AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 116)

Page 116 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
While on the subject of throttle opening we must caution you to make sure that full throttle is
achieved by having someone verify that the throttle blade is opening fully and up against the
stop on the throttle body while you depress the throttle. Also, make sure that nothing interferes
with the full range of the throttle opening, such as a floor mat.
MAP (Manifold Pressure Sensor)
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).
Typical boost or vacuum gauges used in automotive applications use the gauge type of readout.
In automotive engineering, the absolute method of measuring pressure is used. To illustrate the
different ways to read manifold pressure refer to the accompanying chart.