UH-1M
The "Mike" model Huey was a conversion of the existing UH-1C by re-engining it with the 1,400 shp (1,000 kW) Lycoming T53-L-13 powerplant used in the UH-1H. This provided more power to the "C" model for its role as a gunship and also provided engine commonality between the attack and transport helicopters in use in Vietnam at that time.
Read more about this topic: Bell UH-1 Iroquois Variants