Piston Weights

Ilya A. Kriveshko ilya.kriveshko at verizon.net
Thu Mar 13 05:35:31 PDT 2008

Steve Robertson wrote:
> So do those boat anchor Wiseco pistons make good torque :-) 

I understand it's a joke. But pretending for a second you were serious,
let me ask the gear heads on the list: would a heavier *reciprocating*
mass affect torque figures? And how?

My own gut feel is that unlike heavier rotating mass, heavier
reciprocating mass would actually reduce output torque.

Heavier *rotating* mass would increase the mean torque over the cycle,
by borrowing from the sharp and short power peak and spreading it around
the ~3x wider valley. Normally, the sharper the power peak (i.e. the
quicker it changes), the more of its energy gets dissipated via material
deformation in the drive line. Lowering and widening the peak allows the
various stresses to rise and fall at a slower rate, thereby wasting less
energy on component deformation, and delivering more of it to the output

However, it seems that heavier *reciprocating* mass would not have the
same full-cycle peak spreading effect. It would rather amplify the
harmonic of positive and negative inertial torque effects that is
overlaying the combustion torque curve. If anything, that would produce
more sharp combined torque rises and falls, creating peaks and valleys,
producing more stress, sapping energy and wearing components. The torque
measured on the output shaft would probably fall.

Anyhow, that's how I explain it to myself. Barring confusing terminology
and lacking clarity, do I have the right picture in mind?

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