A test schedule was established to study the repeatability of a CFR cetane rating engine. The effects of the three manual settings (fuel consumption, injection advance, ignition delay) were analysed. Results show that these parameters influence the compression ratio determined by the standard procedure but do not affect the cetane rating itself significantly, within the scope of the study. Potential improving of the current isodelay method is likely to be found by means of analysis of other parameters like fuels, atmospheric conditions, engine speed and load, design, etc.A linear relationship between engine compression readings and test parameters was established, allowing to quantify the link between cetane and ignition delay. Further testing at constant compression ratio showed that more precise measurements are needed to make the use of a variable delay method possible, but such procedure will always be disturbed by nonlinearities, especially at low cetane levels.