1 Atmosphere (as a unit of pressure) = 14.7 psi = 34 ft water column = 76 cm/Hg = 29.92 in/Hg = 1.013 bar = 1,013 mbar
Many japanese manufacturers will specify a range of manifold absolute pressure in mm/Hg. Guages are rarely found in this increment, but are avaliable in cm/Hg.
Manifold vacume should be between around 150-250 mm/Hg. Each manifold must be matched to the other manifolds with a minimum of difference not to excede about 20 mm/Hg at idle.