The RD-173 engine is an improved modification of the engine RD-170 (RD-171). The RD-173 used new materials, an advanced design of the turbopump and the injector head. The mass of the engine was reduced.
Chamber pressure and thrust could be increased. Further was worked for the increased reliability of the engine. Descendants of the RD-173 -- the engine was not used in original -- are the engines RD-180 and RD-171M.