Installation manual

90-898305

1-54

Speed / Density Theory and Operation

Speed/Density Theory

Outboard OptiMax engines operate on the fuel injection strategy called

“Speed/Density”. This means that the ECM primarily looks at the engine’s

speed and the intake manifold’s air density in order to calculate the correct

amount of fuel to inject.

The engine requires an air/fuel mixture of approximately 14:7 to 1 in the

combustion chambers.

Since the system doesn’t control air flow, it must determine how much air is

flowing through the engine in order to calculate the correct amount of time to

fire the fuel injectors. The net result is that there must be 1 part of fuel for

every 14.7 parts of air going through the engine.

Since the engine is basically an air pump, we know that an engine is capable

of pumping a certain (maximum) amount of air at any specific rpm. The actual

amount of air it pumps (at a specific rpm) depends on the density of the air in

the intake manifold. The air density (in the intake manifold) will vary depending

on rpm, throttle plate position and barometric pressure.

If the air density in the intake manifold is known, the actual amount of air

flowing through the engine (the “Air Mass” or “Mass Air Flow”) could be

calculated. This calculated (and the actual) air flow is a repeatable function,

meaning that at a specific rpm and a specific manifold absolute pressure

reading, the air flow through the engine will always be the same.

However, in the speed/density system we do not actually calculate the actual

air flow. Instead, the ECM measures the rpm and the air density, then refers to

a programmed “lookup table” in the ECM’s EEPROM. This lookup table will be

programmed with the correct fuel injector information for every rpm and

density reading. The programming engineer has to come up with these

figures, because the ECM is not actually calculating the Mass Air Flow.

The speed-density system depends on the engine being unmodified (from its

original production state). If we change the volumetric efficiency of the engine

in any manner, the amount of air flow for a given rpm and air density will

change, causing the ECM to deliver the incorrect amount of fuel. Any

modification to the following components will influence the air flow through the

engine, throwing the speed-density system out of calibration.

a) Pistons and combustion chambers (anything that changes the

compression ratio).

b) Changes to intake and exhaust port size, as well as “porting and

polishing”.

Notes

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