Installation manual

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Speed/Density Operation

To determine the density of the air in the intake manifold, we need to know the

intake manifold vacuum, which we measure with the MAP (Manifold Absolute

Pressure) sensor. It is important to remember that a MAP sensor measures

the manifold pressure above absolute zero (like a barometer), while a

conventional vacuum gauge measures the manifold pressure below the

current atmospheric pressure. The use of the Manifold Absolute Pressure

Sensor allows us to compensate for variations in atmospheric pressure due to

weather and altitude changes. A conventional vacuum gauge would not

provide us with this needed information.

NOTE: While the temperature of the air does affect its density, not all engines

use an IAT (intake air temperature) sensor. If no IAT is present, then the ECM

assumes 75 degree Fahrenheit for all density calculations. If an IAT is

present, then the ECM can more accurately determine the air’s density.

However, the amount of correction the IAT adds is a relatively small amount

(approximately 10% maximum change in fuel flow).

In review, our standard, unmodified production engines flow a repeatable (and

therefore “known”) amount of air at any specific engine rpm and manifold

pressure. With this knowledge, the ECM can be programmed to deliver the

correct amount of fuel from the combination of the speed sensor (distributor

signal) and density information (from the MAP sensor).

It is often said that the speed-density system runs “in theory alone”, since the

ECM doesn’t really know how much air is flowing through the engine, it is just

assuming it knows how much (based on the repeatability of airflow theory). In

reality, the system is simple, rugged and works extremely well. But, the ECM

cannot compensate for changes in volumetric efficiency of the engine.

Mass Air Flow Systems

Mass Air Flow systems actually measure the amount of air (or “Air Mass”)

entering the engine, so they generally can compensate for modifications or

changes to the air flow through the engine. While these systems are generally

considered more accurate, they are generally not as robust (and cost

effective) as the speed-density system. Mass Air Flow systems are typically

used in automotive applications to meet stringent emissions and fuel economy

requirements.

NOTE: The speed-density system is more than accurate enough for our

marine applications. The additional reduction in emissions and the resulting

increase in fuel economy (with the Mass Air Flow system) are negligible. But

when automotive manufacturers must meet emissions standards, they often

have to take small improvements where they can find them.

Notes

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