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Whilst writing an answer to this question, I remembered something I don't really understand.

The partial vacuum in the inlet manifold is caused by the piston descending with the inlet valve open during the "suck" phase of the Otto cycle. It is this vacuum that causes fuel to be pushed into the intake, the greater the vacuum, the greater the fuel added to the inlet charge and the greater the power.

Since the needle valve is on the manifold side of the throttle butterfly along with the input to the MAP gauge, why does the manifold pressure increase with power? I would have thought more power, more "suck", lower pressure; more pressure, less "suck", less fuel.

I'm obviously wrong since that's not how it works but why?

The answers to this question get close but I'm still missing something (and I'm sure will soon be embarrassed by the obviousness of the answer).

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  • $\begingroup$ Possibly related: aviation.stackexchange.com/q/9313/167 $\endgroup$
    – Steve V.
    Commented Apr 2, 2015 at 2:50
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    $\begingroup$ You don't need a vacuum to draw fuel through the jet. Air passing by at speed is enough. I've got an airbrush that has a compressed air nozzle, with a paint nozzle in front. When I open the air valve, paint is drawn up from the paint nozzle into the airstream. $\endgroup$
    – Hobbes
    Commented Jun 10, 2019 at 18:22
  • $\begingroup$ Manifold pressure does not actually increase with power. With a constant speed propeller when you increase rpm, the power will increase, but the manifold pressure will decrease. $\endgroup$
    – Jan Hudec
    Commented May 19, 2020 at 9:31
  • $\begingroup$ @Jan Hudec, I'm pretty sure he's talking about moving the power lever, not the prop control. $\endgroup$
    – Michael Hall
    Commented Aug 24, 2020 at 4:39
  • $\begingroup$ @MichaelHall, when one says just ‘power’, I generally consider it to mean the actual power generated by the engine no matter how it's control levers are set. And for spark-ignition engine the usual term is ‘throttle lever’, because it controls the throttle, that term even appearing in the question. $\endgroup$
    – Jan Hudec
    Commented Aug 24, 2020 at 5:14

9 Answers 9

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Manifold pressure is actually a measure of the vacuum pressure between the throttle and the cylinders. The more the throttle is opened, the closer that manifold pressure returns to atmospheric pressure.

The amount of fuel provided to the engine depends on the pressure difference between the manifold and the throttle body. The throttle body pressure will decrease as the throttle is increased, because of greater rate airflow across the venturi (faster air -> lower pressure). The manifold pressure will increase as throttle is increased, due to the greater flow of fuel/air mix into the manifold (greater mass of air flowing into a fixed volume -> higher pressure). So the pressure difference will get larger as the throttle is increased.

This link has a pretty good description of what's going on.

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  • $\begingroup$ Great link. I have that printed out in my fight bag from lessons I took a while back in a DA-40. $\endgroup$
    – jt000
    Commented Mar 31, 2015 at 20:33
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    $\begingroup$ I think you are looking at it backwards. It isn't reduced pressure that causes less fuel flow. It's less fuel flow that causes the reduced pressure. Going back to the straw analogy from below, imagine sucking on a very wide straw. It would be difficult to create any "suck" (reduce pressure) but when the opening is narrowed, the pressure decreases (vacuum increases) --- Looking at any fluids system can be confusing, and I'm not sure that seeing it as a cause and effect is really accurate at all. $\endgroup$
    – Joel M.
    Commented Mar 31, 2015 at 20:53
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    $\begingroup$ I think this statement from the link is probably the most helpful: "Manifold pressure depends on ambient pressure, the position of the throttle plate, and the speed at which the pistons are moving up and down." We can only directly affect one of these variables (throttle). Increasing throttle increases RPM (with fixed pitch), but because our inlet is larger, the pressure will be raised more by the inrushing air than it is decreased by the faster "vacuum pump" action of the cylinders. $\endgroup$
    – Joel M.
    Commented Mar 31, 2015 at 21:03
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    $\begingroup$ I think I've found my answer. My mistake is in the original question. The fuel inlet is not on the manifold side of the butterfly, it's on the intake side! So, wider throttle setting means a reduced pressure in the intake, but an increased pressure in the manifold, which is what draws the fuel in! With this knowledge, and the answers here, it's starting to fit together. The straw analogy helps! You, and @jt000, might consider editing your answers along these lines. I don't feel they are yet complete enough to accept. Cheers $\endgroup$
    – Simon
    Commented Mar 31, 2015 at 21:05
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    $\begingroup$ The diagrams in that link are wrong. They have the throttle butterfly upstream of the carb venturi and it is actually between the venturi and cylinder. The choke is at the position indicated, and it cannot function as a throttle. $\endgroup$
    – Pilothead
    Commented Aug 17, 2018 at 16:17
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Manifold pressure is the pressure in the fuel\air mixture between the throttle and the engine. When the throttle is at low power, then it is preventing fuel\air from flowing which causes a reduction in pressure. This is because the engine is trying to pull fuel\air, but throttle is preventing it much like sucking on a straw stuck into thick milkshake. When the throttle is wide open, then the fuel is able to freely flow & you should see normal pressure just like outside (i.e. 29.92).

In other words, the pressure is actually decreased when at low power & at "normal" pressure when at full power.

For more info check out this link

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    $\begingroup$ Thank you. I'll wait to see other answers, but I respectfully think you might be wrong. Certainly on the systems I've seen, MAP is measured in the inlet manifold and has no connection with the fuel lines. $\endgroup$
    – Simon
    Commented Mar 31, 2015 at 20:06
  • $\begingroup$ The inlet manifold is what passes fuel\air to the cylinders. It would have to be connected to the fuel lines, wouldn't it? en.wikipedia.org/wiki/Inlet_manifold $\endgroup$
    – jt000
    Commented Mar 31, 2015 at 20:11
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    $\begingroup$ No, the fuel line feeds the float chamber in the carb. The vacuum outside the needle valve is what draws the fuel in. The fuel line(s) does not connect with the manifold. I've read that link and I still feel thick :) I'm just not getting it. $\endgroup$
    – Simon
    Commented Mar 31, 2015 at 20:12
  • $\begingroup$ Ah, now sucking on a straw in a thick milkshake is beginning to do it for me! So as the butterfly opens, the milkshake gets thinner? But if that's the case, why does fuel flow decrease with increased vacuum? $\endgroup$
    – Simon
    Commented Mar 31, 2015 at 20:14
  • $\begingroup$ The engine is trying to pull fuel, but the throttle is preventing it. If there was nothing impeding fuel flow, then the pressure differential would be near 0. In cause\effect terms: cause: fuel flow is decreased, effect: pressure decreases (not the other way around). $\endgroup$
    – jt000
    Commented Mar 31, 2015 at 20:24
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Manifold pressure controls power production and the throttle controls manifold pressure. Pressure does not increase because more power is being made, rather, more power is made because opening the throttle (increasing pressure or reducing vacuum) allows the engine to pump and burn more fuel/air mixture.

A carburetor contains the throttle for controlling airflow into the engine. It also contains a venturi which meters the amount of fuel mixed into the air. The ratio of gasoline to air must remain approximately 14:1 in order to burn properly, so as airflow increases through the venturi vacuum in the throat also increases, pulling enough more fuel to keep the ratio correct. More fuel without more air or vice versa would cause the engine to quit.

enter image description here

Also shown is the choke plate, which is used to get extra fuel into a motor before it is up to operating temperature. Closing the choke adds manifold vacuum to venturi vacuum to pull more fuel into the intake. Fuel does not fully vaporize when the motor is cold and it will not burn if not vaporized, so the extra fuel makes enough vapor for the motor to run. As the motor comes up to operating temperature, the choke is opened so the normal amount of fuel is mixed with the air.

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I'm answering my own question since the existing answers are good, but do miss a key point in my question which ultimately was the source of my confusion.

I mistakenly thought that the fuel needle valve is on the manifold side of the butterfly. Therefore, I could not understand why more vacuum (less power) did not result in more fuel (more power).

The reason is that the valve is on the intake side of the butterfly and therefore, the more open the butterfly, the lower the pressure on the intake and therefore, the more fuel.

So, increasing throttle opens the butterfly which lets more air into the engine faster, which causes a drop in pressure (because of the venturi) in the intake which draws more fuel. Sorted!

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  • $\begingroup$ I think you may have the misconception that power is increased by increasing just the amount of fuel. Actually, power is increased by packing more fuel/air mixture into the cylinders. The fuel/air ratio shouldn't vary all that much. Just adding more fuel would make a richer mixture, which actually decreases power. $\endgroup$
    – Fred Larson
    Commented Apr 1, 2015 at 17:04
  • $\begingroup$ @FredLarson Thanks, but I do get that. More air, means more fuel means bigger charge. Just increasing fuel would lead to a rich cut or overheating. My only misunderstanding was on which side of the butterfly the fuel valve is. $\endgroup$
    – Simon
    Commented Apr 1, 2015 at 17:18
  • $\begingroup$ Ok, good. I'll leave my comment to help anyone else who may be confused. $\endgroup$
    – Fred Larson
    Commented Apr 1, 2015 at 17:32
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Talk about obfuscating the issue!

Throttle plate - a variable valve to restrict air flow.

Throttle Body - a duct with a converging diverging throat. ie, a venturi. the throttle plate controls the diameter of the venturi.

Manifold - a tube connecting throttle body to cylinder.

The same air flowing through the manifold flows through the throttle body, governed by the throttle plate. The converging shape of the venturi causes that air to speed up. the same volume of air passes through the narrow venturi, as flows through the large manifold.

As per bernoullis theorem, velocity, pressure and density are all interchangeable. the same density or mass of air flowing through a smaller duct has higher velocity, therefore lower pressure. The atmosphere then pushes fuel into the venturi/throttle body through the jet into this low area zone - The spray gun principle.

The manifold pressure shows the position of the throttle plate and how its governing air flow into engine, and therefore the power the engine will produce, the pressure differential being equal to the square of the airflow. this vacuum is completely separate to the venturi itself, which sees a pressure differential between its narrowest point and the surrounding larger duct (whether this is referenced to atmospheric pressure or not) simply due to air flowing through it having the higher velocity.

The highest manifold vacuum is actually when compression braking... especially on these fancy new efi systems with intake air control and fuel cut on decel... they can close the throttle nearly 100%... and the engine is spinning much faster than at idle...

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Well all pilots know that increasing throttle, or opening the throttle increases the manifold pressure so the above explanation is incomplete. As throttle opens the induction system is open to ambient air pressure so it rises. The volume of air is increasing and being forced into the manifold of fixed size so pressure is increasing and indicated as much on the MAP gauge. The MAP is meassured in the intake manifold not the venturi. When off the pressure throughout the induction is ambient. But then turned on the cylinders suck air out of the manifold, but the closed throttle is choking off the air and so the MAP drops. Then opening the throttle removes the choke and the MAP can rise up to ambient. Or more if turbo. As more air is sucked into the manifold of fixed cross section by opening the throttle its pressure goes up.

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The intake stroke creates a vacuum in the cylinder. This vacuum allows the fuel air mixture into the cylinder. That constant cycle keeps the manifold pressure lower than the atmospheric pressure while the engine is running. The amount of mixture available to the cylinder is controlled by the throttle valve. The more the valve is open the greater volume of mixture available to the cylinder. The amount the throttle is open determines the pressure in the manifold. That is why manifold pressure is the lowest at idle and the greatest when the engine shut down.

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I like to look at it like this: there is a correlation between air pressure and air velocity. When velocity is high, pressure is low. When velocity is low, pressure is high. So, when we open the throttle, we are letting more air flow into the engine. As the engine RPM increases, the velocity of the air flowing into the manifold gets higher. Internally, the pressure gets lower and this is how they are correlated. The pressure in the manifold drops as the velocity of the air increases. By knowing what the manifold pressure is related to the ambient pressure, we can tell the amount of air that is flowing into the engine. If we know the amount of air flowing into the engine, we can choose the correct fuel amount (or the computer does).

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There's two vacuums that matter. There's the vacuum beyond the throttle plate (manifold pressure) which pulls the air from the plenum, and there's the vacuum created by the venturi in the carb (venturi vacuum). You can have high manifold pressure and still have high venturi vacuum. The venturi is configured so that it is created by the FLOW thru the venturi, mostly independent of the pressure of the air around it. Differential pressure (vacuum) in the venturi is created by FLOW. Higher flow means higher venturi vacuum. Because there is very little restriction before the throttle plate, the plenum pressure doesn't vary too much.

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