Does Length Matter?

Installs & Reviews Khiem Dinh for Moto IQ

Someone recently asked, “Why a longer rod?” Someone else answered, “Longer dwell time.” I never really thought about it before, so I thought I’d look into it. Using some basic math, we can prove that a longer rod does indeed create a longer dwell time around top dead center.

As you can see in the diagram, the piston with the longer conrod spends more time around top dead center (0 degree crank angle) due to a slower acceleration rate towards and away from TDC. A longer dwell time around top dead center makes better use of the combustion pressure and turning the pressure into torque; a rule of thumb is that you want peak combustion pressure around 15-17 degrees after TDC. Furthermore, the combusiton pressures are higher while the crank angle is lower resulting in less side loading of the piston into the bore. Less side loading results in less friction between the piston and bore freeing up power and reducing wear. The reduced piston acceleration also improves piston ring life as the lower acceleration equals less force on the rings (remember F=ma?).

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