um ▁levels ▁are ▁common ▁in ▁spark - ign ition ▁internal - comb ust ion ▁engine ▁int ake ▁manif olds ▁at ▁" id le " ▁spe eds ▁and ▁loads ▁rendering ▁water ▁useless ▁as ▁a ▁liquid - column ▁man ometer ▁test ▁medium . ▁Even ▁at ▁lower ▁vac u um ▁levels ▁it ▁ev apor ates ▁to ▁readily ▁and ▁quickly ▁to ▁provide ▁consistent ▁vac u um ▁read ings . ▁ ▁" Ab solute ▁pressure " ▁also ▁has ▁an ▁atmos pher ic ▁pressure - in dependent ▁alternative ▁called ▁" g au ge ▁pressure " ▁and ▁knowing ▁which ▁is ▁in ▁use ▁in ▁technical ▁information ▁is ▁cru cial ▁to ▁proper ▁use ▁of ▁indicated ▁pressure ▁data . ▁Since ▁atmos pher ic ▁pressure ▁is ▁a ▁given ▁and ▁a ▁" constant " ▁a ▁" boost ▁gauge " ▁or ▁man ometer ▁cal ibr ated ▁to ▁indicate ▁ 0 ▁p si ▁or ▁in .- H g . ▁at ▁sea ▁level ▁on ▁a ▁standard ▁day ▁allows ▁" boost " ▁OR ▁" v acu um " ▁to ▁be ▁directly ▁displayed ▁and ▁" read " ▁as ▁" g au ge ▁pressure ". ▁Ab solute ▁pressure ▁requires ▁subtract ing ▁ 1 4 . 7 ▁p si ▁from ▁" boost ▁pressure " ▁or ▁adding ▁it ▁to ▁indicated ▁" v acu um " ▁to ▁determine ▁" g au ge " ▁or ▁actual ▁int ake ▁manifold ▁pressure . ▁ ▁At ▁idle ▁speed ▁and ▁load ▁with ▁nearly ▁closed ▁thro tt le ▁" but ter f lies " ▁and ▁during ▁part - th rott le , ▁light - load ▁operation , ▁even ▁super charg