Stupid Vacuum Question
10-26-2005, 05:18 PM
#1
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Stupid Vacuum Question
Seems like many different functions in our cars are connected with the vacuum system to some degree.
Here's my stupid question: What is the source of the vacuum? Where is the negative pressure generated to begin with?
Here's my stupid question: What is the source of the vacuum? Where is the negative pressure generated to begin with?
10-26-2005, 05:26 PM
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Manifold vacuum is created by the throttle plate blocking the flow of air into the cylinders on the intake stroke.
This is the reason why vacuum decreases when you mash the pedal down, the throttle plate opens and more air enters until finally outside airpressure nears the pressure inside the intake manifold.
Diesel engines have no throttle valve, the only fuel metering is performed by the differing amount of fuel that is injected into the cylinder. This is why diesel engines don't produce manifold vacuum and either have an external vacuum pump or the systems run off other power sources.
This is the reason why vacuum decreases when you mash the pedal down, the throttle plate opens and more air enters until finally outside airpressure nears the pressure inside the intake manifold.
Diesel engines have no throttle valve, the only fuel metering is performed by the differing amount of fuel that is injected into the cylinder. This is why diesel engines don't produce manifold vacuum and either have an external vacuum pump or the systems run off other power sources.
10-26-2005, 05:31 PM
#3
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No such thing as a stupid question.
The short answer is that the engine provides the vacume. I have never looked into exactly where its drawn from on my 928 but it is normaly drawn from the intake. There is intake vacume and venturi vacume. In some (american muscle) engines with big cams there is a lack of vacume so some use a pump to run vacume accessories. Thats why in old cars, like my 59 Ford, the windshield wipers will slow down or stop briefly when you rev the engine. I hope this explanation is helpfull. Maybe someone can point you to the exact location on your 928. If I remember to look tonight I will post tomorrow.
The short answer is that the engine provides the vacume. I have never looked into exactly where its drawn from on my 928 but it is normaly drawn from the intake. There is intake vacume and venturi vacume. In some (american muscle) engines with big cams there is a lack of vacume so some use a pump to run vacume accessories. Thats why in old cars, like my 59 Ford, the windshield wipers will slow down or stop briefly when you rev the engine. I hope this explanation is helpfull. Maybe someone can point you to the exact location on your 928. If I remember to look tonight I will post tomorrow.
10-26-2005, 09:47 PM
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10-26-2005, 09:52 PM
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Originally Posted by bigs
Seems like many different functions in our cars are connected with the vacuum system to some degree.
Here's my stupid question: What is the source of the vacuum? Where is the negative pressure generated to begin with?
Here's my stupid question: What is the source of the vacuum? Where is the negative pressure generated to begin with?
10-26-2005, 09:54 PM
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Originally Posted by Rich9928p
Manifold vacuum is created by the throttle plate blocking the flow of air into the cylinders on the intake stroke..............
This is the reason why vacuum decreases when you mash the pedal down, the throttle plate opens and more air enters until finally outside airpressure nears the pressure inside the intake manifold.
This is the reason why vacuum decreases when you mash the pedal down, the throttle plate opens and more air enters until finally outside airpressure nears the pressure inside the intake manifold.
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10-26-2005, 10:02 PM
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Originally Posted by Tony
amazingly, some of us witness positive pressure in the manifold
10-27-2005, 11:48 AM
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For a U.S. 83S.
Which hose off the intake is the master vacuum source?
Does this "master hose" off the intake go to the reservoir uninterrupted or is it tapped along the way?
Are there any other source hoses or do they all branch off from the single master or the vacuum reservoir?
Got pix for spidery cars?
Which hose off the intake is the master vacuum source?
Does this "master hose" off the intake go to the reservoir uninterrupted or is it tapped along the way?
Are there any other source hoses or do they all branch off from the single master or the vacuum reservoir?
Got pix for spidery cars?
10-27-2005, 12:34 PM
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(Bigs: I'll join you in the "no dumb question" thread) OK, assuming the engineers, esp Pcar engineers, don't/won't do anything to decrease performance, we must assume pulling vacuum from the intake doesn't affect performance. How come? The SC crowd does the inverse, doesn't it?, pushing air into the cylinders faster than it normally would be sucked in. So is the vacuum system using such a small % of the whole that it it is negligible?
10-27-2005, 01:36 PM
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A picture:
10-28-2005, 03:34 AM
#12
Dave
I never was able to open your vacuum info page but I work on what may be the worlds slowest computer, plus I'm on a ship so the signals are linked by satellite and somewhat unreliable. Please repost the link and I'll try again.
Bigs
The primary big-bore vacuum source, as others have stated, is the Y-connector on the left side of the intake which worf's pic illustrates very clearly. It supplies vacuum to the brake servo (booster). From the booster smaller vacuum lines are tapped off from a 4-way connector for HVAC and intake flappy valve actuator. The vacuum reservoir is also connected to the 4-way. Independent of the primary vacuum source there are several secondary sources on the throttle body for minor functions including fuel regulators.
Colin. 89GT
I never was able to open your vacuum info page but I work on what may be the worlds slowest computer, plus I'm on a ship so the signals are linked by satellite and somewhat unreliable. Please repost the link and I'll try again.
Bigs
The primary big-bore vacuum source, as others have stated, is the Y-connector on the left side of the intake which worf's pic illustrates very clearly. It supplies vacuum to the brake servo (booster). From the booster smaller vacuum lines are tapped off from a 4-way connector for HVAC and intake flappy valve actuator. The vacuum reservoir is also connected to the 4-way. Independent of the primary vacuum source there are several secondary sources on the throttle body for minor functions including fuel regulators.
Colin. 89GT