I would keep the lid on. Having it off essentially defeats the purpose of the open-ended filter (to a degree). With the lid on you will still build some negative pressure in the runners and be able to get cool air from in front of the radiator support. This benefits you from a stand-still, while the open end will give more benefit at speed.
FocusFest 2013: Best of Show
Gateway FFW: Show Class Outstanding
September '13 ROTM
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