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

[KRAM36]

Can someone explain what the function of the Venturi Tube on a Tiptronic is? Is it sucking in air or pushing in air into the intake system? What does it do for the Tiptronic trans?

In my picture you can see that it connects into the intake system at 2 spots and both are after the MAF sensor.



I tried Googling it several ways, but you know how Google has become. It only pays attention to Boxster and Tiptronic, so I get results for Boxsters for sale with a Tiptronic instead of the answer to the question I asked.

Thanks!

[NewArt]

Good question! I replaced mine since it was old and brittle and kept breaking. (Sound familiar?) I hadn't realized it was only in the tip. I know it is part # 96611017000 and costs just over $100. I understand the venturi principle. I too would like to know what a hundred plus dollar plastic tube, that I had to replace, does in my car!

[78F350]

I know that the front connector is the vacuum line for the brake booster. Not sure why the tips take that from the aft side of the engine. I forget what the other end connects to. (Drove a Del Sol today and no Boxsters in sight.)
Maint to do? Swapping in a used 2.7

[The Radium King]

dude. i googled 'boxster tiptronic vacuum' and this was the first thing that came up (refer to post #10):

air intake/vacuum line routing - Pelican Parts Technical BBS

[KRAM36]

Originally Posted by The Radium King

dude. i googled 'boxster tiptronic vacuum' and this was the first thing that came up (refer to post #10):

air intake/vacuum line routing - Pelican Parts Technical BBS

This post?

Nivaldo, your car is equipped with a Tiptronic transmission and the vacuum switching valve you're identifying opens the coolant supply line to the transmission cooler. The tube in your hand connects from the purple circle to the blue circle (technically from the vacuum reservoir). The yellow tube connects to the green circle (which leads to a thing Porsche calls the 'flat-base valve on the transmission'). Repair literature cautions that transmission cooling will be inadequate if you don't get things hooked up properly.

He is referring to the secondary vacuum lines. I'm curious about the venturi tube. As you can see the 2 spots where it connects into the intake system after the MAF sensor. I want to know what this tube is doing and how the engine is compensating for whatever it does to the air volume. It's effects are obviously not picked up by the MAF sensor.


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[KRAM36]

Originally Posted by NewArt

Good question! I replaced mine since it was old and brittle and kept breaking. (Sound familiar?) I hadn't realized it was only in the tip. I know it is part # 96611017000 and costs just over $100. I understand the venturi principle. I too would like to know what a hundred plus dollar plastic tube, that I had to replace, does in my car!

Yes mine broke too as I pulled it from the connection to the intake pipe. Luckily I found one on eBay for $75 shipped, even came with the intake pipe. This one is much more flexible at the Y connection.

[The Radium King]

sorry, i confused the two. a venturi in this context is usually associated with vacuum. this looks like a bypass around the throttlebody at idle, using an existing port on the manifold (the manifolds are the same piece reversed, so the brake booster port is available). at idle air is drawn from the intake tube into the manifold (throttlebody is closed and there is a vacuum in the manifold as a result). it moves past the y fitting and creates a vacuum. where does the third end of the venturi system connect to? probably to do with idle control and the torque converter, or maybe just to create additional vacuum for additional functions.

[KRAM36]

Originally Posted by The Radium King

sorry, i confused the two. typically a venturi in the format here is used to create a vacuum. like the aos pipe that connects at the throttlebody, the air rushing by the end of the tube creates a low pressure area and consequent vacuum. this is how the aos creates vacuum on the crankcase. in this case, i would suggest that the tip needs more vacuum than the system can provide typically, so additional connections are made to create additional vacuum (you get vacuum before the tb from the air rushing by, but no vacuum here when the car is idling; in reverse, you get lots of vacuum in the manifold when the tb is closed, but less when it is open). where does the third end of the venturi system connect to?

The third connection goes off to some place right next to the air conditioning compressor. I have yet to track down what it connects into as at the time it wasn't on my mind. I can find out tomorrow though.

[NewArt]

Originally Posted by 78F350

I know that the front connector is the vacuum line for the brake booster. Not sure why the tips take that from the aft side of the engine. I forget what the other end connects to. (Drove a Del Sol today and no Boxsters in sight.)
Maint to do? Swapping in a used 2.7

Yes, when I dropped my engine last year, I remember reading in my Bentleys that I needed to "remove brake booster vacuum line", which was at the front of the left intake manifold. Except it wasn't.

[The Radium King]

ah - the third end goes to the brake booster. prolly just to give addnl vacuum to the brake booster because on a tip you stay on the brakes at idle whereas on a manual you would just release the clutch.

[KRAM36]

This is interesting. See on this engine where there is a connection to the intake plenum and then runs down to the fuel rail? EDIT: Visual misinterpretation on that, it does not run down to the fuel rail.



This spot is capped off and the venturi tube uses that spot as a mounting point. Where it goes from there would be the brake booster?

I'm not sure if this is a Boxster engine either as my air compressor is way more to the right. That last bolt on the intake plenum was not fun to get to. Lots of stuff in the way, not a clear shot like this picture shows.


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[78F350]

Originally Posted by The Radium King

ah - the third end goes to the brake booster. prolly just to give addnl vacuum to the brake booster because on a tip you stay on the brakes at idle whereas on a manual you would just release the clutch.

I think you figured it out. I should have been clearer in my first post rather the posting a confusing link.
The overall function is that it is the brake booster vacuum line.

On a manual transmission there is only an elbow fitting at the left front of the intake manifold (no. 23):


On the Tiptronic there is only a blank cover in that position. The Venturi tube connects to the right rear intake manifold and additionally to the pre-throttle intake, then terminates at the front left to connect to the booster line. The effect is greater vacuum.

[The Radium King]

so the venturi tube ties in to #12 and #14 here (why i don't have these on my car):

Intake Distributor

[KRAM36]

Originally Posted by The Radium King

so the venturi tube ties in to #12 and #14 here (why i don't have these on my car):

Intake Distributor

It ties into #6 on the plenum and and then to the intake pipe.



If this is for the brake booster, why did they do it this way on the Tiptronic cars? Do they require more vacuum? Why run all the way across the engine like they did? How does the engine compensate for the extra vacuum?




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[78F350]

Originally Posted by The Radium King

so the venturi tube ties in to #12 and #14 here (why i don't have these on my car):

No, I don't think it ties to 12 or 14.

If this is for the brake booster, why did they do it this way on the Tiptronic cars? Do they require more vacuum? Why run all the way across the engine like they did? How does the engine compensate for the extra vacuum?

Good questions. My guess on why it runs the way it does on the Tiptronic is that the right rear intake manifold is closer to where it taps the intake tube vacuum. I can't make a good guess why it uses more vacuum though. Maybe it's only significant at lower rpm for braking at idle as Radium suggested.

[The Radium King]

aha, i see now. proportions seemed off to me and was trying to figure a way for that line to get all the way to the front of the engine.

[KRAM36]

EDIT: Scratch this post.

[KRAM36]

EDIT: Scratch this post.

[KRAM36]

Pelican Technical Article: Replacing the Boxster Brake Booster and ABS Controller - 986 / 987

The Boxster is equipped with power brakes that utilize excess vacuum from the engine in order to assist with the pressure needed to apply the brakes.

Since the venturi tube connections are after the MAF sensor, how does the ECU compensate for the loss of air since the MAF sensor told the ECU how much air was coming into the engine?


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[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?