L6 engines generally resonate at frequencies between 200 and 300 Hz (source: Heldt). These orders occur throughout, and well above the maximum RPM of the 292 crankshaft.
The engine speeds for these orders can be calculated from the resonant frequency, where “Hz” is the crankshaft’s resonant frequency, and N is the order number, using this formula:
RPM = Hz × 60 ÷ N
Easy to try, results may be difficult to evaluate:
1. suspend either crank by a loop of wire/ rope etc. through a flange hole.
2. strike a counterweight with a brass or lead hammer (not leave a mark).
3. with microphone pointed at it.
4. wired to an oscilloscope.
5. if a single strong trace appears from several trials, it will show the harmonic frequency.
6. the 2 cranks should show different numbers, for an L6 probably between 200 and 300 (V6, V8 will be much higher).
7. the higher frequency is stiffer and will experience a specific harmonic order at a higher RPM.
8. the actual frequency determines where a dangerous order (the 2nd, 2-1/2, and 3rd) will occur.
9. for example, if 237 Hz is detected, the (worst) 3rd order is 237 × 60 ÷ 3 = 4,740 RPM. The 2nd order is 237 × 60 ÷ 2 = 7,170 RPM. The 2-1/2 order is 237 × 60 ÷ 2-1/2 = 5,688 RPM.
10. the other dangerous orders for an L6 are the 6th at 2,370 and 9th at 1,580, but they're much weaker. The 1st is very strong but far away at 14,220.
