What's the function of the Venturi Tube on a Tiptronic?

[78F350]

Originally Posted by KRAM36

Went out and disconnected the line to the brake booster. It works opposite of how I thought it does. The line to the brake booster pulls air from the booster, a massive amount of air. So the engine isn't losing air, it's getting air added to it.

So you have the venturi tube adding air and the AOS adding air after the MAF sensor, how does the ECU actually know the amount of air going into the engine?

Not quite. The brake booster is a closed system. The booster uses the force of the vacuum to move a diaphragm when you apply the brakes, but is not open to allow intake of new air. The volume of air passing the MAF is still to TOTAL volume of air being drawn for combustion (Except for the SAI system).

There are some good "How it Works" diagrams and videos if you Google it.

You got me curious about the whole venturi tube. Why just on automatics and what does it do....

It seems like it is common on German cars: VW, Audi, BMW, and in most applications the venturi is called a "suction jet pump".
There is some good reading about it:
How does the BMW E39 sucking jet pump (aka suction jet pump) work & how does it fail? - Bimmerfest - BMW Forums

Post #4 in this thread addresses it: *DIY* - B5 1.8T Vacuum/Check Valve/SAI/PCV - Delete & Simplification