I have only installed a Blunami in one steam engine, an MTH Hudson. I get a pretty consistent 4 chuffs per rev as well. I only had to adjust the rate CV a few integers to get it close. The engine has a 94xx Pittman. Back EMF is OK though maybe not as good as ERR, still close enough. I have no complaints in that compartment. Only with not being able to sync the smoke fan.
Pete