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XDI200 Manual Version 0.1
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©2017 Electromotive, Inc.
If when revving the engine, the engine rpm oscillates a little bit around the decel rpm while falling,
reducing the reset value should help to prevent this. If the engine falls below the idle target and
almost stalls, a larger reset value will help.
Fuel Pump Turn-On Time
Set Constant for the Fuel Pump Turn-On Time:
This feature is used to turn the fuel
pump on when the ignition is turned on. The fuel pump will run for the specified number of
seconds, or until engine cranking occurs. Use this value to prime your fuel system.
8.5.2 Absolute vs. Gage Pressure
A MAP sensor is an absolute sensor. This means that its readings are referenced to a full
vacuum. As such, the lowest reading attainable on a MAP sensor would be a full vacuum. (In
practice, however, most MAP sensors only go down to about 10kPa. Gauge pressure is
referenced to ambient atmospheric pressure. When we say that an engine is running 15psi
(103kPa) of boost, it means that the intake manifold is seeing 15psi of positive pressure. Since
standard atmospheric pressure is approximately 14.7psi (101.3kPa), the absolute manifold
pressure for this engine would be 29.7psi (205kPa or about 2 Bar). Thus, a 2-Bar sensor is
necessary for this engine. Refer to the formula below if you are confused about the difference
between gauge and absolute pressure.
Ambient (atmospheric) Pr Gauge Pressure = Absolute Pressure
When discussing absolute pressure, there is no distinction made between
vacuum and pressure.
When discussing gauge or relative pressure, vacuum readings will be
negative values and boost readings will be positive. Alternatively, in the US
measurement system, vacuum readings can be expressed in inches of
Mercury and boost can be expressed in pounds per square inch.
8.5.3 Units of Measurement
In the US, units of vacuum are typically measured in inches of mercury (in-Hg). Units of
pressure are typically measured in pounds per square inch (psi). Since there is a distinction
made between vacuum and pressure readings, these units are generally not used for absolute
measurements.
In most other countries, the metric system in used for pressure measurements. In this
system, the kilopascal (kPa) is the standard unit of measurement. The distinction must be made
to whether the measurement is in absolute kPa or gauge kPa. Typically, this is done by writing
“kPa, abs” for the former, and “kPa, gauge” for the latter. For the purposes of this section, we will
treat all kPa reading as
absolute
readings.
Table 11
gives conversion data for US and metric
units. Remember:
Ambient (atmospheric) Pr Gauge Pressure = Absolute Pressure
