# Manifold pressure question

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Simple question, why is less manifold pressure required for a given power setting as altitude is increased?

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The equation is volume of air in cylinder is proportional to actual air pressure minus manifold pressure. As you go higher you are opening the throttle wider but that is offset by the falling external pressure.

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For those who don't like to live on a knife edge.

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My understanding is that manifold pressure is absolute pressure - it is not "gauge pressure" which is relative to sea level.

So, for a given air density, for a given suck- the vaccum will vary- IE higher altitudes, lower air density, lower vaccum (Bernoulii).

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10 hours ago, Thruster88 said:

Simple question, why is less manifold pressure required for a given power setting as altitude is increased?

Colder air at standard temperature?

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1 hour ago, aro said:

Colder air at standard temperature?

That sounds plausible.

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A bit of semantics here.

We think of pressure as a positive force and see it when we introduce air into a tyre. We increase the pressure of the air inside the tyre and we see that pressure force the tyre to change shape. In this case we measure the difference between the force exerted by the atmosphere (1013 hPa, or 14.6 lbs/sq in, or 29.7 in Hg at seal level) and the pressure exerted by the introduced air on the inner wall of the tyre.

If we go the other way we can reduce the pressure within a container to below that of the outside air pressure. We call the difference between the outside atmospheric pressure and the inside of the container, a vacuum, and the way we reduce the pressure is to suck. The engine sucks air by increasing the volume of a cylinder by moving an "airtight" piston from the top to the bottom of a cylinder. This movement, when timed with an open intake valve draws air from the atmosphere, through the intake manifold to the interior of the cylinder. If you introduce the inlet of a pressure gauge into the intake manifold you can measure the pressure exerted by the moving air.

The drop in pressure is due to the venturi effect, although an intake manifold does not look like the venturi tube we are more familiar with. In fluid dynamics, an incompressible fluid's velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressure must decrease in accord with the principle of conservation of mechanical energy (Bernoulli's principle). Thus, any gain in kinetic energy a fluid may attain by its increased velocity through a constriction is balanced by a drop in pressure.

As an expert in creating poorly worded questions, I suggest that the question should be reframed, but I don't know what the correct question should be.

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After some more thought and research:

The maximum power that can be developed, in the cylinders of a particular piston engine, increases or decreases directly with the density of the air in the intake manifold, and air density decreases as altitude increases — or temperature increases.

Power produced is proportional to  the air density at the intake manifold, the cylinder displacement and compression ratio, the number of cylinders, and the rpm. Of those  items, only  the  air density at the intake manifold and the engine rpm alter, or can be altered, during flight. With a normally aspirated engine and a propeller whose pitch is not variable in flight, the throttle  controls manifold pressure, which then determines rpm.

Density is directly proportional to pressure. As pressure decreases, with temperature constant, density decreases. Air density will decrease by about 1% for a decrease of 10 hPa in pressure.

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In a normally aspirated engine , manifold pressure will always be below ambient unless the engine is stopped. When it's running, the throttle, by being able to restrict the flow can reduce the MP. When it's fully open the difference will only be whatever the venturi (or Dashpot) needs to draw fuel out of  a jet

Mass flow determines power (Air and fuel mixed) Temperature being lower helps here and MAYBE?? the less pressure opposing the exhaust flowing from the engine at altitude helps there. A fixed prop needs less power to drive it at the same revs  at higher altitudes. Nev

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Facthunter, I must disagree with this:

39 minutes ago, facthunter said:

the throttle, by being able to restrict the flow can reduce the MP.

When the throttle is "closed" the pipe through the carby and into the manifold is closed. Air cannot move. Therefore the pressure in the manifold will approach the value of the outside air pressure. As the throttle opens, air begins to move through the tube and since pressure is inversely proportional to fluid velocity, the pressure drops. Because the pressure inside the manifold is less than the pressure of the surrounding atmosphere, air pushed in from outside and moves faster past the fuel jet, taking more fuel into the cylinder thus producing more power.

You'll notice in the table presented that at some time we reach full throttle open, so the manifold pressure cannot get lower at that particular density altitude. Also, I think that Thruster might be misunderstanding the purpose of the table. To me it is a quick reference for RPM and manifold pressure setting required to obtain a desired power and fuel burn rate at various altitudes from a CSU equipped engine/propeller combination.

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Is that chart showing horsepower or percentage of horsepower.

I cannot understand that the engine is shown as 100hp at SL, 2400rpm and 19.3MP and also 100hp at 15000' same rpm but 16.3MP. That is lower MP and other things except air density being the same, including fuel flow.

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28 minutes ago, Yenn said:

Is that chart showing horsepower or percentage of horsepower.

I cannot understand that the engine is shown as 100hp at SL, 2400rpm and 19.3MP and also 100hp at 15000' same rpm but 16.3MP. That is lower MP and other things except air density being the same, including fuel flow.

Yes it shows hp or the percentage of the full 180hp. How the engine can produce the same hp with less manifold pressure at higher altitudes is the question I am asking,

OME the purpose of the chart is to set power in my RV6-A.

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2 hours ago, Thruster88 said:

the purpose of the chart is to set power in my RV6-A.

I thought so. It's a reference chart that you can pull out of your bag while flying and use it to bring the needles on the dials to the figures that will give you the power you want.

Now to answer the question. Why is less manifold pressure required for a given power setting as altitude is increased?

Here is a chart made from one column of data: 100 HP at 2300 RPM

At sea level, the air pressure in the International Standard Atmosphere  is 29.9 inches of mercury (Hg). We know that the moving pistons reduce the pressure in the cylinders, producing a lower air pressure in the fuel intake system. If the engine is running at a steady 2300 RPM at sea level the manufacturer tells us that the manifold pressure will be 19.8 ins Hg. The difference between the pressure inside the manifold and the free air outside it is 10.1 ins Hg.

Now let's get up to circuit height in the same atmosphere. The air is less dense, so the engine at 2300 RPM can only reduce the air pressure to 19.6 ins Hg. However, at the same time the free air pressure has dropped to 28.85 ins Hg. The difference between inside the manifold and outside is 9.25 in Hg. The difference between inside and out at 1000 ft compared what that difference is at sea level is 0.85 ins hg.

As we gain altitude, both Manifold Pressure at 2300 RPM and the outside air pressure drop. HOWEVER, if you look at what happens as the aircraft goes higher, you can see that for each 1000 ft rise in altitude, the difference between Manifold Pressure and Free Air pressure from one height to the next remains at a constant 0.8 ins Hg. (Allowing for rounding errors in the published data and in the values calculated from it.) The difference is made constant by opening the throttle body more and more as the plane goes higher, until the throttle plate is in its full open position. That is one of the determinants of the service ceiling of a fixed pitch prop aircraft. The constant speed propeller lets the pilot "change gear"

Why is less manifold pressure required for a given power setting as altitude is increased?  The question needs to be: "Why does manifold pressure reduce with increase in altitude for a given engine RPM/power output?"

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OME  the engine is an air pump which when it's turning, sucks air into the inlet ports. The greatest suction (lowest MP) is when a good condition motor is IDLING. It can be an indicator of an engines health. When the motor is stopped the MP equalises with ambient fairly quickly. Any time it's operating the MP is below ambient due pressure drop through the carb and throttle position. The throttle restricts flow (lowers MP) to control engine output. Any Carnot cycle motor, the HP (output) is proportional to the MASS airflow or to make it easier Fuel flow if the ratio is right. Engines  (pistons) drop off power as they climb. and you have to use  a wider throttle position till you eventually are at WOT after which the power drops off and you can't do anything about it. without a blower. Nev

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6 hours ago, old man emu said:

When the throttle is "closed" the pipe through the carby and into the manifold is closed. Air cannot move. Therefore the pressure in the manifold will approach the value of the outside air pressure. As the throttle opens, air begins to move through the tube and since pressure is inversely proportional to fluid velocity, the pressure drops.

Manifold pressure is measuring the pressure in the inlet manifold, after the carb/venturi, throttle etc. it is measured relative to zero i.e. a vacuum. It is lower than ambient when the engine is running because the pistons are drawing air out of the inlet manifold.

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We are talking about an engine that is maintaining RPM despite moving into lower pressure air (gaining altitude). Does that imply that there is a CSU involved and that it is adjusting the pitch to maintain RPM?

Tell me. If a prop has a certain pitch to RPM configuration for cruise at sea level, does the pitch get coarser or finer for the same RPM when cruise is at higher altitudes?

Once again we are being bamboozled by words - the Humpty Dumpty Effect.

When we use the word "pressure" we imply an application of Force. We increase the air pressure in a tyre and it forces the tyre casing against the ground. The Boss piles our desk with work and we feel pressured.  When it comes to the present topic, air pressure relates to the mass of air in contact with an area of material. Since this mass of air is acted upon by Gravity, it has a force proportional to the to the area - hence we use pounds per square inch, or Pascals, which is the pressure exerted by a force of magnitude one newton perpendicularly upon an area of one square metre. That's a small value, so we use Hectopascals (hPa) which is 100 Pascals.

In an engine we are trying to measure how the mass of air that the sucking pistons are removing from the intake manifold. Vacuum means any volume containing less gas particles, atoms and molecules (a lower particle density and gas pressure), than there are in the surrounding outside atmosphere. So what we are really trying to measure is a vacuum.

The term "Manifold Pressure" gives a mental picture of a strong Force. This picture is based on our experience of tyres, toy balloons and pressurised spray cans. In order to understand what we are actually measuring we should refer to that gauge as a Manifold Vacuum Gauge. Maybe it should also be graduated from a starting point of Zero and ending somewhere about 35 ins Hg. Picture is a Boost Gauge used to measure increased pressure due to the function of a turbo or supercharger. The principles that make it work can be reversed to allow it to read manifold vacuum. Engine manufacturers could then produce performance tables of Manifold Vacuum/RPM combinations for various power outputs.

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1 hour ago, old man emu said:

In an engine we are trying to measure how the mass of air that the sucking pistons are removing from the intake manifold.

No.

The manifold pressure gauge measures approximately how much air is available in the manifold for the engine. It is a measurement relative to zero i.e. a vacuum.

It is an indication of how much air will be drawn into the cylinder in an intake stroke, but it doesn't measure how much is being used.

If the engine is not running:

- at sea level, the gauge measures about 30 inHg

- parked at an airport at 5000', it will read about 25 inHg.

- launch your Cessna into space and the manifold pressure gauge will read zero.

With the engine running, the gauge will read somewhere in between. The pistons are sucking the air out of the manifold, and the throttle limits how fast it can be replaced. The manifold pressure gauge can't tell the difference between low manifold pressure due to flow into the manifold being restricted by the throttle, and low pressure at wide open throttle because you are at altitude. Nor can the engine.

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There's already correct terminology out there but it sometimes gets misused, You pull as close as possible to a vacuum before your recharge an air conditioner. You won't get a vacuum in the inlet manifold, unless you are in space.  Manifold pressure is just what it is. Pressure existing in some engine plumbing.  Without a pump, blower or supercharger you rely on engine "suction" to get fuel and air in there and in those circumstances The MP will always be lower than the ambient pressure at the time.

BOOST is supplementary forcing of the air into the motor. beyond what the atmosphere provides where you are.. .  Normally the throttle is upstream of the supercharger. Occasionally with low boost used It's before the carb., and the carb gets pressurised  When the throttle is not fully open you are wasting the effect of the supercharger  to some extent. Nev

Edited by facthunter
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3 hours ago, old man emu said:

Vacuum means any volume containing less gas particles, atoms and molecules (a lower particle density and gas pressure), than there are in the surrounding outside atmosphere.

32 minutes ago, facthunter said:

You won't get a vacuum in the inlet manifold, unless you are in space

You'll get close to a "perfect" vacuum in Space, but we won't get a perfect vacuum in a manifold. We know from observation and experience that we will have a bit of a vacuum because the pistons are increasing the volume of the cylinder on the induction stroke. We know that "vacuum" is a relative term. It is zero when the number of gas particles at the measuring point is the same as the number is the outside atmosphere. As the number of gas particles deceases at the measuring point, the difference between outside and inside is called "vacuum", and is usually reported (shown on a recording device) as a smaller value than the outside air pressure  -a negative sense. (Without getting into a discussion of "Boost", it is essentially the measure of variation above the outside air pressure - a positive sense).

How this measurement of the mass of air being drawn into the cylinder is important in engine performance is dependent on the stoichiometric ratio of air and fuel. Note that here we use the term "air" to mean all the various gases whether combustible or not. The main combustible gas under normal temperatures is Oxygen, which constitutes only 21% on average. The other 80-odd% of gases in the air mass don't help produce power.

The theoretical perfect stoichiometric ratio between the mass of air in a cylinder and the mass of fuel  for petrol is 14.7 units of air to 1 unit of fuel. Any mixture greater than 14.7:1 is considered a lean mixture; any less than 14.7:1 is a rich mixture. In practice the ratio is 14.1:1.

Note that the stoichiometric ratio is calculated from the mass of air. When air is sucked into the manifold, it attains a velocity. Mass x velocity = Force. Force/Area = Pressure. Therefore, Pressure is related to air mass, which is related to the stoichiometric ratio of the fuel/air mixture, which results in the release of energy, and energy x time = power.

I still don't know Why is less manifold pressure required for a given power setting as altitude is increased?  Is the question worded correctly?  Should it be this? "Why does manifold pressure reduce with increase in altitude for a given engine RPM/power output?"

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Your definition is WRONG. A vacuum is  Essentially a place devoid of matter. Hard to achieve but there's no little vacuums and bigger ones. Higher and lower pressures exist and are understood.. Pressures are obtained by molecules hitting a surface and bouncing off it. You can have more molecules or faster molecules to get pressure. More dense or hotter, I've answered your question by the concept of mass airflow or fuel flow. Power is proportional to fuel burned properly. I think the power charts are simplistic and only an approximation and I wouldn't take them too literally. MP going higher for any reason will increase the torque you can get, other things remaining equal. The LOAD (propeller) is reduced in thinner air. in a flying situation.( Density altitude.) like wing lift.

Universal Gas equation P1 x V1 /T1  is a constant, in appropriate units (Temp is in Kelvin) Nev

Edited by facthunter
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50 minutes ago, old man emu said:

You'll get close to a "perfect" vacuum in Space, but we won't get a perfect vacuum in a manifold. We know from observation and experience that we will have a bit of a vacuum because the pistons are increasing the volume of the cylinder on the induction stroke. We know that "vacuum" is a relative term. It is zero when the number of gas particles at the measuring point is the same as the number is the outside atmosphere. As the number of gas particles deceases at the measuring point, the difference between outside and inside is called "vacuum", and is usually reported (shown on a recording device) as a smaller value than the outside air pressure  -a negative sense. (Without getting into a discussion of "Boost", it is essentially the measure of variation above the outside air pressure - a positive sense).

How this measurement of the mass of air being drawn into the cylinder is important in engine performance is dependent on the stoichiometric ratio of air and fuel. Note that here we use the term "air" to mean all the various gases whether combustible or not. The main combustible gas under normal temperatures is Oxygen, which constitutes only 21% on average. The other 80-odd% of gases in the air mass don't help produce power.

The theoretical perfect stoichiometric ratio between the mass of air in a cylinder and the mass of fuel  for petrol is 14.7 units of air to 1 unit of fuel. Any mixture greater than 14.7:1 is considered a lean mixture; any less than 14.7:1 is a rich mixture. In practice the ratio is 14.1:1.

Note that the stoichiometric ratio is calculated from the mass of air. When air is sucked into the manifold, it attains a velocity. Mass x velocity = Force. Force/Area = Pressure. Therefore, Pressure is related to air mass, which is related to the stoichiometric ratio of the fuel/air mixture, which results in the release of energy, and energy x time = power.

I still don't know Why is less manifold pressure required for a given power setting as altitude is increased?  Is the question worded correctly?  Should it be this? "Why does manifold pressure reduce with increase in altitude for a given engine RPM/power output?"

Just to be clear we are talking constant speed propeller, and a fixed percentage of power at increasing altitude. The propeller will get coarser with increasing altitude.

To remove the mystery of manifold pressure let's look at a turbocharged seneca 2 engine, it can be operated with more than atmospheric pressure in the manifold yet it still requires less manifold pressure at 10,000 feet than it does at sea level to achieve 75% power(165hp). No need to talk vacuum with the turbo or the naturally aspirated.

So the question remains, why is less manifold pressure required at higher altitudes to get a fixed amount of power?

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Is the question "Why is less manifold pressure required? is really not a question at all. There is less pressure available.

I still fail to see how an engine can run the same power at the same rpm and also fuel flow at greatly varying pressures.

Can someone really explain it?

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Lots of correct stuff in this discussion, but  OME introduces velocity when it is not required, and force= mass times acceleration not mass times velocity. Yes the venturi effect is needed to explain the inner workings of the carburetor, but not this question which as has been said is not clearly written.

We need to distinguish between Absolute pressure and Gauge pressure.

Gauge pressure is the difference between the internal and external pressures as applied to the gauge, and it is the only one we actually see in practice.

Only if the external pressure is removed can a gauge read absolute pressure. There may be gauges with a built-in vacuum chamber and these would read absolute pressure. But our common gauges do not do this.

Think of your car tyres. You pump them up to 30 psi gauge but the atmospheric pressure at the service station is 15 psi.

Then you send the car to the moon where there is no atmosphere. The gauge will now show 45psi which has always been the absolute pressure in the tyre.

In my Jabiru, at cruise at 2000 ft, the "vacuum " gauge shows -20kPa, while the atmospheric pressure is 100kPa. So the absolute pressure in the inlet manifold is 80kPa. If the "vacuum " increased to -30kPa then the absolute pressure would reduce to 70kPa.

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37 minutes ago, Yenn said:

Is the question "Why is less manifold pressure required? is really not a question at all. There is less pressure available.

I still fail to see how an engine can run the same power at the same rpm and also fuel flow at greatly varying pressures.

Can someone really explain it?

In the case of the turbo seneca the pilot could increase the manifold pressure by opening the throttles at 10,000 feet from 32.4 inhg to the sea level setting of 35.5 inhg but it is not required,  why?

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We should really be talking about Manifold-air Density.

2 hours ago, facthunter said:

MP going higher for any reason will increase the torque you can get, other things remaining equal.

I agree with that. Increase MP > increase fuel/air mass > increase in pressure on the piston head from from combustion > greater torque on the crankshaft. Greater torque applied at a given rate (RPM) gives greater power.

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