Having the proper Emulsion Tubes will enable you to get the best out of your Webers! Just tell us what size you need! Sometimes the Jets, Venturis and E-tubes are sized using a trial and error method. There's just too many variations between Porsche engines to state absolutely that a particular combination of Jets, E Tubes, Venturis, etc. will be perfect for your 911. Sometimes you will need try different jets, etc to see what will work the best.