The Stovebolt Site Forums The Shop Area Engine Shop why does vacuum go down when rpms go up?

Ok, so we all know that manifold vacuum goes down when the engine accelerates (well, at least those of us with vacuum wipers). But why? I've been pondering on this for quite some time. It seems to me that, as the engine accelerates, it will be pulling in more air, thus increasing the vacuum! I would love to hear an explanation of this counter intuitive phenomenon!

You and me both brother!

Vacuum varies with load, not RPM. If you nurse the speed up gradually, with light throttle in neutral, the vacuum stays high. Goose the throttle with the car in gear, especially the higher gears where the engine can't accelerate quickly, and the atmospheric pressure rushing past the open throttle plates just kills any vacuum that's there. Once the speed comes up, and the load stabilizes, the vacuum will go up again.
Jerry

It's called Bernoulli's Law. The speed of air increases when air flows through a venturi (a thing that is kind of shaped like a hourglass). Air pressure decreases as the air speed increases. Fuel is held in a chamber vented to atmosphere. Air passing through the venturi when the engine is running creats a low-pressure area while there is higher atmospheric in the float bowl. That's how fuel gets sucked into the carb. The throttle controls the amount of air flow with the throttle plate. So, if you open the throttle plate under acceleration your vacuum "bleeds" off. Let off the throttle and the venturi gets more constricted and low atmospheric pressure or vacuum is there again. Got it?

Pulling in more air, decreases the vacuum, not increases.

If you suck the air out of a plastic bag and cut one corner, you have less vacuum inside the bag.
The bigger the cut, the less vacuum....

The cut in the bag acts somewhat like your throttle plate opening.

Tylerwatts: I can't explain like like these gents have but there is one thing you can do to help. I think it was an "option" but there is a "cannister" that was available to hold more vaccumn, or pressure??? but by using it your wipers would stay constant. I'm sure there are folks that know what I'm talking about and know more about it but I have seen them a few times along the way.

There might be a simpler way to explain this. Consider a vacuum cleaner. If it's running and you place your hand over the end of the nozzle, you can feel the vacuum go up (not just the drag on the motor). As you remove your hand, the vacuum goes down again. Your hand acts like the throttle in the carb - open the throttle and the built up vacuum has no resistance. Close the throttle (place your hand over the hose)and the vacuum goes back up again.
On my truck the wipers go like the band when I am up at cruising speed and let off the throttle, using the engine to brake. That is also when the blue smoke pours out...
Karl

Chipmunk, We use to call that cannister a "dead air chamber." All it did was buffer changes in input or output much like a capacitor does in power supplies. Sort of like having a compressor with a 25 gallon or 50 gallon tank. The compressor will cycle more often with the 25 gallon one. So, the bigger your cannister the less change you will see in wiper speed for the same amount of change in vacuum input. "On the road again"...I don't believe it's the amount of air but the speed of the air and the differential pressure drop across the venturi.

I'm gonna try another way to explain this.

There is 14.7 psi of atmospheric air pressure at sealevel, right? Okay. This air pressure is forcing itself on the throttle plates in the carburetor when they're closed.

The engine's basically an air pump. So when it's running at idle, the idle circuit allows just enough air and fuel in to keep the engine running. But it's pumped most of the air out of the intake manifold AFTER the throttle plates. So you have vacuum. But it's vacuum as compared to the ambient air pressure. Your cam grind has something to do with how much vacuum you pull because of overlap. Torquey cams like those in truck engines are what they call "mild", which will have a higher vacuum than a drag race cam with lots of overlap.

But anyway, Once you open the throttle and the plates in the carburetor open up, all that ambient air pressure around you wants to force itself into the manifold and engine because nature hates a vacuum. Your engine gets more fuel with that extra oxygen because bournelli's theorem draws the fuel into the airstream. The engine responds by increasing in rpm, and because the Earth's atmosphere has just forced itself into the intake manifold, you will experience a drop in pressure simultaneously.

Does that make more sense?

When do we get our diplomas?

OK,,with engine at idle the carb throttle plates are closed so the cyl,s are still trying to take in air but throttle is closed thus it is creating a high vacum in the intake(((18 inches)))when you increase engine speed you have to push foot pedal it opens carb throttle allowing more air into the intake thus the high vacum falls because throttle is opened allowing air to flow freeley thru carb and intake so your vacum is down so the wipers slow or stop until you close throttle to allow vacum to increase,,so under full throttle you have very little vacum and high air flow,,at idle you have high vacum in intake and very little air flow...now I need a nap!!!!doc

God, you gotta love this site!
Beltfed busted out with the Bernoulli Principle!
I solved this problem temporarily as my 235 does not run nor does the 46 have a wipers.
Note to self, archive date 3-20-07
Cavebull

all you guys make it too complicated, it's just as simple as electrical smoke .... vacuum [suction] makes the wipers work, mashing the gas lets the suction out, wipers quit

smoke good, suction good, where they belong .... don't let them out

Bill

I think we lost Tyler. He'll be careful before he opens a topic like this again. Where is CarbKing when you need him? I can venture a guess: he is sitting home reading this and laughing so hard the beer is running out his nose. He's laughing because we are shooting at a mouse with a 10 gauge, all us amateurs. But I think we can put away all those big Italian words. No matter what Red says, the vacuum comes from the pistons going up and down and the valves opening and closing. The counter intuitive part of this is that if the pistons were strictly a vacuum-creating machine, the faster they went the greater the vacuum would be, and the faster the wipers would go. This would be true if you could find a way to increase RPMs without opening the throttle. With the throttle closed, the vacuum has only two ways out, one way is the little hose to the wipers, the other is the even smaller idle jet, so the wipers get plenty of suction. When the throttle is open, then the great big main throat vents all that vacuum out to the atmosphere. At that time a big volume of air rushes through the throat and you can start talking about Bernoulli and Venturi, these are the guys that decide how the much fuel gets sucked into the motor. But with the throat (throttle) open the vacuum will rather vent through the throat than that little hose to the wipers.
How long can we keep this topic open before we all go to sleep? Tyler, are you there? Do you say "Uncle?"

Sorry Red, you didn't say anything about where the vacuum comes from, that was another mistake from me, one of many I'm sure

my intuition sez it's not speed, but volume that counts, besides everybody knows the vacuum is installed at the factory, just like the smoke

Bill

Don't they top off the vacuum along with the blinker fluid when you get a fill up?!

I'm installing electric wiper motors, just so I don't have to think about this anymore . . .



-Michael

Yes, you just have to worry about letting the smoke out of the electrics. Brian

Thanks to Tyler for this post. I LOVED ALL THE ANSWERS!! Vacuum has be one of those great mysteries now forever demystified. Thanks guys!!

Mine has factory installed squeeks.

I was sitting today pondering "why is there air?"

Boy, am I glad I didn't ask that question here on the forum.

BTW: I did learn something from all the explanations above. Thanks.
(I don't think I could repeat it well enough to explain it to anyone else tho!)

Brian

I have to throw my two cents in on this one because I think you guys are making this to hard. Remember, these are old engines - they operate on simple principles.

To get right to the heart of manifold vacuum, you need to understand two things: 1. Where does the vacuum come from? 2. Why does vacuum change?

1. First it is important to point out that some might be confusing manifold vacuum with venturi vacuum - this is how your carb mixes air and fuel in the proper ratio. Manifold vacuum is created when a piston moves downward during the intake stroke, thereby expanding cylinder volume above the piston and creating an area of low pressure. As a result, air (and fuel) is pulled into the piston through the mainfold and carb. Since the pistons move up and down at different times, manifold vacuum is essentially constant.

2. Engine speed is limited by the amount of air (mixed with fuel) that is allowed into the pistons - this is controlled with a simple butterfly valve below the carb. When the throttle is opened, more air (and fuel) is allowed into the pistons and the engine speeds up. In theory, vacuum would entirely disappear and the engine would operate at atmospheric pressure as the restriction to airflow (the butterfly) is removed. Note that the carb would still work because, remember, it operates on the venturi principle. In practice, however, we all know that the engine maintains a vacuum. However, it is not always clear what will happen to vacuum as the vehicle accelerates - this is a function of several variables, but is largely determined by the load on the engine and the parasitic drag that the air (and fuel) encounter as it flows through the carb and manifold. Two examples: 1. Remember the last time you floored the old bolt up a big hill with a load in the back? What happened to vacuum? It was probably almost nothing. Why? The pistons were moving up and down more slowly which creates less vacuum to start with. However engine speed was limited by load. Since the throttle was wide open, and there was no restriction to the amount of air that could flow in, vacuum pressure should approach atmospheric pressure. 2. Now consider the last time you were late for work and you flogged the old bolt down the highway. Vacuum was lower than what you see at idle. Why? Theoretically, it should again approach atmosheric pressure since the restriction to the airflow is now removed (i.e. throttle wide open). In practice, however, parasitic drag (turbulence) in the manifold becomes the limiting factor to airflow speed and a vacuum is maintained.

Odds and ends: If my rambling made any sense, it should be pretty clear how a turbocharger or supercharger works. Both systems work to increase airflow into the engine. Imagine you have a turbo that produces 7 lbs of boost. What that means is that your engine (at full throttle) will be operating with 7 lbs less vacuum than it would without the turbo. In many cases, this means the engine will be operating above atmospheric pressure.

"All things being equal, the simplest solution tends to be the best one." - William Ockham (c. 1285–1349)

Now y'all start explain gravity to us, please....

I guess according to William Ockham you could say "vacuum sucks".

Enough already; pucker your lips like your going to whistle, poke your finger in that pucker and suck like hell, you'll note high vacuum and no air flow (throttle plate closed). Now pull your finger out while still sucking hard--you'll get high air flow but no vacuum (throttle plate open). You might do this experiment when no one else is around or they might think your "funny".

Two things on this topic... One I have tube I use to suck from the wipers as I drive in the rain this keeps the wipers at a constant pace it looks kind of funny but it works... Two when its only misting out I use my wipers the regular way I think this is where someone thought of Intermitent wipers on then off etc. Those are my thoughts. Peter

Be lucky you don't have an older Model T that had manually operated wipers!

Converting to electrical wipers is an excellent upgrade. I've got it on my truck.

My Corvette though, has vacuum-operated headlights that come up. When they are restored, they work well. It's a little weird having one headlight come up at a time, but it does work nicely. It's just that with my huge cam, I kind of have to be slowing down to raise my headlights...not speeding up. A lot of guys like to try to upgrade to electrical headlights, but vacuum is a lot lighter than electric motors, so I keep it vacuum.

But that's off the beaten path...

God..KC..give us a break,,now I,m confuse and I thought I new all....doc

Love's me some RainX.

doc41,

I'm with you!

Stuart

Sorry doc, I was just trying to help. Next time I'll save my $.02 and put it toward parts for the old bolt. Kyle

Great explanations (some easier to understand) thank you very much. Now I feel comfortable with why the engine loses vacuum and I'll start ponderin' something else. BTW, I'm very happy with my original vacuum wipers, and I live in an area where it rains alot. What really "sucks" is my defroster...

Tyler

Oh Boy, now we're going to get into heat transfer coefficients and thermodynamics for Tyler's defroster, just our cup of tea. In just a short time we'll have him roasting marshmellows with it. Someone hand me a coat hanger. Bob

HEHEHE!!!!!!!!doc

So why in the world would I dig up an old thread, well, blame it on the search function LOL.

Even tho it was pouring the rain, I broke my old rule and took the ole Bolt out in it anyway. It still has it's original vacuum wipers which got me to pondering this very same question as the OP did back in 07, as I tooled along in the rain.
I was thinking about floating this subject out in a new thread but the search function found this most excellent thread that pretty much covered all the bases. I think I'm going to try a vacuum canister now.

Bernoulli's Principle, man you gotta love this stuff....

But this thread didn't answer the other related question that came up, what about a warm weather defroster?

Dave

Occam and Bernoulli in the same thread. Fun stuff here. "Vacuum" here we go. I got up early for this? OK try this. The "vacuum" we refer to is the difference between ambient air pressure and the "lesser" pressure in plenum below the carb. At W.O.T.(wide open throttle) that difference should be ...very small. The engine should be getting all the air that will flow into it. The pressure above the 'venturi" will be almost the same as the pressure is below it. The venturi still works. It changes volume to create the difference needed to pull fuel out of the bowl through the jet into the engine. Now close the butterfly. Pressure under the carb, goes way down. Difference between the air outside(ambient) stays the same, Hoo Ya we got vacuum! We would also get a lot of fuel in the engine. That is why we have all them other jets and stuff in the Carb. Enjoy.

Very small is typically 1.5" to 3" of vacuum, smaller carburetor = higher number.
Slightly more power by going below 1.5", but unless the carb is very sophisticated (Weber DCOE & IDA, Dell'Orto DHLA, Mikuni PHH, Solex, &c.) the venturi signal is too weak for accurate metering.
EFI can work as far down as .25": really big TB.