randygrein at comcast.net
Thu Mar 13 06:19:12 PDT 2008
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
> 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
> the ~3x wider valley. Normally, the sharper the power peak (i.e. the
> quicker it changes), the more of its energy gets dissipated via
> deformation in the drive line. Lowering and widening the peak allows
> various stresses to rise and fall at a slower rate, thereby wasting
> energy on component deformation, and delivering more of it to the
> 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
> more sharp combined torque rises and falls, creating peaks and
> producing more stress, sapping energy and wearing components. The
> measured on the output shaft would probably fall.
> Anyhow, that's how I explain it to myself. Barring confusing
> and lacking clarity, do I have the right picture in mind?
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