The answer is in the ME 7.3.1H Alfa 2.0l BTS in module BGKV 1.30 in the section "Detailed description of function" headed "Physical relationship of the conversion factor" (Page 72 of 1327).

I haven't worked it up because I initially thought it might just be the Universal gas constant (8.3145 J/mol K) * 293.15 K = 2479 which is within 4% of the actual conversion constant value of 2578 assuming SATP (Standard Ambient Temperature and Pressure) conditions then correcting to the funktionsrahmen standard conditions of 273.15 K and 1013 mbar pressure. I guessed you'd get 2578 but it doesn't quite work.

Have a look at the attached article "Improved Cylinder Air Charge Estimation for Transient Air Fuel" which gives some further insight into this topic.

TTQS

Thanks for the article. will go through it.
My english Function frame says KUMSIRL only. Have attached formula screenshot.