Rob Edwards

Question for the collective brains: How the heck does the venturi tube/Y-connector/'ejector' piece that connects the intake hoses to the brake booster work? It is present on every 944, 911 SC and 928. I am trying to understand why that piece (which is listed as 035 133 753 A on the 78-86 PET sheets, I assume it's the same for 87-95) is as complicated as it is. On the Zombie, Mark had the brake booster vacuum hose plumbed directly into the rear corner of the intake, so it saw manifold vacuum at all times:



AFAIK Mark never ran out of brakes.....

But the factory venturi is a lot more complicated. Front end to upper manifold plenum, rear end to the MAF elbow Y, which has a tiny (3mm restrictor) hole to the hose that runs to the ISV, and the other leg is open to the MAF elbow.

Venturi piece together:




Pulled apart:



FWIW, there's an o-ring at the red arrow, which is utterly petrified on this particular example. I wonder whether this is additional low hanging fruit for those chasing manifold vacuum leaks?


Front view, showing ~3mm hole and 'ribs' on tip:



I am not sure how close the 3 mm hole fits up against the opening in the front half of the venturi assembly. Obviously it doesn't seal in the opening or the hose to the booster would be bypassed.

So- what's the theory of operation? Is it to limit the amount of vacuum that the brake booster sees? At idle (closed throttle plate), the front is seeing full manifold vacuum, so there's air rushing from back to front thru the venturi, creating vacuum for the booster (?)



At WOT, throttle plate is open, MAF elbow's probably not flowing much air into the rear half of the venturi circuit, ISV is closed off, but there's still enough manifold vac via the front that the booster's being evacuated?

I'm sure the last two paragraphs are all dead-*** wrong, so how's this thing work under varying airflow conditions? Why so complicated, Franz?

Koenig928

I'll take a stab at it, I bet Porsche engineers were trying to increase the vacuum pressure in the booster under certain conditions.

A venturi tube follows basic aviation theory, "as velocity increases, pressure decreases" at the restriciton point. Bournulli's principle to be exact, it's one of the ways to explain how lift is produced.

Been flying for 25 years...

Mrmerlin

to answer this,
the fitting is called a venturi tube and incorporated into an ejector.

This part is used to create a suction or pull a vacuum compared to ambient air,
this will then be connected to the booster to have maximum vacuum available,
considering intake fluctuations when the throttle is opened or closed.

This way you wont have the intake fluctuations exposed to the booster or the vacuum tank.

With this part, it will maintain a higher vacuum or suction than the engine intake is supplying.

The O ring should be replaced so outside air doesnt sneak past and reduce its suction capacity.

Fitting one of these ejectors to Marks car will increase the suction available for the booster.

NOTE on Marks fitting is there a one way valve to limit the suction vacuum that the booster will see
IE hi suction VS low suction or pressure as when at hi RPM the throttle is closed and the intake loses its suction.

NOTE this throttle opening/closing along with RPM changes engine operation from a suction pump to an air pump inside the manifold

Rob Edwards

That makes sense, thanks guys- I thought as much, was just wondering whether I'd missed anything more subtle.

Stan, I do wonder how many venturi's o-rings are dried up. If the venturi comes apart with minimal resistance, I'd guess that it's not up to sealing against 18-20 in Hg of vacuum. (?)

And yes, there is a 1-way valve on the racer's booster hose.

Leon Speed

I think "venturi" is misguided since it doesn't act as a venturi at all. In a venturi tube the pressure difference is created at the narrow passage. I see this thing much more simple: vacuum in the intake causes vacuum in the brake booster. The injector as Stan calls it equalizes vacuum differences caused by different operating conditions (idle: low vacuum, WOT: high vacuum).

FredR

Rob,

The chaps have it pretty much covered. The system is configured this way so that vacuum is maximised. The venturi pipe itself accelerates the gas [air] flow such that the pressure is equalised with the vacuum in the plenum. This creates a localised relatively high velocity of the gas as it comes out of the nozzle and this in turn creates a very localised zone of low pressure on the outside of the nozzle thus creating the maximum amount of vacuum possible even when the throttle is fully closed.

Whilst playing around with my system recently I noticed that the fit of the nozzle inside the carrier was not exactly tight so I sealed it with silicon RTV as I could see no practical reason to need the ability to remove the insert.

Regards

Fred

FredR

Arjan,

A venturi is exactly what it is- it creates the "venturi effect".

The high vacuum occurs at idle with the throttle closed

Regards

Fred

Leon Speed

Right, I got those reversed. In that case, a venturi would make sense.

FredR

Aryan,

I suspect the real object of this kit is to keep the fullest vacuum possible on the booster at all times. Once the engine is down to idle revs no problem but when coming off throttle and stomping on the brakes immediately as racers and hooligans usually do there is probably a degree of lag.

It would be interesting to see a vacuum gauge connected to that system and see how it pulls vacuum when the throttle is open or partially open. I suspect it will be close to a full vac all the time. Indeed I might just try that.

Regards

Fred

Mrmerlin

Fred thats not a bad idea for more data points,
and the system operates as you have written

dr bob

Sand for the brain gears:

The Venturi/eductor/ejector is there to maintain vacuum for accessories when driving with the throttle open. Consider that the original market for the car was as a high-speed tourer, with extended deep throttle operation. You still want the vacuum accessories to work, like AC vents and such.

Brakes get full manifold vacuum regardless, unless you apply them with the throttle wide open. Then you only get one or two boosted stops before the booster pressure comes up to atmospheric pressure. At the time you'd apply the brakes, vacuum is highest at the same time that velocity through the ejector is lowest.

PorKen

I'm with Dr. B, vacuum created by airflow under full throttle (autobahn) operation.
Not very useful in the states, I reckon.


I noted an increase in pedal firmness when hauling it down from >130 to <60 (over and over and...) after replacing it with ~20" of Gates 15/32 vacuum booster hose.

GregBBRD

Another question regarding this "venturi" piece....or whatever you guys decided it actually is.

The hose that runs up to the boot between the throttle plate and the MAF has a very small restrictor in it (about 1/8"). Is this hose location important or does it just need to be located in the air before the butterfly? Specifically, does the air being pulled through the throttle body create a suction on this hose or is that hose simply there to "equalize" intake vacuum?

ptuomov

It's a "jet pump" or "venturi jet pump." At least that name gets the most google hits, and who's the authority if not the internet...

I believe that the restrictor is there to make it flow the right amount of air. The right amount is little enough that the hose is under strong vacuum with the throttle plate closed without the idle surging, but still enough with throttle open for the air flow to operate the "venturi jet pump."

With part throttle and boost, the venturi jet pump mode is marginal. There's not enough mass flowing fast enough to always overcome the manifold boost. That's not the greatest thing in a boosted car with the cruise control set to high-way speed and a long uphill slope. Been there, done that, got a nice French cuff shirt. [Edit: FredR says that this venturi jet pump only supports the brakes, not the general vacuum system. So my problem has an even simpler explanation.]

By my logic, it doesn't need to be close to the throttle plate. Anywhere upstream of the throttle (but after MAF if one is used) with large enough cross-sectional area should be fine.

GregBBRD

Yes, the internet is such a great thing...people generally know just enough to be dangerous.

You, however, strike me as having quite a few brains cells, without the internet.....

Do you think that the little orifice has air drawn out of the hose by the column of air passing by it....creating a vacuum at low intake manifold vacuum conditions or it just a pressure equalizer, so that there is not excessive vacuum on the booster under high intake manifold vacuum conditions?