SD Standard (Apocalypse)
A compact anti-grav unit suitable for Hoverbikes and smaller vehicles only.
Source: X-COM Apocalypse Ufopedia
The power of this engine is wanting, so it's best to swap out this engine with something better to improve acceleration. Typically, it is best to sell the SD Standard and upgrade, but if you have to use it, then it is a capable engine only for the Hoverbike, and even then only in very early combat engagements (before Alien Fast Attack Ships start appearing).
Despite its small size, this engine can power even a Hawk Air Warrior, but at very restricted acceleration.