The S-2 graf unit was a rough equivalent of the warp drive used by Starfleet ships such as the Constitution-class in the same era. The engine normally allowed a D7-class battle cruiser to catch up with a ship traveling at the speed of warp 9. (TAS: "The Time Trap"; TOS: "The Enterprise Incident" ) According to the Star Trek: Starship Spotter, other Klingon starships, at least the Bird-of-Prey, K't'inga-class and the Vor'cha-class all use different types of dilithium conversion graf units as warp nacelles.