Methods and systems are provided for coordinating cylinder deactivation adjustments with changes to individual cylinder piston displacement. In doing do, the benefits of variable displacement and variable compression ratio may be synergized. An engine can be operated with some cylinders deactivated while active cylinders operate with knock addressed while spark timing is at MBT for a longer duration.

An ignition system controlled by a powertrain controller via a dwell line carrying a dwell signal has a state machine to monitor a dwell period between consecutive dwell signals and to adjust a rate of a variable clock signal accordingly. A multi-bit counter is clocked by the adjusted clock signal. The counter has a first set of bits to establish an integration period and a second set of bits coupled to a resistive ladder generating a stair step signal. A current sensor provides a current signal proportional to the ion current. A comparator compares the current signal to the stair step signal. An ion current counter is incremented during the integration period whenever the current signal indicates an ion current magnitude greater than the stair step signal. The accumulated count at the end of the integration period is reported to the powertrain controller as a measure of the ion current.