Improved, high Specific Mechanical Energy extrusion systems (20) are provided including a single or twin extruder (22) and an upstream preconditioner (24). The extruder (22) includes an elongated barrel (26) with at least one elongated, axially flighted, rotatable screw assembly (58) therein. The barrel (26) includes a mid-barrel variable restriction valve assembly (32), and the screw assembly (58) has a disrupting/homogenizing screw section (66) therein upstream of the valve assembly (32). The screw section (66) includes a plurality of alternating pitch screw parts (82-92). Preferably, the barrel (26) also has an atmospheric vent (36) downstream of the valve assembly (32).