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We compared motoring friction loss, output performance at WOT (wide open throttle) and specific fuel consumption of big-end bearings on engines having identical specifications between the case of using plain bearings and rolling bearings to investigate the effect of the lubricating oil supply rate on these parameters in an attempt to improve output through reduction of friction loss for big-end bearings of small, high-output motorcycle engines. Testing was performed using a 125 cc, 4-cycle, single cylinder engine at high engine speeds mainly above 10,000 rpm.

Four-, five- and six-valve engines having two, three and four intake valves, respectively, were fabricated. Fifteen types of output characteristics were then investigated using these engines by varying the intake specifications to examine the relationship of the intake valve mean effective area and the number of valves needed to attain the required maximum output. Improvement of combustion in the light load range at low speeds was also examined by comparing the cylinder flow of four- and five-valve engines. As a result, it was found that the maximum output of multi-valve engines is generated at essentially the same mean intake Mach number regardless of the number of valves, and proportional to the intake valve mean effective area for an identical single cylinder volume. In addition, the result of five-valve engines with the larger intake valve mean effective area than four-valve engines showed that they could attain both the satisfactory mean flow coefficient and tumble.