wotswot, I recently went from a 16 x 4 to a 15 x 4 for faster spool up. It's powered by an O.S. 91 four stroke, stock exhaust, and recently on 25% pro pattern fuel. As it turns out, the 15 x 4 is signifacantly louder. With the first prop several people commented on how quiet it was. Of course I was floating around at part throttle alot. Even at full throttle it wasn't very loud. Seems the extra 700-800 rpms really makes a difference.