d enginetorque (T.sup.i) over available ignition timings (spark) for homogenous operation at the maximum lean homogenous air/fuel ratio (a/f.sub.max.sup.homogeneous) at the current operationg conditions defined by, for example, engine speed (RPM), fresh airflow, exhaust gas recirculation amount, and any other variables known to those skilled in the art to affect engine indicated torque.