Air Charge

Air charge is the percentage of air molecules within the cylinders compared to how much the cylinders would hold at one atmosphere. Anything above 100% air charge is under boost. The ECU uses it as the primary axis for most lookup tables in place of load on other platforms. Rather than targeting a set amount of boost, air charge is targeted instead so that atmospheric pressure and temperature are taken into account.

Boost pressure and air charge are closely related, but they are not the same. Boost pressure is a measurement of the force the air is exerting on it's surrounding space compared to atmospheric, whereas air charge is an actual measurement of air molecules within the cylinder. Both are increased by the turbocharger, but boost pressure does not take into account the temperature of the air nor the RPM and flow of the engine and thus cannot be used to target a certain amount of torque output.

Targeting air charge and using it as an index for most tables is the primary means by which the Bosch ECU safely makes torque in any environmental condition.