Pressure Control Of Spark Timing

The distributor centrifugal advance mechanism provides a spark timing which is too advanced when turbocharger is operating at high load. To counteract this, the distributor vacuum advance unit has a double function.

Under normal operating conditions it can advance the spark by a maximum 15°. At high pressures it retards the spark timing by a maximum 8°, when compressor pressure reaches 5 psi (.35 kg/cm² ).