Lightweight flywheel options and selection criteria.

Here's some data to stir the pot :)

Although not a comparison of actual flywheel weights as the flywheel/clutch in these tests were exactly the same, here's instead a comparison of entire engine rotating assy weights. Actual Racepak data was collected from my personal 2525lb driver/car, also made some rough calculations using that data...
...Crankshaft weight difference about 6.9lbs.
...Bobweight difference was about 3.27lbs

NA sbc power.
Exact same heads/intake/carb/cam/compression/timing.
Exact same car/weight/trans/flywheel/clutch/gearing/tires.
Exact same location under nearly identical conditions.
Can't get any more real world than testing an engine in an actual car on an actual road.

With Scat 9000 cast crank (49lbs), steel rods, 1863g bobweight, WOT no-load rev rate of 8500rev/sec.
...1st gear WOT from 4500 to 5500- 1975rev/sec accel rate.
Calculations show 116.18ftllbs absorbed by the rotating assy, 383.82ftlbs twisting the trans input shaft.
...3rd gear WOT from 4500 to 5500- 535rev/sec accel rate.
Calculations show 31.47ftlbs absorbed by the rotating assy, 468.53ftlbs twisting the trans input shaft.

With Scat F43 lightweight crank (42.1lbs), aluminum rods, 1492g bobweight, WOT no-load 4500-5500 rev rate of 11,515rev/sec.
...1st gear WOT from 4500 to 5500- 2217rev/sec accel rate.
Calculations show 96.27ftllbs absorbed by the rotating assy, 403.73ftlbs twisting the trans input shaft.
...3rd gear WOT from 4500 to 5500- 541rev/sec accel rate.
Calculations show 23.49ftlbs absorbed by the rotating assy, 476.51ftlbs twisting the trans input shaft.

Both engines make the same torque when operating steady state @ 5000rpm, the differences really show up when the engine is gaining or losing rpm at a quick rate against WOT.
...WOT from 4500 to 5500rpm in 1st gear, the lightweight rotating assy hit the transmission's input shaft with about 19.91ftlbs more torque.
...WOT from 4500 to 5500rpm in 3rd gear, the lightweight rotating assy hit the transmission's input shaft with about 7.98ftlbs more torque.

Full diameter flywheel/clutch weight is typically centered farther from the axis of rotation than the crankshaft, making it an even more effective place to gain/lose weight in the rotating assy.

Grant