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MAP sensors measure absolute pressure. Boost sensors or gauges measure the amount of pressure above a set absolute pressure. That set absolute pressure is usually 100 kPa. This is commonly referred to as gauge pressure. Boost pressure is relative to absolute pressure - as one increases or decreases, so does the other. It is a one-to-one relationship with an offset of -100 kPa for boost pressure. Thus a MAP sensor will always read 100 kPa more than a boost sensor measuring the same conditions. A MAP sensor will never display a negative reading because it is measuring absolute pressure, where zero is the total absence of pressure (it is possible to have conditions where negative absolute pressure can be observed, but none of those conditions occur in the air intake of an internal combustion engine). Boost sensors can display negative readings, indicating vacuum or suction (a condition of lower pressure than the surrounding atmosphere). In forced induction engines (supercharged or turbocharged), a negative boost reading indicates that the engine is drawing air faster than it is being supplied, creating suction. This is often called vacuum pressure when referring to internal combustion engines.
In short: most boost sensors will read 100 kPa less than a MAP sensor reads. One can convert boost to MAP by adding 100 kPa. One can convert from MAP to boost by subtracting 100 kPa.
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