Ok I give up... Why would you build a V-7??
An engine is never in 'balance' by the way..
it is only an approximation that shakes the least at the rpm of interest..
Did the inventor of the V-7 give any thought to the stresses when you have the
missing cylinder not fire and the have a time lapse before the next firing??
I think you will get something like..
put..put..put..put..put..put..........bang...
The instantaneous loading on the crank and drive train may not like this 'gap'
Oh on the 'overbalance' it is not an over balance or under balance it is merely
a
shift in the frequency of interest. A trade off between where you think it is
important to shake the least. I would think that at max power output I would
want
my engine to shake the least there are no awards for 'best idle' last i checked.
But if you want to be kind to the bearings you would want the least extra stress
on them when they are already heavily loaded. The one gram tolerance is doable
in everyday production work now also.
Dave Dahlgren
ardunbill@webtv.net wrote:
> Hi Folks, been reading the messages about V8 and V7 crankshaft balancing
> with interest. Some of the ideas seem confusing to me. I thought I
> would quote an authority so the novices reading the discussion might
> understand what is being talked about.
>
> P.E. Irving, "Automobile Engine Tuning", Temple Press, London 1963,
> Chapter 13, "Balancing", p. 141: "V8 crankshafts require dummy weights
> on each pin; each weight must be equal to the combined weights of the
> TWO big-ends, plus the top-end weight of ONE connecting-rod and its
> piston, rings, etc., ..."
>
> In this procedure, bobweights as above are attached symmetrically on
> each crankpin and the crankshaft then rotated at 1000 rpm in a balancing
> machine. The crankshaft is then corrected by drilling holes or adding
> "heavy metal" in the counterweights until it is in balance.
>
> It is assumed that all four sets of pistons and con-rods have an equal
> weight to 1 gram and good luck achieving this in practice!
>
> The weights of the big and small ends of the con-rods have to be
> measured horizontally on a frictionless device to get accuracy.
> Speedway Motors sells such an item for this purpose, how good it is I
> know not.
>
> I imagine that for a V7 a steel sleeve clamped on the crankpin, taking
> the place of one big-end, to seal the oil hole in the crank and take the
> side thrust from the working con-rod, and the said sleeve weighed 50% of
> the original balancing bobweight, then the crank balance would remain
> correct for all practical purposes.
>
> I suppose that the previous messages about 50-51-52% balancing have a
> relationship to the above information, and perhaps people would verify
> that for me.
>
> The man who does my balancing seems to be willing to do it in accordance
> with Irving's model. It's his machine, and I can't run it.
>
> Cheers from Ardun Bill in the Great Dismal Swamp, Chesapeake, VA, where
> it's been in the high 60s the last four days so I could get my bike out.
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