Lighter flywheel or crank frequency change can be roughly estimated if the frequency is known. In general terms, the extra mass reduces the frequency in inverse proportion to the square roots of the weights:
ω ~ 1 ÷ (M2^.5 ÷ M1^.5)
If the crankshaft and its attached components (damper, flywheels, bolts, &c.) weighs (example only) 100 lbs. (M1), and a 10 lb. lighter flywheel were substituted (new weight: 90 lbs., M2) the frequency would be 1 ÷ (100^.5 ÷ 90^.5), or 94.874%. If the original frequency were 250 Hz., it decays to 237 Hz, &c. and the RPM drop at each order is proportionate.
