That as engine speed increases, the net forces on the connecting rods decreases up until a critical rev range? What happens is the combustion pressure acts against the inertial force from the piston/rod. So as the inertial forces increase, the compressive force from combustion is negated. Past a critical speed however, the exponentially increasing inertial forces will surpass the compressive combustion forces and the net stress in the connecting rod at tdc is actually in tension not compression!

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I should add, if you decrease the mass of the connecting rod/piston you delay the critical minimum stress to a higher engine speed. Likewise, if you also decrease structural integrity of the components while lightening, structural failure of the connecting rod at lower speeds becomes more probable.

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