The Pumping Loss
A so-called "pumping loss" is also cited as causing extra work for the engine to do under partial load conditions because the pistons have to work harder to suck in the fuel-air mixture when the throttle position results in low intake-manifold pressure. Under this scenario, deactivating some of the cylinders allows the remaining active ones to have less manifold vacuum to overcome during the intake stroke.
Read more about this topic: Active Fuel Management
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