Originally Posted by CJ Chet
Manifold vacuum is taken BELOW the carb and is constant. Port vacuum is variable and controlled by the carb (taken off of the carb itself).
That's half right...
can be one taken one of two places...
1. Would be Plenum vacuum, taken just below the carb throttle blades.
This is total manifold vacuum, and will be "Relatively" constant, high anytime the throttle blades are closed and the engine isn't heavily loaded.
2. Would be taken from a runner out on the manifold closer to the cylinders.
This will be a 'Pulsing' type of vacuum and will only be a high vacuum when the intake valve is open.
The correct answer should have been,
is the vacuum created by the collective low pressure cells created when the pistons drop during the intake strokes.
IT is a constantly changing and pulsing vacuum that responds to engine speed, engine loading and throttle opening.
Manifold vacuum is very high at idle and any time the throttle blades are closed,
Is available at high volume for powering things like vacuum assist brakes and heating/cooling passage controls.
Spark Ported Vacuum
is a vacuum sample that is taken at the Venturi int he carb or fuel injection throttle body, and is a very smooth type of vacuum signal that responds only to the amount of air being drawn through the carb...
Which makes it ideal for most ignition vacuum sources.
This is a low volume source because any changes in the amount of volume at the throttle body can drastically change the fuel/air mixture in a carb.
Only Ignition vacuum sources and carb internal passages usually use this type of vacuum.