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is done, but to have the user start thinking about the conceptual process that they are about to begin during tuning of the
MPI PRO-JECTION System.
In general, the goal of the tuning process should be to develop a fuel map that corresponds to a slightly "richer than
stoichiometric" (or chemically perfect) fuel mixture. It is preferable to run the engine slightly rich particularly at high throttle
settings. Significant engine damage can occur quickly from overheating inside the cylinders, if extended running is done at a
high throttle position and with a lean fuel mixture. Many engines will make slightly more power running at a slightly lean
condition but the user should be discouraged from running at this condition. Holley engineers have found that the best engine
performance can be felt in a "band" around the optimum fuel mixture. A minimum amount of performance difference can be
found within the extremes of this band and Holley recommends that the user tune to the rich side of this band. This will generally
result in a slightly rich map (around 10-20%) that will give very good performance without sacrificing performance, economy,
driveability, or engine durability.
13.2 Oxygen Sensor Effect on Performance
For some users, it may seem like a waste of fuel to map the system richer than stoichiometric. For those users who may be
running a fairly stock engine and are interested in fuel economy, they should keep in mind that running a system in closed loop
(relying primarily on oxygen sensor-fuel mixture feedback) will safely adjust all part-throttle operation back to stoichiometric.
Therefore, the MPI PRO-JECTION systems will give optimum fuel economy during normal engine operation. However, any of
the MPI PRO-JECTION systems will revert to open loop operation at wide open throttle settings which rely on the users fuel
tuning process. With this in mind, at high throttle settings, the system needs to be tuned richer than stoichiometric.
13.3 Engine Configuration Related Concerns
The Holley MPI PRO-JECTION systems have been designed to provide the versatility needed to be custom tailored to most any
applications and will provide very good performance for a variety of engine sizes and engine types. In addition, the user can
make the tuning process much easier and understandable by knowing a little about their engine and understanding how the fuel
system delivery interacts with their engine requirements.
The system allows the user to custom tune the engine fuel curve to match the engine requirements. The system allows the user
to visually see the Injection Volume fuel map using Holley software and a PC connected to the MPI PRO-JECTION ECU. An
example of this Injection Volume fuel map is shown below. The user should notice that the fuel delivery will generally increase
steadily as the MAP Pressure (or engine load) increases or vacuum decreases. This can be seen by looking at a particular
engine speed on the graph and following that same engine speed line in the direction of increasing MAP Pressure. The increase
in this direction at a given speed will be fairly linear or be a fairly smooth straight line that is always increasing. However, the fuel
map curve looks very different when looking at constant map pressure values and looking across increasing speed values. The
fuel curve in this direction will generally steadily increase to a peak and start to drop off above some speed. In the example
figure, this speed of peak fuel injection amount is approximately 3500 RPM. The peak Injection Volume amount will correspond
to the engine speed where maximum torque is developed or in more technical terms, the peak engine volumetric efficiency.
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