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