The secret to an airfoil is to make sure you are doing a good, efficent job of threshing ( good concaves and bars--don't grind and pound--thresh),run plenty of air, and I tell my customers to open the bottom sieve at least TWICE as wide as the book calls for---when you shut the bottom sieve down, you are shutting off the air to the airfoil. We build the All Crop AFC here---and it is rare to see one come back if guys will do these things to their machines