I dont go as far as pulling off the filter, but with the nw up front pulling off the grill is good for a consistent .1 to.2 tenths at the track. Put it back on and slows .1 to.2 tenths like money. Rob started this first and recommended it and it's worked. Unfortunately my best in the winter of 12.52 was with the grill on, so can't wait till Nov. rolls around. 178mm, NW is all the mods i have. My DA is no where near you back east guys. It doesn't look good, but it's a quick and easy mod.