The dual racor filters. I made this manifold myself years before racor started selling them as complete units, although I wasn't the first person to do so. The ones you can buy now have fewer fittings than this, but this does the same thing... it lets me isolate and change one filter while running on the other, and has a vacuum gauge down stream of both which gives me some indication of how hard she's pulling. 2 1/2" lead core foam insulation from Soundown is in. Next