Its important to note that everything has a natural frequency, that can be picked up by cyclic examination.
Like an earth quake on gravel or sand, it can trigger abscene responses that you'd never expect.
If you use a 29 frames per second, you can make a helicopters blades seam to stand still.
https://www.youtube.com/watch?v=qLiumVfzGrA
Down here, our power is 60HZ, and fluro tubes in work shops where you use Mazraks to machine, it creates a serious issue with the machined stock looking like its not moving...
Noise, Viration and Harshness engineers at GM and Ford did all the work on six cylinder crankshafts. Vibration periods are essentiually based on dimensions and material frequency. Adding forged steel to a stock crank design raises its natural frequency, so does welding on counterweights, increasing the main or crank pin diameters. The 300 industrial crank and in some cases, even SVO blocks were designed to cope with extreame duty. Our user
super4ord's pictures on the 300 industrial big six heavy duty components
The Fox and 1978 onwards Granada/Monarch crankshaft has a different part number to the earlier versions. 3MA, IIRC. Still cast iron, but Ford did some NVH work on it. Forged cranks are usually weighted differently to the cast iron cranks, and normally weigh an extra 13 pounds on a 3 to 3.25" stroke six with a 4.08 bore spacing. A fully counterweight, 12 counterweight crankshaft weighes another 13 pounds up on the stock 6 counterweight cranks.
No factory Ford forged crank exists for the Falcon six except for some of the early four bearing cranks in 144's and 170's. When Ford went to seven bearing, everything was exclusively 6 counterweight nodular iron.
Since the Australian GM H 138/149/161/173/179/186/202 engines use the same 4.08" bore spacing as the Falcon six you can get a 149 /179M/179HPcrank from Australia that is forged, but its only 3.0" stroke and has 1.90" crank pins and 2.2" mains. You cut off the nose, wled up the main bearings to 2.4" and spigot weld a Ford crank flange on it, and use the 350 crank balancer since the cranks noses are similar. It then fits a Ford 200 or Australian 250 engine.
General Motors Holden engines in Australia were trial made form US protoype engines, so there was a forged crank made in Canada for GM Austrlaia for seven years. This Forged crank, IIRC, was made 145 hp for the twin carb X2, 2-bbl "S", and the 160 hp triple Zenith Stromberg CD150 carb GTR's, from 1963 to 1970. The long stroke 190 hp 3.3 liter 1971 202 XU1 GTR came out with a 3.25" stroke crank, but this time cast iron. To make it survive, Holden took the main bearings up to 2.5", but It suffered constant issues with two torsional vibration periods, although it never failed much in service, even in 500 mile endurance races with 215 to 235 hp engines. Even with the cranks main bearings ground back down to 2.2" to be used as a cheep 8% stroker on the smaller engines, it would take any level of abuse without failing. It was just blqqdy rough.
GM took stock of the cast iron 202 crank vibration periods (typically noticed in back to back comparions with Ford and Chrysler and Leyaland in line sixes which were much smoother). Vibration was most noticable on the stock 260 degree cammed 3.3 liter 1-bbl carb version, 135 hp gross or 102 or 118 hp net, and that came in at about 4500 rpm, and its that period that shakes everything loose, flywheel bolts, dampener bolts. Extensively raced every avenue of Australian Motorsport, and it was later replaced with a 12 counterweight XT5/XT6 crank starting 1980, and that lasted to 1987 in the last Bedford CF trucks.
That fully counterweighed crank raised the vibration periods to a very mild one at 5300 rpm, and another bigger one at over 7500 rpm, a zone no 3.3 liter six cylinder engine really runs into
The EF Falcon 4.0 egine in a similar way went to a 12 counterweight crank, and Ford used this from 1993 to 1998, and then downgraded it to a 10 counterwight design for the later Intech and Barra engines.
The same issues exist with single and double row timing chains, aluminum gear drives, steel timing chains, phenolic fibre gears, Pierrili or Uniryol belt drives. They all have periods where they shake. It might be 10000 rpm, it might be 3800 rpm like the cam belt on my twin cam 188 hp Nissan Skylined engine Stagea. But everything shakes if you find the right period.