INLET DESIGN IS MORE IMPORTANT, THAN TAIL PIPE.. GOOD CLEAN ,SMOOTH AIR IS WHAT YOU WANT.. INLET LIP IS CRITICAL !! INLET SHOULD GET SMALLER IN CROSS SECTION AS IT GETS TO FAN SHROUD. PIPE OUTLET DETERMINS AIR SPEED. MOST ARE 3" TO 4" 3.5 GOOD FAN, AIR SPEED ON THE GROUND ,,165,MPH.. DONE ALOT OF TESTING IN THIS AREA!!!RD