How does the Venturi tube work?

[crashr]

Can someone explain to me how the venturi tube on a 3.2 engine works. I don't see how this acts as a venturi whatsoever? It is pulling a vacuum from the brake booster loop and the airbox elbow into the intake manifold.

If there is a change in pressure or an air leak in the brake booster loop how does this effect the flow of air being pulled from the airbox elbow?

[crashr]

Monday Morning bump

[crashr]

No one knows?

I guess my main question is:

Will a change in pressure in the brake booster loop change the amount of pressure (create more vacuum) being pulled through the peice which connects to the air box?

[RWebb]

here's a bump for you

[ischmitz]

Without having the exact answer to your question it reminds me of the aspirator-type vacuum pumps that we had in our chemistry labs. It's basically a venturi connected to the faucet. See here for details: http://en.wikipedia.org/wiki/Aspirator

Since the source for the vacuum in your engine is the intake plenum you could connect any consumers (brake booster, etc) staight to the intake port. My assumption is that the venturi-style setup it to minimize intake pressure fluctuations as a result of a vacuum consumer suddenly coming online. So in other (layman) words this loop has an air mass circling around and the circling air pulls the vacuum on the break booster. So when you engage the breaks this doesn't cause the intake vacumm to break down but only the circulating air mass to slow down.

Not sure if this makes sense. It is only a guess.

Ingo

[KTL]

Here's some pics so people know what you're referring to

[Hladun]

I haven't looked inside one, but I assume it's an application of Bernoulli's equation. You accelerate flow and that produces a lower pressure (more vacuum here). It's the same as how a carb works to suck in gas. EFI engines don't suck enough.

[crashr]

So does the vacuum being pulled from the airbox (long metal vacuum hose) ever change? Why would you want to create a suction on the airbox?

[Ferrino]

If you're not using a brake-booster (for example, a 3.2 transplanted into a pre-77 car), can you dispense with the venturi plumbing circled below and cap off the intake manifold and throttle-body elbow?

[KTL]

Manifold yes. Throttle body no. That should be connected to oil tank vent, unless you instead use a catch can for the tank

[Ferrino]

Thanks Kev. I should clarify that I was referring to capping the throttle body elbow where the long tube enters, not the big silver thingy that accepts lovely oil from the tank.

[VFR750]

A Venturi is a type of tube that goes from one diameter, then gracefully tapers to a smaller diameter, then gracefully expands back to the original diameter.

As the air flows through the smaller section the static air pressure drops. If you mount a port 90 degrees to the wall of the small section, the tube only senses the lower static pressure. In our case this would be a vacuum compared to ambient air pressure.

This is how a carburetor Venturi can suck fuel into an engine.

[VFR750]

The static air pressure drops because the air accelerates as it goes through the smaller diameter.

[KTL]

Yep my mistake. Plug that port in the throttle body elbow that connects to the long tube. I think that long tube is connected to the throttle body elbow to allow for some brake vacuum when the throttle plate is closed? Regardless, you don't need it for your non-boosted brakes.

Make sure to flatten the intake manifold spacers/insulators on some fine sandpaper & a sheet of glass or whatever. Those plastic insulators are notoriously leaky.

Also I would suggest separating the intake at the rubber union so you can flush it out. The intake usually has some oil sitting it in from a previous overfill of the oil tank. Notice how the primary section of the manifold halves are "sumped" because the primary area is slightly below the secondary tubes. That sump captures quite a bit of oil.

[rick-l]

I wonder how much idle air goes through there and if the idle air control will compensate (and have the base idle adjustment range) if it is plugged?

[Derek911]

Originally Posted by Hladun

don't suck enough.

We are still taking about the vacuum system... Right?

[T77911S]

Originally Posted by rick-l

I wonder how much idle air goes through there and if the idle air control will compensate (and have the base idle adjustment range) if it is plugged?

thats what i was wondering. maybe its there because the ICV can not provide enough air at idle.
its still metered air, its just bypassed the throttle.

it could also be there for when you let your foot up from hi RPM's so there is not a large vacuum or so the engine still gets air when you let off the gas.

[Derek911]

KTL hit the nail on the head. Vacuum boost for the brakes that is somewhat independent of throttle plate. As you can see it bypasses the throttle plate so that brake boost stays uniform through out throttle range.. Including being closed... Which is when the brake is used.

[rick-l]

Originally Posted by Derek911

KTL hit the nail on the head. Vacuum boost for the brakes that is somewhat independent of throttle plate. As you can see it bypasses the throttle plate so that brake boost stays uniform through out throttle range.. Including being closed... Which is when the brake is used.

It only provides a vacuum boost when there is a difference in pressure across the throttle plate (for air flow) which is maximum at idle. At wide open throttle it is zero.

I wonder how much this improves the vacuum in the booster? like does it go from -7 psig to -9 or try to go close to 0 psia?

[Ferrino]

I didn't consider that the long pipe would be another idle air source. Surely the main idle air circuit (via the ICV) should be designed to be sufficient by itself?