Quote:
quote:Originally posted by IRPT
Quote:
quote:Originally posted by Big Al
Quote:
quote:Originally posted by IRPT
Refers to a manifold absolute pressure sensor, a variable resistor used to monitor the difference in pressure between the intake manifold at outside atmosphere.
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Actually - this is "absolute" pressure, which is not in relationship to the outside atmosphere. That would be relative pressure, which is what a boost gauge measures.
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From PTDIY:
The MAP serves as a PCM input, using a silicon based sensing unit, to provide data on the manifold vacuum that draws the air/fuel mixture into the combustion chamber. The PCM requires this information to determine injector pulse width and spark advance. When MAP equals Barometric pressure, the pulse width will be at maximum.
The MAP sensor input is the number one contributor to pulse width. The most important function of the MAP sensor is to determine barometric pressure.
Read full article here:http://www.ptdoityourself.net/ptmapsensorguide.html
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Barometric pressure = outside atmosphere
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The sensor does NOT read manifold vacuum. It reads manifold absolute
pressure. Vacuum is again a relational representation.
Using the absolute pressure, engine RPM, and a predetermined volumtric efficiency table - the engine controller is able to accurately estimate the amount of air-mass the engine is ingesting.
This of course is used to manipulate the about of fuel metered. The engine load (volumetric efficiency) is also used for spark control and other attributes.
The controller can accurately meter fuel not knowing the barometric pressure. However this added info can help fine-tune the control.
So what happens when this sensor fails and the TIP sensor cannot be used instead? Hopefully the engine controller infers the volumetric efficiency based on RPM and throttle position. However this is not nearly as accurate for a dynamic engine situation.
Pulse width is proportional to pressure. The higher the MAP, the greater the pulse width. In a turbo engine such as ours, this is often much higher than atmospheric pressure - especially at altitude.