While I agree with your pictures, and your ideas about airflow, smooth edges, the flaps on the Nascar roofs, etc. etc. i think the issue is bang for the buck, risk vs. reward, noticeable improvements etc. etc. I think it's awesome that you or anyone else wants to fabricate a part that isn't available. Whether it works or not doesn't matter. That's what this hobby is all about. ....but all the examples you give are the .01% of the performance envelope. In the "average" situation, with your average hobbyist hotrod, I can't see it making a difference. At 200mph sure. Again, I save "average" situation. While I agree that all the small difference do add up, if it REALLY mattered, all thirdgenners would drive Firebirds and not Camaros, because technically they ARE more aerodynamic....in a lab you can see it, in the real world, you never will.

...but I'm 100% serious when I say that you should make it. I mean most the cars nowadays that DO have them don't make much of a difference either. The vette I think is the perfect example of having several race bred features that when pushed to the limit by an experienced driver, DO make a difference, but.....for 99.9 of the people that drive em, they never actually do anything.

 

Air pressure and air flow effect a vehicle at every speed. I see what you're saying about not really being effective unless you're at high speed, but honestly, I noticed the biggest improvement at regular highway speeds. If you were going faster, it was more difficult to tell what was happening, but I could notice a big difference in handling while changing lanes and in high winds and such in normal speed conditions.

 

What was the angle and length of your diffuser?

 

The flat pan surface was about 20" front to back, about 3' wide and sat at about an 8° angle up in the rear. Went from halfway on the muffler to the rear of the car, had a soft section between the muffler and back bumper, the little pic with the pink outline shows it well.