Manifold pressure(aircraft) vs engine vacuum(automotive)

cowman

cowman

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This popped into my head while I was driving home and I can't seem to resolve it. I suspect it's a matter of just not understanding what scale/measurement is being used for manifold pressure but... here we go.

For these examples I'm only talking about naturally aspirated engines.

I work on cars quite a bit and we often measure what car mechanics call engine vacuum with a gauge. It's a measurement of the (negative) air pressure in the car's intake manifold. In cars the vacuum(again a negative number) is highest at idle and decreases(increases in pressure) as the throttle opens because more air is allowed to rush in. This is measured in inches of mercury. Pretty simple to understand.

So my airplane has a manifold pressure gauge which at some point I just translated in my head as basically measuring the same thing(pressure in the intake manifold) and never really thought about it beyond that. The gauge on my plane is also measuring inches of mercury. But.... as I increase the throttle the number on the gauge goes UP and aside from operationally using it to set power I have no idea what that means. It doesn't say vacuum and there's no - indicating negative... and if it was absolute value the number should still go lower as the throttle is opened right? Surely an NA airplane engine doesn't have positive pressure in the intake manifold does it? What does this number actually mean. This is gonna bug me until I understand it now.
 
The vacuum gauge measures the difference between ambient and manifold pressure.

The MP gauge just shows the direct reading of pressure. If you subtracted the ambient air pressure, it would read the same as your vacuum gauge.

They dont do it that way because the ambient pressure decreases as you climb. The difference would be the same (very small at high throttle) even though you're still losing power.
 
Jim K said:
The vacuum gauge measures the difference between ambient and manifold pressure.

The MP gauge just shows the direct reading of pressure. If you subtracted the ambient air pressure, it would read the same as your vacuum gauge.

They dont do it that way because the ambient pressure decreases as you climb. The difference would be the same (very small at high throttle) even though you're still losing power.
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That makes sense, I knew it had to be something along those lines but I never heard it spelled out or thought to consider it before. For the most part it's just a number we reference to set power
 
MAP is the better term. Manifold Absolute Pressure. When the engine is not running you will see the current barometric pressure on the gauge.
 
I had to dig through my photos for a pic showing the MP with the engine off. Sure enough showing about 29” (upper left, partly in view). I am now aware I never referenced MP before starting.
IMG_9677.jpeg
 
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Jim K said:
The vacuum gauge measures the difference between ambient and manifold pressure.
Click to expand...

They both do that. One reads in gauge pressure while the other reads in absolute pressure.
 
Dan Thomas said:
MAP is the better term. Manifold Absolute Pressure. When the engine is not running you will see the current barometric pressure on the gauge.
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You probably know this, but for anyone reading. MP with the engine off will be the uncorrected pressure, so at elevation it’s going to be off from the reported atlimeter by about 1mp per 1k above sea level..

All
 
Also, in the car world, boost is in PSI. So they mix and match, inches of mercury below ambient and PSI above.

Aircraft MP are in inches all the time. But any over the ambient (about 30 inches at sea level) is boost pressure.

Both my car and my plane are turbocharged and intercooled. The aircraft is a max MP of 39 inches. Car is max 12 PSI boost.

So airplane is about 4.4 PSI boost and the car is about 54 inches of MP.
 
Simply put, they're basically the same gauge, just marked differently for historical, human factors, and practicality reasons.
 
Pinecone said:
Also, in the car world, boost is in PSI. So they mix and match, inches of mercury below ambient and PSI above.
Click to expand...

Those are the common units of measure for the boost gauges people view, but not universal (my current car's stock boost gauge isn't). And every factory ECU I've worked on have the pressures in absolute and the unit of measure in kPa or bar. Some aftermarket units may be in other units of measure but it seems like the general default is kPa as well.
 
RussR said:
Simply put, they're basically the same gauge, just marked differently for historical, human factors, and practicality reasons.
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Not quite. A vacuum gauge rests at zero when there is no vacuum, and when there is vacuum, the reading is giving the differential between the ambient air pressure and the vacuum. The vacuum gauge's case is vented to allow ambient pressure to work on the outside of the Bourdon tube or diaphragm capsule within the gauge.

The manifold pressure gauge reads in absolute pressure, and it does so by having the pressure measured against spring force in the gauge, not against ambient air pressure.

Which is why the MP gauge reads ambient air pressure when the engine isn't running, or even when the gauge is sitting on the bench, not connected to anything. The spring has driven the needle up to read ambient. A vacuum gauge says nothing until vacuum is applied to it.

Vacuum gauges have more in common with air pressure gauges found in a shop. PSIG means "pounds per square inch, gauge" to tell us that is it reading air pressure that is above ambient pressure. It will read zero just sitting in ambient pressure.
 
There are two basic types of pressure readings: Gauge Pressure and Absolute Pressure. Gauge Pressure is balanced against the Atmosphere and the Atmospheric Pressure exerted by the air around us, whereas Absolute Pressure is balanced against a complete vacuum. At sea-level an Absolute Pressure gauge open to the atmosphere and calibrated in Inches of Mercury will read about 30 inches depending on the actual barometric pressure, and will vary directly as the Atmospheric Pressure changes. A gauge pressure gauge will read zero regardless of the change in Atmospheric Pressure.

Manifold pressure gauges are Absolute pressure gauges and measure the Absolute pressure inside the intake manifold in
Inches of Mercury. Let's say before you start the engine the manifold pressure is 30, and after you start and are running at idle the manifold pressure is 10. A vacuum gauge will read 20 inches of vacuum. As you climb at WOT the vacuum gauge will read about 1.5-3 inches, depending on how restrictive your induction system is. That number will be the same regardless of altitude, but a manifold pressure gauge will vary with altitude showing you how much air pressure is in the manifold at all times.

Hope this helps.
 
cowman said:
This popped into my head while I was driving home and I can't seem to resolve it. I suspect it's a matter of just not understanding what scale/measurement is being used for manifold pressure but... here we go.

For these examples I'm only talking about naturally aspirated engines.

I work on cars quite a bit and we often measure what car mechanics call engine vacuum with a gauge. It's a measurement of the (negative) air pressure in the car's intake manifold. In cars the vacuum(again a negative number) is highest at idle and decreases(increases in pressure) as the throttle opens because more air is allowed to rush in. This is measured in inches of mercury. Pretty simple to understand.

So my airplane has a manifold pressure gauge which at some point I just translated in my head as basically measuring the same thing(pressure in the intake manifold) and never really thought about it beyond that. The gauge on my plane is also measuring inches of mercury. But.... as I increase the throttle the number on the gauge goes UP and aside from operationally using it to set power I have no idea what that means. It doesn't say vacuum and there's no - indicating negative... and if it was absolute value the number should still go lower as the throttle is opened right? Surely an NA airplane engine doesn't have positive pressure in the intake manifold does it? What does this number actually mean. This is gonna bug me until I understand it now.
Click to expand...
Aircraft manifold gauge reads ABSOLUTE pressure. Open throttle allows HIGHER absolute pressure in the manifold. Automotive manifold gauges are reading amount of VACUUM in the manifold. Open throttle creates LESS VACUUM.. for whatever reason the auto industry chose to measure vacuum so more vacuum shows a higher reading on the gauge
 
mondtster said:
Those are the common units of measure for the boost gauges people view, but not universal (my current car's stock boost gauge isn't). And every factory ECU I've worked on have the pressures in absolute and the unit of measure in kPa or bar. Some aftermarket units may be in other units of measure but it seems like the general default is kPa as well.
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Do you work on mainly foreign cars? Or domestic?
 
I think the dc-6's with water injection could do about 65".
 
Cars use MAP sensors for their ECU manifold pressure inputs. They don't use vacuum sensors. They want Manifold Absolute Pressure.
 
Two things, that may be different, is what they measure and use in the engine management computer, and what they display. :D

But reality is, you could take an aircraft MP gauge and remark the face to show inches of vacuum. Or in kilograms per square kilometer. :D
 
cowman said:
This popped into my head while I was driving home and I can't seem to resolve it. I suspect it's a matter of just not understanding what scale/measurement is being used for manifold pressure but... here we go.

For these examples I'm only talking about naturally aspirated engines.

I work on cars quite a bit and we often measure what car mechanics call engine vacuum with a gauge. It's a measurement of the (negative) air pressure in the car's intake manifold. In cars the vacuum(again a negative number) is highest at idle and decreases(increases in pressure) as the throttle opens because more air is allowed to rush in. This is measured in inches of mercury. Pretty simple to understand.

So my airplane has a manifold pressure gauge which at some point I just translated in my head as basically measuring the same thing(pressure in the intake manifold) and never really thought about it beyond that. The gauge on my plane is also measuring inches of mercury. But.... as I increase the throttle the number on the gauge goes UP and aside from operationally using it to set power I have no idea what that means. It doesn't say vacuum and there's no - indicating negative... and if it was absolute value the number should still go lower as the throttle is opened right? Surely an NA airplane engine doesn't have positive pressure in the intake manifold does it? What does this number actually mean. This is gonna bug me until I understand it now.
Click to expand...
I had a little trouble getting my head wrapped around the word pressure at first also. The cylinders are sucking air out of the manifold. But it’s really quite simple. Sucking lowers the pressure.
 
A lot of pilots never really learn what a MAP gauge tells them.

Most PPL training aircraft are fixed pitch and have no MAP gauge. With no hands on application, many students learn just enough to pass the written, then flush it.

Thus, many pilots first encounter MAP when they transition to a CS prop, often as part of a complex or high performance endorsement. That POI is at the discretion of the CFI.

Based on my own experience, the MAP gauge is addressed mainly in the context of operating the prop, ie keeping MP lower than RPM. (yes, we all know, no lecture needed)

I had been a pilot for over 20 years when I noticed that MAP equaled ambient pressure when the engine was off, and decided to understand the gauge a bit more. Installing my engine helped a lot, because now I could visualize where the sensor was.
 
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luvflyin said:
I had a little trouble getting my head wrapped around the word pressure at first also. The cylinders are sucking air out of the manifold. But it’s really quite simple. Sucking lowers the pressure.
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But it is still pressure. That pressure is just lower than ambient.

Unfortunately, many people don’t understand that and the commonly used term vacuum confuses most people. We’d all be better off if everything was measured in absolute pressure so the misunderstandings would be eliminated.
 
luvflyin said:
The cylinders are sucking air out of the manifold. But it’s really quite simple. Sucking lowers the pressure.
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That is half the equation. The other is how much the throttle plate restricts air flowing into the manifold to meet the demand of cylinder suction. At WOT, less restriction = higher pressure. At idle, more restriction = lower pressure.

MAP tells you how much air you are allowing the engine to receive. The closer you are to ambient pressure, the less you are restricting flow.
 
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Pinecone said:
Warbirds do that an more. :D
P-51 max was 70 inches with 115/145 fuel.
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I've seen the max quoted anywhere from 61 to 81 inches depending on power level: continuous, military, war emergency, etc.
I'll add that below 40 inches the engine sounds like you're shaking a box of ball bearings. That engine wants to be under boost. That's just my non-expert opinion having all of 0.7 hours in one. Take it FWIW
 
Pinecone said:
But reality is, you could take an aircraft MP gauge and remark the face to show inches of vacuum
Click to expand...
There is a difference. The vacuum is measured against ambient pressure, while MAP is absolute.
 
MRC01 said:
I've seen the max quoted anywhere from 61 to 81 inches depending on power level: continuous, military, war emergency, etc.
I'll add that below 40 inches the engine sounds like you're shaking a box of ball bearings. That engine wants to be under boost. That's just my non-expert opinion having all of 0.7 hours in one. Take it FWIW
Click to expand...

It also depends on the P-51. Later models were generally approved for more MP. Same with nearly every Allied fighter. Though... generally, there were cautions in the manuals to use lower MP when not using the best fuel available. Allied fuel ranged from 120 to 150 octane. And I think the Soviets were lower than that.
 
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