How do I know if RPM restrictions should exist for my prop/engine combination?

I have an O-360-A4K with an MT 3-blade electric prop. I feel the engine is very smooth at all RPM, and only gets more smooth if anything at lower RPM (2,100, for example). Prop is balanced to .05IPS. How do I know if a certain RPM range should be restricted?

Take a look at your prop manual or check with the manufacturer ... The Hartzell manual has a list of prop + engine combos and any continuous operation speed restrictions for that combo.

My understanding is that the restrictions are there to keep away from known RPMs that excite a resonance that are specific to the prop + engine combo. The resonance itself may in a different range (2nd, 3rd, 4th harmonic etc) than the restricted RPM.

The prop balance IPS is not a factor in the restrictions.