What decay rate controls
Ignition timing on a Nexus isn't always allowed to jump straight to a new value. The rate tables limit how fast it can move. The Decay Rate table covers one direction: moving from more advance to less advance (retarding). It's set in degrees per engine cycle, so a value of 10 means timing can fall by at most 10 degrees each engine cycle.
Its partner, the Attack Rate table, limits movement the other way, toward more advance. That's the one tuners use more often, because slowing down how quickly timing comes back in can smooth out tip-in and transitions.
Why it usually stays at default
When the tune calls for less timing, you generally want that retard to happen right away, since pulling timing is the safe direction. That's why the decay rate defaults are high, somewhere around 50 degrees per cycle, which in practice lets timing drop almost instantly. The function is disabled by default and supports up to 3D mapping if you do need it.
We'd only lower it for a specific drivability reason, and we'd confirm in logs that timing still retards fast enough under load. See ignition tuning for the full picture, or base ignition for the main timing table.
