See link . . .

http://www.bolhuijo.com/airflowtest/

 

The stock filter extends much deeper into the intake manifold box therefore giving it a lot more surface area. If k&n did the same thing it would have faired better. Anybody want's to donate their AFE intake for testing??

 

It means oem filter are probably the best for your car and KN filter aren't that great unless you don't want to clean out a filter so often.