How does altitude change your baseline?
#1
How does altitude change your baseline?
Ok, so I was confused with something. I recall seeing someone saying that a healthy Miata engine should be pulling (at least) 20 inches of vacuum at idle but cannot find the thread now. I started to dig into this because my car used to get decent gas mileage, and has been getting worse. (I am realizing now that it is because I did not account for my altitude and I am tuning it worse because I did not factor in air pressure)
Mine sits at 16.5, so I was worried. However, I am also at 5500 ft above sea level
Sea level with ISA is 101.3 kPa, and barometric readings show it is about 81-82 kPa depending on temperature here.
Because I am taking aviation classes atm, I know ISA (international standard air pressure) at 15C is 29.92 or 1013.1 hPa at sea level, and for every 1000 ft, roughly you loose 1 in of pressure.
So my 16.5 at 5500ft would be about 22 at sea level. (16.5 - the 5.5 for the 5500 ft altitude difference) Google finds several threads where people discuss the theory but all the threads always end with a cliffhanger as there is zero followup.
So it was only after looking at some of this other stuff, I realized that I was looking at my engine and thinking there was a problem, where there is not.
It is inches of pressure in relation to the outside pressure, which in my case starts off low, and I have been trying to figure out why i was not pulling the full 20 inches and thinking my engine was blown.
Now my question, is my thought process with this incorrect, and if it is wrong, what is correct?
You can see in this chart, for my altitude the pressure ratio x100 is also right around what would be that 82 kPa area as well
Mine sits at 16.5, so I was worried. However, I am also at 5500 ft above sea level
Sea level with ISA is 101.3 kPa, and barometric readings show it is about 81-82 kPa depending on temperature here.
Because I am taking aviation classes atm, I know ISA (international standard air pressure) at 15C is 29.92 or 1013.1 hPa at sea level, and for every 1000 ft, roughly you loose 1 in of pressure.
So my 16.5 at 5500ft would be about 22 at sea level. (16.5 - the 5.5 for the 5500 ft altitude difference) Google finds several threads where people discuss the theory but all the threads always end with a cliffhanger as there is zero followup.
So it was only after looking at some of this other stuff, I realized that I was looking at my engine and thinking there was a problem, where there is not.
It is inches of pressure in relation to the outside pressure, which in my case starts off low, and I have been trying to figure out why i was not pulling the full 20 inches and thinking my engine was blown.
Now my question, is my thought process with this incorrect, and if it is wrong, what is correct?
You can see in this chart, for my altitude the pressure ratio x100 is also right around what would be that 82 kPa area as well
#3
I would think that the vacuum on an engine is a fixed relationship of combustion volume irregardless of atmospheric pressure.
My thought process:
At idle, the throttle plate would be closed, and air would be metered by the IAC. To maintain a steady idle, the motor would need a fixed amount of Oxygen and fuel for a fixed speed. I think the IAC would just let more volume in at elevation to have requisite Oxygen. The Volume of the combustion would be identical, and Idle speed is standard, so the Vacuum should be identical at Sea Level and High Elevation.
Maybe? I'm sure Sav, Emilio, Vlad, et al. will correct me if there's a hole in my reasoning.
My thought process:
At idle, the throttle plate would be closed, and air would be metered by the IAC. To maintain a steady idle, the motor would need a fixed amount of Oxygen and fuel for a fixed speed. I think the IAC would just let more volume in at elevation to have requisite Oxygen. The Volume of the combustion would be identical, and Idle speed is standard, so the Vacuum should be identical at Sea Level and High Elevation.
Maybe? I'm sure Sav, Emilio, Vlad, et al. will correct me if there's a hole in my reasoning.
Last edited by Chooofoojoo; 12-21-2014 at 05:42 PM. Reason: grammar
#6
On startup will vary between 82 and 83 depending on temp and weather patterns
Local airport is 5622 ft
When warmed up it will idle at 25-26 kPa no load
My view is that the 25-26 kPa I am pulling when shown as vacuum shows about the -16.5
That 16.5 is vacuum in reference to the external air pressure.. so if I were artificially change that outside pressure refrence source to sea level pressure, I would have the 20 inches of vacuum..
This is where I start confusing myself. Engine starts with less base air pressure, so all vac and boost pressures will change with that. So say I was boosting to 14 psi, that is about one standard atmosphere ( at sea level) , so I start with
82 kPa base + 100 kPa boost = 182 kpa or my 14 psi of boost at 5500ft
Someone at sea level boosts to 14 psi:
101 +100 = 201 kPa
so about 19 kPa difference
My gauge would need to be showing about 16-17 psi to match the same kPa atmosphere entering the engine.. because even "boost" is in reference to outside air pressure
Local airport is 5622 ft
When warmed up it will idle at 25-26 kPa no load
I would think that the vacuum on an engine is a fixed relationship of combustion volume irregardless of atmospheric pressure.
My thought process:
At idle, the throttle plate would be closed, and air would be metered by the IAC. To maintain a steady idle, the motor would need a fixed amount of Oxygen and fuel for a fixed speed. I think the IAC would just let more volume in at elevation to have requisite Oxygen. The Volume of the combustion would be identical, and Idle speed is standard, so the Vacuum should be identical at Sea Level and High Elevation.
Maybe? I'm sure Sav, Emilio, Vlad, et al. will correct me if there's a hole in my reasoning.
My thought process:
At idle, the throttle plate would be closed, and air would be metered by the IAC. To maintain a steady idle, the motor would need a fixed amount of Oxygen and fuel for a fixed speed. I think the IAC would just let more volume in at elevation to have requisite Oxygen. The Volume of the combustion would be identical, and Idle speed is standard, so the Vacuum should be identical at Sea Level and High Elevation.
Maybe? I'm sure Sav, Emilio, Vlad, et al. will correct me if there's a hole in my reasoning.
My view is that the 25-26 kPa I am pulling when shown as vacuum shows about the -16.5
That 16.5 is vacuum in reference to the external air pressure.. so if I were artificially change that outside pressure refrence source to sea level pressure, I would have the 20 inches of vacuum..
This is where I start confusing myself. Engine starts with less base air pressure, so all vac and boost pressures will change with that. So say I was boosting to 14 psi, that is about one standard atmosphere ( at sea level) , so I start with
82 kPa base + 100 kPa boost = 182 kpa or my 14 psi of boost at 5500ft
Someone at sea level boosts to 14 psi:
101 +100 = 201 kPa
so about 19 kPa difference
My gauge would need to be showing about 16-17 psi to match the same kPa atmosphere entering the engine.. because even "boost" is in reference to outside air pressure
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