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It causes a loss in HP because it adds additional cross sectional area. I am not sure, but it looks like the model doesn't use a very highly contoured underside, and may not have used a moving road surface or spinning tires either. The reason people put the front air dams on their cars is to prevent air from traveling under the cars "aerodynamically dirty" underside. If you have a smooth underbelly, an air dam is just adding area, but for the those that don't, it actually helps.
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