It's not unique to in-line engines (although a longer crank is more at risk), and it's not the same RPM for all of them.
The harmonic orders (bad RPM spots) will differ among engine designs, chiefly based on crankshaft rigidity.
Stiffer: short OAL, big main bearing journals, big rod bearing journals, short stroke
Weaker: long OAL, small main bearing journals, small rod bearing journals, long stroke.
The number of main bearings matters, but not intuitively. More bearings sounds good, but it also makes the crank longer.
Once the harmonic frequency of the crank is identified (very complex math or very good scientific testing), the orders are simply a math function, where Hz is the frequency, and N identifies which order:
RPM = Hz × 60 ÷ N
Most L6 engines are between 200 and 300 Hz (V8 are higher, V6 still higher).
Using 250 Hz as an example:
Order # RPM
1 15,000
2 7,500
2-1/2 6,000
3 5,000
4 (not relevant)
5 (not relevant)
6 2,500
9 1,667
12 1,250
As you can see, not all occur during useful engine speeds: 1, 9 and 12.
For an L6, the most dangerous are in this order:
worst = 3, 2-1/2, 2, 6, least = 9
Lightening the crank generally raises the RPM somewhat.
The real cure is a SEMA damper tuned to those RPM points.
Here are some effects of using a V8 damper, tuned to a common frequency of 350 Hz.
Order # RPM
1 21,000
2 10,500
2-1/2 8,400
3 7,000
4 (not relevant)
5 (not relevant)
6 3,500
9 2,333
12 1,750
Not too much similarity...
