Discussion and questions related to the course EFI Tuning Fundamentals
in the equation of air density we must write absolute pressure as 14.7? or it differ if we are tuning in another atmospheric pressure
Absolute air pressure (atmospheric) is about 14.7psi, or 1Bar or 101kPa in metric units. The exact amount depends on altitude and weather, and use the units that match the equation you are using. You need absolute pressure to calculate air density, yes.
Gauge pressure (atmospheric) is 0psi.