Decades ago in Popular Mechanics or Mechanix Illustrated, I saw a design for an aerodynamically efficient exhaust tip. Since I have no hope of finding the article (which I may still have) I took the opportunity to model it in Wings 3D:
The theory was that the cross sectional area at the middle of the cone is equal to the cross section of the tailpipe. Considering one of Julian Edgar's articles at
Ballistic Bellmouths and the
Supertrapp muffler, is there something worth pursuing here?