I love working with the guys at EC'd....but, like mentioned above. If you are running modified mani's, dual cold air...going to 61mm may run you lean.... I don't even see why anyone would want to run the 61 if they weren't intending to do these other mods including fuel... There aren't shortcuts here... one mod leads to another and if you aren't building that way...go with the larger pulley...stay in the safe range, or be willing to monitor your engine... I am sure if you discuss this with anyone at EC for your tune, they will explain it as well...