SD Turbo (Apocalypse)
A large anti-grav unit designed for military vehicles or commercial transports.
Source: X-COM Apocalypse Ufopedia
Phoenix Hovercars should be fitted with this engine whenever possible in order to maximize their speed and maneuverability. Performance is, however, lacking if used in the Valkyrie or Hawk Air Warrior.
If unavailable for purchase, one can buy a Valkyrie Interceptor and rip out the engine, selling the Valkyrie afterwards.