▁compression ▁ratio ▁is ▁much ▁less ▁than ▁the ▁measured ▁compression ▁ratio . ▁ ▁Under ▁light ▁load , ▁the ▁thro tt le ▁is ▁not ▁fully ▁open , ▁and ▁the ▁cyl ind ers ▁receive ▁less ▁than ▁a ▁full ▁charge ▁of ▁air ▁on ▁each ▁int ake ▁stroke . ▁ ▁The ▁pressure ▁and ▁temperature ▁generated ▁at ▁comb ust ion ▁are ▁therefore ▁less ▁than ▁full ▁load ▁and ▁the ▁therm od ynamic ▁laws ▁which ▁apply ▁to ▁all ▁heat ▁engines ▁dict ate ▁that ▁the ▁engine ▁will ▁then ▁be ▁operating ▁at ▁less ▁than ▁its ▁maximum ▁possible ▁thermal ▁efficiency . ▁ ▁Thus , ▁a ▁high - power ed , ▁large - dis p lacement ▁engine ▁is ▁highly ▁in efficient ▁and ▁waste ful ▁when ▁being ▁used ▁for ▁normal ▁driving ▁conditions . ▁This ▁is ▁the ▁motiv ation ▁for ▁cyl inder ▁de activ ation , ▁to ▁effectively ▁spread ▁the ▁work ▁load ▁of ▁the ▁engine ▁over ▁fewer ▁active ▁cyl ind ers ▁which ▁then ▁operate ▁under ▁higher ▁individual ▁loads ▁and ▁therefore ▁at ▁higher ▁efficiency . ▁ ▁P ump ing ▁loss ▁A ▁so - called ▁" p ump ing ▁loss " ▁is ▁also ▁c ited ▁as ▁causing ▁extra ▁work ▁for ▁the ▁engine ▁to ▁do ▁under ▁partial ▁load ▁conditions ▁because ▁the ▁p ist ons ▁have ▁to ▁work ▁harder ▁to ▁su ck ▁in ▁the ▁fuel - air ▁mixture ▁when ▁the ▁thro tt le ▁position ▁results ▁in ▁low ▁int ake - man if old ▁pressure . ▁Under ▁this ▁scenario , ▁de activ ating ▁some ▁of ▁the ▁cyl ind ers ▁allows ▁the ▁remaining ▁active ▁ones ▁to ▁have ▁less ▁manifold ▁vac u um ▁to