re: engine weight

[bearbvd@domain.elided (Greg Hermann)]

At 11:49 PM 7/30/02, borrani@domain.elided wrote:
>**weight between 1300, 1600, 1750, and 2000
>engines?  **
>
>The 1300 and 1600 will certainly be lighter with their
>single counterbalance crankshafts.  Those cranks weigh
>nearly 1/2 the weight of their fully counterweighted
>brethren.
>I suspect the fully counterweighted crank allows for
>smoother spinning at higher revs.  Adriansen's
>book  'Alleggerita' shows an Autodelta 1600 crank that
>was fully counterweighted.  I've always wonderered if
>the 1600's found in later european '70's-and-on Alfas
>ever utilized a fully counterweighted crank, could be
>the hot setup!
>
>Steve Schaeffer
>Seattle, WA
>'65 GTA 1600 w/single counterweighted crank

The eight weight cranks (of equal stroke) should really not be particularly
heavier than their four weight brethren. The purpose of the extra weights
is to split the counterweight to either side of the crankpin and rod
big-end mass, and thus reduce the bending stress and deflection in the
crank (and thus also reduce main bearing loads and stress and deflection in
the not-too-stiff aluminium block). Total counterweight mass in a four
weight vs. eight weight crank ought to be about equal.

The 1750 and 2000  (long stroke) motors went to the eight weight crank
exclusively, rather than just for racing, because the forces and
deflections involved get bigger with the longer stroke.

Greg
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