AEM 30-71XX Infinity Stand-Alone Programmable Engine Mangement System Full Manual - Manual (Page 292)

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© 2014 AEM Performance Electronics
Infinity User Manual
MAP (Manifold Absolute Pressure)
Before any discussion is held of what a MAP sensor is, it is necessary to understand what
manifold pressure is. The definition of pressure is the force per unit area, thus it is an intensive
quantity formed as a ratio force and area. So if a 100-pound force is exerted on a piston that
has a total area of 100 in^2 the pressure acting on each square inch is 100lb/100 in^2 or 1 PSI.
If the same force were to be applied to a piston with an area of only 1 in^2 the pressure exerted
on the piston would be 100 lb/ 1 in^2 or 100 PSI. Now consider if a 100-pound person stepped
on a nail that has a tip that is only .010” diameter. This would yield a pressure of 10,000 pounds.
(100 lb / .010 in^2 = 10,000 pressure)
Realistically, there is no such thing as “manifold vacuum,” just low manifold pressure. The
average air pressure exerted on Earth under standard conditions is 14.7 psi or 101.325
kilopascals (kpa). An engine ingests air by creating a differential of pressure across the engine
via the movement of the pistons in their bores. When a piston moves down the bore, pressure in
the bore is reduced. When the inlet valve opens, the awaiting relative higher-pressure air above
the inlet valve enters the chamber and fills the void caused by the motion of the piston in the bore.
On an engine without a means of forcing air into the engine, the most this pressure can be is
whatever the atmospheric pressure of the day is. At sea level the average pressure is 14.7 psi.
It is common to refer to low manifold pressure as vacuum, which is how we will refer to low
pressure for the purposes of this discussion. Pressure is measured in two ways: one is absolute
pressure, and the other is gauge pressure. The difference between these is where the zero point
of each scale starts. With absolute pressure, zero is a complete void of all pressure. With the
more common gauge pressure, zero is at standard atmospheric pressure (14.7 psi). Anything
below 14.7 psi is referred to as vacuum and anything above that is referred to as boost or
positive pressure.
Closed or very small throttle angles are associated with low manifold pressure (a vacuum on the
gauge measuring style), and large throttle angles or full throttle is considered high manifold
pressure (0 on the gauge measuring style).