[California98Civic]

Thanks for all the pictures and thinking about what you are doing. I looked at them all and your notes. To remind: I am not expert and I have no formal training. I just own a car that I have modding and testing on the basis of research and my own and others testing.

I think your photos confirm for your specific car configuration what we broadly know for these three box sedan and coupe car bodies. A large turbulent wake develops behind the car, especially behind the trunk (and the rear window, though dust won't show it). The wheel wells and the underside in general are major contributors to such turbulent separation.Your dust shows that the angle of the curves on the body panels at the rear, such as at the driver and passenger sides of the rear bumper cover, become too sharp, too fast, and very quickly. At first, there is perhaps no dust because there is relatively little turbulence, and then the dust deposits show up as the turbulence grows.

Better separation edges should help, but despite the dust, these cars were pretty well optimized from the factory. So, testing experience here with 6th and 7th gen Civics (similar to your Acura) has shown it is hard to improve at the tail with small changes. A rear flat spoiler, a box cavity, wheel well covers, side skirts, grill blocking, an airdam, or more ambitiously a full undertray with a carefully tuned rear difuser angle. I think it is important to study placement and shape of the mods and to test them as best you can. Go bigger than a small separation edge; try a box cavity of some sort on combination with wheel well skirts and a diffuser.

I am planning to test a flat rear decklid spoiler again this summer. But I have a ton of work and family responsibility to wade through before that it really possible.