Sorry, I don't remember where I learned this from, but someone noticed that there is quite a gap between the fan and the cage on the stock exhaust fan.  Adding a nut decreases that gap and hopefully improves efficiency of the fan.

Had to trim the top of the fan to screw the plastic cap in.Nut was just a bit too tall.  I had to shim the cover with two cardboard pieces so the fan would clear the cover.