[beetle1303]

I feel the need to make smthng straight for the ease of everybody understanding:

Imagine, a wing mounted somewhere and air flowing through it. IF the wing's plate is flat (not the profile) ie is straight, like the ones in the pics in this post, the air leaving the trailing edge will flow straight, evenlt across the wing's width.

Vortices will be generated between the side plates and the wing's plate, and around the corners of the side plates. IE rule of thumb, vortices need edges, protrutions to be generated.

Eatoniashoprat,
you got it the other way round, lift is generated, by attached flow, or low pressure cavities...

Now, a lip may reduce local lift, and disrupt the attached air flow ( to the car's body), but how effective it will be depends on the angle of the lip, and its height,

effvee, am i silly asking or it is facing the front of your car???

Chris