At slow speed gravity should allow them to close, or the fan sucks them shut, and that will allow the suction from the fans to only get air from the core. With the holes the fans get their air from the path of least resistance. This is the one on my car and the flaps are rubber that is probably double the thickness of a bicycle inner tube, held on by little plastic rivets. Simple yet effective. Sorry to hijack Young gun just some tips help make what you have already done a great job on more effective.