. ▁A ▁reading ▁of ▁+ 6 ▁meant ▁that ▁the ▁air / f uel ▁mix ▁was ▁being ▁com pressed ▁by ▁a ▁super char ger ▁blow er ▁to ▁ 2 0 . 7 ▁( r ounded ▁figure ) ▁p si ▁before ▁entering ▁the ▁engine ; ▁+ 2 5 ▁meant ▁that ▁the ▁air / f uel ▁mix ▁was ▁being ▁com pressed ▁to ▁ 3 9 . 7 ▁p si ▁– ▁ 1 4 . 7 ▁p si ▁atmos pher ic ▁pressure ▁added ▁to ▁the ▁" boost " ▁pressure ▁of ▁ 2 5 ▁p si . ▁Typ ically ▁" absolute ▁pressure " ▁is ▁indicated ▁in ▁inches ▁of ▁mer cur y ▁because ▁" absolute ▁pressure " ▁is ▁dependent ▁on ▁multiple ▁atmos pher ic ▁influ ences ▁and ▁can ▁be ▁used ▁to ▁indicate ▁and ▁measure ▁" v acu um ", ▁or ▁pressure ▁below ▁atmos pher ic ▁pressure , ▁as ▁well . ▁Mercur y - filled ▁man om eters ▁( dev ices ▁capable ▁of ▁meas uring ▁press ures ▁above ▁and ▁below ▁atmos pher ic ▁pressure ▁also ▁known ▁as ▁vac u um / boost ▁gau ges ) ▁that ▁apply ▁pressure ▁directly ▁to ▁a ▁column ▁of ▁liquid ▁have ▁much ▁greater ▁vac u um ▁range ▁when ▁the ▁fluid ▁is ▁much ▁more ▁dense ▁than ▁water . ▁ ▁Sub ject ing ▁water ▁to ▁press ures ▁above ▁or ▁below ▁atmos pher ic ▁pressure ▁raise ▁or ▁low ers ▁its ▁triple ▁points ▁and ▁causes ▁it ▁to ▁bo il ▁at ▁" standard " ▁temperature ▁if ▁subject ed ▁to ▁near ▁one ▁atmosphere ▁of ▁" v acu um " ▁Such ▁vac u