Porsche 911 2.7/3.0/3.2 Ignition Sensor Ref/Sync Function

Why do you need this system and what is a trigger or ref/sync? 

When converting classic Porsches to modern electronic fuel injection, the engine control unit (ECU) needs to know exactly where the crankshaft is in its 4 stroke cycle and how fast it is spinning.  This allows the ECU to correctly time each spark and fuel injector opening time and duration.  One sensor is pointed at the crankshaft pulley and measures a pulse for every tooth.  There is one missing tooth, and from this the ECU can work out what the crankshaft position is relative to this missing tooth.  The time between pulses gives the engine speed and rate of change.  But, there is one problem, the engine spins two complete revolutions to complete its 4 stroke cycle.  So whilst with this one sensor, the ECU knows where it is relative to top dead center, it does not know if it is on the compression or exhaust stroke.  This is where the reference sensor comes in.  This sensor is installed in the body our module, and measures a single tooth.  As this is installed in the distributor hole, this is geared to run at half engine speed, so will produce a pulse once every 720 degrees.  With these two sensors, the ECU knows the exact position the engine is in its 4 stroke cycle.  And now, we can run accurate sequential injection and ignition.