FH4 Car`s Engine Power Chart - Dependency Torque/Power to RPM

Hi!
Everybody had seen the Cars Engines Power Charts in game. But can anybody describe the system how it was built? I tried to use real Formulas to reach the values, which in-game chart presents. And as expected - the values ​​differ. So, what is the dependency between this Engine`s values for FH4? How to calculate correctly. Only one point I noted - when we find RPM for max torque - it is 20% lower than real world estimates.

I would be really happy if somebody will support me with this chart. And How to build it “on paper” for calculating in Forza.