Engine testing method and system therefor

The performance of an automotive engine 12 is tested by accelerating the engine while imposing a predetermined load 28 on the engine and measuring the rotational speed of the engine. Rotational speed data per unit time of said engine is measured during acceleration thereof after it has started to operate, and rotational acceleration of the engine is calculated by processing said rotational speed data. The calculated rotational acceleration is compared with rotational accelerations of a normal engine and engines with intentionally given troubles e.g. ignition phase lagging or leading, timing belt lagging or leading, spark plug failure or compression leakage, and whether the performance of the engine is acceptable or not is determined, and if unacceptable the cause of a trouble of the engine is estimated, based on the result of the comparison. A cylinder 44 maintains tension in endless driving belt 18 being reduced by vibration.