The cetane index is calculated based on the fuel's density and distillation range (ASTM D86).
The older D976, or "two-variable equation" is outdated and should no longer be used for cetane number estimation.
It is, however, still required by the United States Environmental Protection Agency (EPA) as an alternative method for satisfying its aromaticity requirement for diesel fuel.
D4737 is the newest method and is sometimes referred to as "the four-variable equation".
Diesel engine operation is primarily related to the actual cetane number, and the cetane index is simply an estimation of the base (unadditized) cetane number.