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Old 11-30-2014, 11:56 PM   #1
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Default 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


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Old 12-21-2014, 04:46 PM   #2
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Anyone? Is this line of reasoning correct? I see a ton of speculation on the miata.net forums but nothing with hard evidence that this is normal.
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Old 12-21-2014, 06:33 PM   #3
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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.

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Old 12-22-2014, 02:46 AM   #4
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What's your MAP reading with the ignition on/engine off, and what's the reading with the engine idling after warmup with no loads?
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Old 12-23-2014, 02:11 AM   #5
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81-82kpa is normal for that altitude. Your vac is probably fine too, but I don't pay attention enough to that to know off hand. Compression tests will be low compared to sea level also.
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Old 12-23-2014, 03:18 AM   #6
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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

Quote:
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 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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Old 12-23-2014, 10:19 AM   #7
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Yep.
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