Piston Weights

[Randy Grein]

No. Neither reciprocating nor rotating mass affect torque. An increase  
in rotating mass evens out power output making it easier to feather  
the throttle (like spinning coming out of  a corner), and does provide  
a slight increase in acceleration (measured torque and power on a  
Dynojet dyno), but the few grams we're talking about here between  
pistons isn't enough to notice. Piston weight does affect the optimal  
crank balance; go light or heavy enough and you'd increase vibration)  
but the primary impact is on crank/rod stress. Big heavy pistons break  
cranks at high rpms.

Your drive line deformation argument is interesting, but except for  
the clutch basket springs and sprocket cushions there isn't any to  
speak of at this level. Parts that deformed enough to materially  
impact power delivery, even instantaneous power delivery would be  
deforming significantly and break in short order. Yes, there is some  
deformation, which is why we don't make cranks out of aluminum, but  
for the effect you're thinking of to be noticeable deformation would  
have to be plastic and on the order of 10-20 degrees of crank rotation  
every half  engine rotation.

Randy Grein, WMRRA #41

On Mar 13, 2008, at 5:35 AM, Ilya A. Kriveshko wrote:

> 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
> shaft.
>
> 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?
> --
> Ilya
>