borderlands 2 physx problem?

im playing borderlands 2 on a AMD video card but when i go in combat i get a huge FPS drop because of the physx i read on some forums that i have to change something in my willowengine.ini file problem is i cant seem to find it it should be in my map borderlands2/willowgame/config map but its not in there i only see files like defaultgame defaultinput etc... only files that start with default and nothing with willowengine pleas help