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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. Apr 2 '15 at 2:50
  • $\begingroup$ 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 Jun 10 at 18:22
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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 Mar 31 '15 at 20:33
  • $\begingroup$ Thanks, it is a good link. But I still don't get (from this link or any of the answers) why reduced pressure leads to less fuel flow. Surely it's the vacuum that draws fuel from the carb? $\endgroup$ – Simon Mar 31 '15 at 20:46
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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. Mar 31 '15 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. Mar 31 '15 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 Mar 31 '15 at 21:05
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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 Mar 31 '15 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 Mar 31 '15 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 Mar 31 '15 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 Mar 31 '15 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 Mar 31 '15 at 20:24
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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 Apr 1 '15 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 Apr 1 '15 at 17:18
  • $\begingroup$ Ok, good. I'll leave my comment to help anyone else who may be confused. $\endgroup$ – Fred Larson Apr 1 '15 at 17:32
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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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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.

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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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