High Altitude Compensation (HAC) System: Description

As altitude increases, air pressure decreases and the air/fuel mixture becomes richer. The HAC system supplies additional air to the primary low and high speed circuits on all models, and secondary high speed circuits on Celica, Corolla, Corona, and Pickup models. This helps to reduce emissions and improve driveability. Ignition timing is also advanced for better driveability.

The systems include a high altitude compensation valve, a vacuum advance sub-diaphragm, and a check valve. Starlet models use a thermostatic vacuum switching valve (TVSV) which serves to eliminate compensation while the engine is cold.