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Before discussing the detailed mechanics of how the EIS works, we should overview the benefits and general operation of the EIS. The EIS differs from standard magneto systems in one very significant way: TIMING. In a magneto, timing is permanently set; the EIS adjusts timing (or spark advance) based on RPM and manifold pressure. This ability to adjust the spark advance allows the EIS to determine the optimum timing setting which produces the most power with the least fuel.
EIS Spark Advance (Timing) = Mechanical Advance + RPM Advance + Vacuum Advance
The mechanical advance is set during the installation of the EIS timing housing or crankshaft sensor. This setting is usually zero degrees of TDC.
As the engine is started, the unit is set to remain with a zero advance up to 250RPM. After reaching 250 RPM, the EIS will advance the timing to 7 degrees and stay there until the engine reaches 400RPM. After 400RPM, the EIS will advance the timing to the setting correct for your engine (found on the data plate).
The final component for the total amount of timing comes from the vacuum advance, or Manifold Pressure Sensor. The manifold pressure is sensed and calculated in to the total spark advance. These two measurements are used together to determine the most efficient timing setting for the engine. The MAP Sensor will add a maximum of 15” of advance to the total Spark Advance. Refer to the following Vacuum Advance Curve Chart, fig. 1:
If the MAP Sensor (manifold absolute pressure or vacuum) option is not installed, then the vacuum advance value in the above equation would be zero. Without the MAP sensor installed, the advance remains zero up to the 250 RPM. At 250 RPM, it advances to 7 degrees BTDC, and then at 400 RPM, the unit advances to the spark advance setting for your engine (this is set at the factory) and will remain at that setting for operation.
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