A laboratory setup for testing diesel engines at simulated altitudes (pressure and temperature according to international standard atmosphere data) up to 14,000 ft is described. Results obtained are discussed in relation to site rating of engine. Attention is drawn to different bases which may be used in correcting engine performance (for example, constant fuel delivery, smoke density, indicated specific fuel consumption, and constant thermal loading method). The validity of derating formulas when applied to a small open-chamber, 4-stroke, water-cooled diesel engine, running at constant speed, is discussed. The effect of increased speed under constant air-fuel ratio conditions is also examined.