of the cut off boattail compared to the original cross sectional area of recirculation eddy produced by the car.
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