Diffuser
I would like to make a correction to my post on diffusers. I stated that the diffuser is not effective if the underbody is smooth. It should read unless the underbody is smooth. The diffuser can be viewed as a rather weak pump that does not work well if the flow path has high drag due to roughness typical of car underbodies. You will note that all race car underbodies are very smooth.
Also, on the topic of diffusers, if the diffuser angle is small a small drag reduction can occur. Certainly a boattail on the top and sides is better, but the diffuser is out of sight and can add to the boattail. The same comment applies to needing a smooth underbody to get the most from the lower panael of a boattail, which is effectively a diffuser.
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