PolarStar F2 HPA Engine (VFC HK417)Bite Sized Fusion Engine for VFC HK417 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,