Quote:
Originally Posted by Muffo
when the pressure is 0 this is at idle.
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Sorry to call you out, but manifold pressure it at a high vacuum at idle (about 21-22 inHG). When pressure is 0, the throttle plate is wide open, allowing atmospheric pressure into the manifold.
Of course, this is all relative. It's not actually "0" psi at W.O.T., because atmospheric pressure is not actually 0. (A.P. is around 14.5 psi at sea level, and less psi the higher your altitude.) It's "0" psi GAUGE PRESSURE, which you shouldn't ever get confused with atmospheric pressure. When you fill your tires and check them with your gauge, that gauge reads "0" when it's not on the tire valve, but the actual "pressure" all around you is whatever atmospheric is at your altitude, around 13-14 psi. High accuracy digital gauges have to be calibrated every time you use them, due to slight changes in atmospheric pressure all the time.
Another thing that might help to understand this, is when you hear someone talk about what type of MAP sensor they have. There are 1 BAR, 2 BAR, 3 BAR.... MAP sensors. BAR is short for barometric pressure, just yet another unit of measuring pressure.
0 BAR = 0 psi gauge pressure = around 14.5 psi atmospheric
1 BAR = 14.5 psi gauge pressure = around 29 psi atmospheric
2 BAR = Nearly 29 psi gauge pressure = nearly 45 psi atmospheric
They use BAR for vacuum measurements also
-1 BAR = 29.54 inHG = -14.5 psi (which is about as close to a perfect vacuum as you can get here on Earth) Of course, there's no such thing as atmospheric vacuum.
So when you hear someone say they have a 3 BAR MAP sensor, they don't mean it reads nearly 45 psi, but it reads FROM -30inHG up to 30psi, so it SPANS 3 BAR levels.
So when you see a vacuum measurement on my chart, it is only relative to 0 as being gauge pressure, as a reference.
