Piston Weights

Ernest Montague afm199 at earthlink.net
Thu Mar 13 15:54:51 PDT 2008

You are getting decades of experience from a couple of the top engine 
builders around, neither of whom has a degree in engineering. However 
their bikes win races. Take that for what it is worth. I know there are 
a couple engineers on the list. My GF is an engineer.  (PE) Her job is 
to run a company that monitors the growth of invasive species in the SF 

On Mar 13, 2008, at 3:31 PM, Jake Meyer wrote:

> I know I'm a n00b here, but I have to ask - how many people on this 
> e-mail
> list are engineers?  mechanics?  I'm just wondering how many years of
> design/build/repair/modify experience is sitting in my inbox on any 
> given
> day...  I'm a mechanical engineer with only about 7 years experience 
> working
> on cars.  Zero experience on bikes :) - so thanks to everyone for 
> sharing
> your knowledge and experience.
> Jake
> On 3/13/08, KC Gager <kc at brgracing.com> wrote:
>> Spot on Randy
>> The problem with piston weight is the surface area that the rod has 
>> on the
>> crank. It is design for X weight and the oil film will hold X weight. 
>> If
>> you
>> make x heavy old film goes away and boom.
>> KC Gager
>> BRG Racing Products
>> "We Have Sickness For Quickness"
>> KC at brgracing.com
>> http://www.brgracing.com
>> (925)672-5789
>> -----Original Message-----
>> From: Randy Grein [mailto:randygrein at comcast.net]
>> Sent: Thursday, March 13, 2008 6:19 AM
>> To: Suzuki SV650 Mailing List
>> Subject: Re: Piston Weights
>> 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

More information about the SV650 mailing list