F2™ HPA Conversion KitBite Sized Fusion Engine Winner of the "Best HPA Engine" in the 9th Airsoft Players Choice Awards, the dual solenoid design of the F2 provides the same functionality as the legendary Fusion Engine but in a more compact, efficient and flexible design. Dual solenoids allow for the nozzle and the poppet valve to be adjusted independently of each other for unparalleled control over both the loading time and air volume used. Additionally, this makes it