as the Volumetric Efficiency itself changes with altitude. Specifically, with increasing elevation, the turbos need to spin faster and will operate at a higher pressure ratio for a given boost pressure. This results in increased exhaust manifold back-pressure which lowers V.E. The ECU will compensate for the changes in charge density, but since the V.E. has also changed, the fuel mass calculation will be incorrect and your A/F will be off target.