Square flange bolt pattern. Performance calibrated. The ultimate vacuum secondary street performance upgrade. Product Features:Vibratory Polished For Good LooksOptimized Street Calibration That Works Out Of The BoxFactory Preset Electric Choke For Easy StartupsQuick Change Vacuum Housing Cap To Tune For Performance/Fuel EconNo Unnecessary Trips Back To The Parts StoreLimited Lifetime Warranty100% Wet Flow Tested By Holley TechniciansArrives Ready To Run