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Think of it this way, when the engien is idling the butterfly valve for the throttle is closed, but the engine is still sucking in air beacuse the pistons are going down on the intake stroke. Beacuse this butterfly valve is closed it has the same effect as if you were to put your palm on the end of a shop vac while its on, the vacuum inside the hose goes up beacuse of the restriction.
When the throttle is opened the vacuum stays strong but the restriction is less as it opens further. On full throttle the vacuum is still there but the restriction is minimal, hence a lower reading on a vacuum guage, and lower fuel economy. It would be the same as sliding your palm back off of the shop vac more and more until it was removed.
This is why as you push the throttle the engine continues to gain speed, it will suck in as much air\fuel as you will feed it. When the throttle is open all the way but the car is bogged while going up a hill or in too high of a gear, the vacuum reading will be low beacuse the throttle restriction is not there, but the engine is not going fast enough to make the amount of power necessary, another instance of low vacuum on the guage, and low fuel economy.
Lots of things depend on vacuum, such as timing advance, climate control, fuel injection system... This is why vacuum leaks cause so many problems, plus in my situation with a turbo there is the possibility for boost leaks, which is a whole new situation to deal with.
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Zack Silver 83' 242Ti (221k)
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