**Had to modify this post, realized I was using a modified system setup. All the concepts are the same though!
Look for the bold
title below to get just the technical details.
Picked up a 2002 TT 225 not too long ago, my first German car. I had to start digging in right away. Thanks mostly to the folks here, I've been able to get **** done!
So, anyway, I was removing my SAI, repairing/modifying my PCV due to a leaky PCV valve and bad hockey puck (my mod is weird, like super weird, but it works. Will post later.), and generally simplifying/refreshing the vacuum lines. As I removed the mess of hard lines placed under the intake manifold, this crazy check valved venturi'd piece falls apart in my hands. I researched the two systems, power-brake/PCV, and couldn't get any good answers as to why this thing existed. It's called "valve" "suction jet" "suction jet pump" "suction pump" "brake booster vacuum pump" "vacuum pump."
The closest thing I could find to an answer was in the comments here:
Now, after troubleshooting the squishy brake pedal plague (which in my case was air in the brake lines, air in the ABS, and poor vacuum from the ancient brake booster soft line), I wondered if reinserting a suction jet pump would help maintain a solid feel to the brake booster assist pressure. I still didn't grasp what the part did until I was contemplating the high amount of crank case pressure these cars produce. It all came together, and here it is!!
A jet pump operates on venturi pressure differentials, in a similar way to a carburetor. One stream of gas or fluid is accelerated through a venturi, which when applied across or in front of another stream of gas/fluid, creates a negative pressure zone, drawing gas/fluid from the second stream. The "suction" portion of this jet pump comes from the fact that the pressurized stream and secondary stream, also have a vacuum source being applied to the outlet. The vacuum pulls gas through the venturi at even higher velocities, and applies vacuum to the secondary line. Overall, the vacuum applied to the secondary stream is much higher than the system would experience otherwise (dependent of course on the strength of the pre-venturi pressure and post-venturi vacuum).
In the case of the 1.8t PCV/power-brake systems, the positive pressure is crated by blow by in the crank case, and the vacuum is created by the TIP. The combination flow through the suction jet pump creates a very strong vacuum to the brake booster by scavenging the crank case pressure, and applying a constant vacuum, available in variable amounts at different throttle/boost conditions from the vacuum source. To be fair, this only helps slightly, as most braking occurs while the throttle is closed and max vacuum is already available from the intake manifold connection. But responsiveness and non-typical braking can benefit significantly here. I've created a pretty picture that should make everything clear, hopefully even if you don't already have some idea of what I'm talking about, this will explain it:
2002 TT 225
Motoza stage 1+ - love em!
Bosch 550 cc injectors
4 bar FPR
3" 42dd w/ cat
3" Turbo back exhaust - custom setup
Velocity stack intake - homemade
TT 3.2 brake upgrade (found front calipers/brackets refurbished for 200 bucks!!!!!!!)
Kept the engine and battery covers, I like the look
SAI delete and hybrid draft tube PCV w/ 034 valve
Water/meth is next!