SD Elite (Apocalypse)
A high powered anti-grav unit generally restricted to military or police vehicles.
Source: X-COM Apocalypse Ufopedia
A large size of engine sold by Superdynamics. It is a 4x4 size which comes as standard equipment on the Hawk Air Warrior. It has very good performance when used with a Valkyrie interceptor, but there is better.
If unable for purchase, then buy up the Hawk Air Warrior and rip out the engine.