This is the popular "MAC valve" boost control solenoid. Available in 3 and 4-port configurations. Solenoid ships with 3/16" barb fittings and a muffler/filter fitting for the vent port. The 3-port solenoid (shown) is the most popular and works well for most combinations. We recommend the 4-port solenoid for extreme high boost applications. In some applications the 4-port solenoid can give a broader range of boost adjustment than the 3-port is capable of. 3-port solenoid shown. 4-port similar.