HappyTrails

Looking for a suitable manifold pressure gauge, either electronic or mechanical. One that reads in inches hg, not psi. Anyone install such an item? Any suggestions?

bulrid8

Do you mean a vacuum gauge? There are a ton of them on the market.

HappyTrails

No, not a "vacuum" gauge. A manifold pressure gauge. Sort of the same thing, but not really. "Vacuum" in the manifold is an inaccurate description. There is pressure in there under operating conditions. Just less than standard atmospheric pressure. The engine computer does not take a "Vacuum" reading, but a manifold pressure reading.

Comparing manifold pressure before and after certain changes, such as vehicle weight change, performance parts installation etc under unchanged driving circuit and speed will indicate the effects on fuel economy among other things.

Just exploring some ideas here.

hankthetank

if atmospheric pressure is corrected to zero on a vacuum gauge, then you are reading actual corrected pressure in the manifold...just on a different scale. you need a vacuum gauge.

spaz_22

get a vacuum gauge. there is an absence of pressure during normal idling and cruising. Absence of pressure is also known as vacuum. Unless you are boosted there will never be more then 0psi of pressure in the intake.

hankthetank

actually no, just less than atmospheric. Atmospheric pressure is 14.7 psi. A vacuum gauge at zero has 14.7 psi acting on it. vacuum in the manifold is just LESS than 14.7 psi. The scale of reference is what throws people off.

14.7 psi is about 30 in/hg. most automotive vacuum gauges read to 30in/hg, or -1 atmosphere.

spaz_22

if you say so. maybe i should start calling my vacuum lines pressure lines instead.