The effective engine performance is determined on test-bench trails at the clutch or calculated on the driven wheels after subtracting the power losses occuring in the drive chain. Performance figures given for old-timers are to be treated with caution. In the American specification 'SAE-HP', the performance is determined in theory, without the cooling-fan, the cooling circuit, the generator and the back-pressure in the exhaust system. The Italian 'CUNA-HP' is not quite as optimistic, only the air-filter and the exhaust system have been left out. In both cases however, a higher performance figure than that given in 'DIN-HP' is to be expected.