G.M. lab test sheets on Corvair Harmonic Balancer development.
...The basic concept is that if there is more than 1/2 degree twist vibration
(torsional vibration) from one end of the cranishaft to the other, the crankshaft will eventually fail by cracking.
...The goal of a good harmonic balancer design is to minimize those vibrations
to less than 1/2 degree at the problem r.p.m. areas.
special problem areas (frequencies) for Corvair engine (an air cooled six).
frequency in hertz (cycles, or vibrations, per second)
...We were able to adjust the rubber compound to the 'ideal' (for this engine) 222 hz. freqency by experimenting with amounts of additives to the rubber compound.
...An added benefit to reducing rotational vibration of the crankshaft is that unwanted vibrations to the distributor, oil pump, and cam shaft are minimized. This should result in a longer lasting, quieter engine. I imagine that the gears connecting crankshaft to camshaft and to the distributor drive gear will wear better with proper crankshaft dampening.
...Our own testing of used GM harmonic balancers has shown that as the factory rubber begins to harden with age and temperature, the tuned frequency of the harmonic balancer increased as much as 26% with the unit still appearing to be intact and serviceable.
...A look at the test data above reveals that if a balancer started out at 220hz when new, an increase of only 10% will put it at 240hz which is the 1/2 degree crankshaft twist safe limit.
...One might conclude that many apparently serviceable used factory harmonic balancers no longer are capable of protecting against excessive radial (twisting) crankshaft vibration.
..."safe" limit of twist: (1/2 of a degree of twist of the crankshaft from one end to the other).
...w.o.t. means wide open throttle.
...'closed' means closed throttle.
...#6 means 6th order of vibration.
...#3 means 3th order of vibration.
Problem areas for this engine: idle; 2,750 and 4,800 RPM at full throttle.
(notice how the proper harmonic balancer tuning lowers vibration twisting amounts.)
Safe range = < 1/2 degree twist vibration.