Let’s start with the basics. Air pressure or more specifically atmospheric air pressure. As we know, earth’s atmosphere is filled with air. We are born and we die in this atmosphere, it’s our natural environment, and this is why we don’t really notice or feel the weight or the mass of air. But air most definitely has a weight and a mass. All that air, the entire atmosphere, weighs about 5 million billion tons! So how come we don’t get crushed by it? We don’t get crushed because all that weight is distributed evenly over the entire surface of the Earth. The average pressure you feel on your body is about 14.7 psi or 1 bar. But because you can’t escape this pressure, and unless you’ve been to outer space, you never actually spent even a moment of your time without this pressure, you don’t really feel it as you can’t reference how it would feel without this pressure. So what do you think, what’s the place where you can feel the least amount of atmospheric pressure without leaving the earth? That’s the top of Mount Everest of course, the highest point on earth. The highest point has the lowest pressure because it has the least amount of atmosphere above it, the least amount of air weight above it. The highest atmospheric pressure you can experience on earth if at sea level and the lowest is at the peak of mount Everest. But you can also experience very low atmospheric pressure if you contract a virus like Covid 19 and they put you into an isolation room. Many isolation rooms are actually negative pressure rooms. Inside a negative pressure room, the air pressure is artificially maintained at a pressure lower than outside the room, a pressure lower than atmospheric pressure. This is usually done with exhaust system that suck out the air of the room. Because the pressure inside the negative air pressure room is lower than outside it, the contaminated air doesn’t come out of the room when you open the door. Instead, clean air from the outside comes into it because air, like all fluids, always flows from a higher pressure area towards a lower pressure area. Do you know what else works on the same principle as an isolation room inside a hospital? The cylinder inside your engine! When the piston moves down inside your cylinder it creates a vacuum, or an absence of pressure. The cylinder moves down the bore at extremely high speeds and as it moves down it rapidly creates this void, or empty space, that for an extremely brief moment, has no air it, and as such is at a lower pressure than atmospheric pressure. But a turbo or a supercharger is capable of generating significant additional air pressure, and the air pressure inside the intake manifold of a forced induction engine can be double or triple that of atmospheric air pressure. When you’re looking at a boost gauge mounted inside a vehicle you’re looking ONLY at the boost pressure generated by the turbo or supercharger. This means that a boost gauge isn’t showing the actual pressure inside the intake manifold. It’s showing the pressure inside the intake manifold – minus the atmospheric pressure. When you expose a boost gauge to atmospheric pressure it’s going to show a value of zero. This is because a boost gauge and it’s sensor are referenced to atmospheric pressure. The reason behind this is that you’re only interested in what additional pressure your turbo or supercharger is generating, the pressure it ADDS on top of the atmospheric pressure, because that’s what boost is, you’re boosting your engine’s power by adding more pressure than could be generated by natural aspiration i.e. the pressure of the atmosphere. But things are different from the perspective of your engine’s ECU. The ECU is interested in ALL of the pressure, both from the atmosphere and the pressure added by forced induction. It’s needs to know all the pressure because it’s trying to match all of the air mass with the correct amount of fuel. So the MAP sensor that reads pressure inside the intake manifold and feeds data to the ECU is referenced to absolute zero pressure. If you were to expose this sensor to the atmosphere it would read around 14.7 psi at sea level. So boost pressure equals manifold absolute pressure – atmospheric pressure. and manifold absolute pressure is: boost pressure + atmospheric pressure
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