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A method is proposed to quantify the activity of a detergent at a selected engine operating condition by measuring the amount of additive required to reduce nozzle fouling to zero in a well-defined engine test procedure. To determine this critical detergent concentration, a series of engine tests is performed using increasing levels of additive, until a zero level of fouling is achieved. These results are then used to construct a detergent concentration response map, defining unambiguously the additive activity and allowing quantitative comparisons of different detergents or additive packages. Concentration response plots are presented from both bench engine test series and vehicle road trials which demonstrate the wide variability in the amount of different detergents required to achieve comparable deposit control. From these maps the performance of the detergents may be compared in terms of keep-clean and clean-up activity for the range of engines and operating conditions.