Borderline spark

The most spark advance the engine can run at a given point before knock begins.

Borderline spark is the knock-limited timing edge: the most advance an engine can tolerate at a given load and RPM before detonation starts. When borderline timing is less than MBT, the engine is knock-limited and borderline becomes your real ceiling. Many GM/Ford strategies in HP Tuners model this directly with a Borderline Spark table (sometimes split by intake-air or coolant temperature), and the PCM blends high-octane, low-octane, and borderline values to pick commanded spark. In VCM Editor you find these in the Spark tab; in VCM Scanner you can log commanded spark alongside knock retard to see whether you are sitting on the borderline edge. The discipline: a good street tune leaves margin below the true borderline so that hot days, bad fuel batches, or carbon buildup do not tip a knock-free log into damaging detonation.