Randy,
There are several factors.
-Much higher exhaust volume and velocities on the 212.
-The cowling area is HUGE compared to a 205. You have the elephant ears, oil coolers, two exhaust stacks, etc. This causes more disruption than you'd think. During the development of the 427, small adjustments in cowling length and exhaust angle would produce completely different rides/vibration levels/stability at different speeds.
-Symmetrical 205 series blades (if installed), produce significantly different flow of air across the boom (The 412 is different again, which is why the mast cowling extends so high - to condition the air over the boom and decrease 4/rev. Interesting side note about the 412 is that if the elevators are out of rig, the 4/rev increases.
Here's an example of how much impact the change in loading may have: The 412, 212, 205B and some A1s use the same t/r blade. One would think that the 412EP, with 11,900 lb gross weight and oooodles more power would be more stressful on the tail rotor, but there is a configuration of 205 in which the t/r blade life is halved because of excessive loading. Don't ask which one because it's been too long and I don't remember
Wish I had invented the strake
