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Influences of oil-flow and oil temperature on frictional torques of whole engine and main lubricating components were determined by motoring method. Oil-flow rate, temperature and pressure in main lubricating pathes were directly measured under the same conditions. Oil-flow rate and frictional coefficient of crankshaft system were estimated by theoretically analysing Reynolds’ equation. Experimental data were discussed based on oil-flow analyses in lubricating system. Engine frictional torque becomes smallest when the engine is supplied with an optimum oil-flow rate, 2.5 - 3.5 1/min where the ratio of oil-flow rate is 10 - 20 % in crankshaft system, 30 - 40 % in a piston-connecting rod system, and 50 - 60 % in valve system. The optimum oil-flow rate which minimises the frictional torque is 1 - 2 1/min for crankshaft and piston-connecting rod systems, and 3-4. 1/min for a valve system.