David Massey wrote:
>
> My understanding is that the six and eight cylinders have a multiplanar
> crankshaft and are less prone to vibrations. These means balance is more
> critical in four cylinder engines - especially if smoothness of operation
> is desired.
I think this might require some theory. A V-8, with a 4-plane crank, has
a fairly good natural balance of the crankshaft, but in a 90 deg. block,
the rod angles at any given position are pretty strange, so it's
important that piston balance be good and connecting rod balancing be
done end for end, because there are several complex rocking couples
which can be made worse without good balance. This type of vibration can
also be made worse by torsional flexing in the crank.
That said, an in-line four has good primary balance, but lousy inherent
secondary balance (for which some manufacturers, such as Mitsubishi,
have tried to compensate with crank-driven balancing shafts), and that's
where the notion originates that balancing a four-cylinder is a waste of
time. In fact, a careful balancing job on an in-line four (particularly
with the crankshaft and flywheel balanced as a unit) will considerably
reduce vibration. However, by definition, it can't get rid of all the
vibration, because of those secondary imbalances.
For both engines, but particularly for the in-line four, the good news
is that the frequency of the secondary vibrations goes up with the
square of the engine speed, and the amplitude decreases similarly, so
those secondary imbalances become less important as engine speed
increases. That's why the Cosworth DFV engines, though 90 deg. V-8s,
could use an inherently lighter and stronger single-plane crank. At the
speeds those engines typically ran (9,000-13,000 rpm), the secondary
imbalances were of little importance.
For both four- and eight-cylinder engines, though, the principal bugaboo
with regard to destructive vibration is torsional flexing of the crank.
When this flexing creates vibration nodes which are at, or harmonics of,
the natural frequency of the crank, the amplitude can rise to the point
where there's enough vibration energy added to the normal stresses on
the crank that the crank will break. Good balancing can't stop this
torsional flexing, but it can minimize the amplitude of those vibrations
when that flexing occurs.
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto: mporter@zianet.com]
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
`80 TR7 (3.8 liter Buick-powered)
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