Question about idle vacuum...

[Jon k]

Ok I noticed that my engine pulls 37 or 38 kPa according to MAP sensor, but my vacuum gauge shows about 18 - 20" of vacuum... I converted Kpa to " Hg and i get "10"... whats going on here?

[Jon k]

Ok I noticed that my engine pulls 37 or 38 kPa according to MAP sensor, but my vacuum gauge shows about 18 - 20" of vacuum... I converted Kpa to " Hg and i get "10"... whats going on here?

"Inches of vacuum" denotes the number of units (in inches Hg (mercury)) less than ambient pressure. kPa is based on an absolute reference point - 0 kPa = a full vaccum. Roughly, 100 kPa is equal to 0 "inches of vac" at sea level, 0 kPa would be equal to 30 inches of vac (not pressure!). 37 kPa is equal to 10.9 inches Hg absolute. 30(ambient sea level in Hg)-10.9(37 kPa)=19 inches of vacuum. I've made this slightly confusing on purpose so you can get your head wrapped around the difference in both units and direction of measurement by virtue of thinking about it....

-Mike

Mike - youre not all that confusing I just didn't think hard enough. So in order to make a correlation between what the MAP sensor is reading vs what my vacuum gauge is reading, I take 100 (or MAP with car off... 98) - 37 = relative, convert that 60 or 61 to in/HG and get 18.6 - 19... thanks a bunch!