[ducatisl]

It's pretty simple really-

I use:

[PID.6512.PSI]-([PID.51.PSI]-[PID.11.PSI])

In other words:

Rail_Pressure = Delta_Pressure_Injector-(Atmospheric_Pressure - MAP_Pressure)

I won't get into the crappy choice of variable names, but essentially Rail_Pressure is that 58psi goal the fuel system aspires to keep by varying Delta_Pressure_Injector for the conditions of Atmospheric_Pressure and MAP_Pressure. This should be obvious when you consider a normally aspirated car at WOT: MAP values approach atmospheric and that term on the right gets very close to zero, and the data logs will show that Delta_Pressure_Injector get very close to 58 psi

HP Tuner variables can automatically switch to a desired units by specifying the units after the PID in programming struct fashion, ie - variable.<units>, which is important in this case because we always look at MAP and atmospheric pressure values in kPa, but for fuel, we look at lbf/in2.