I compared the drag factors on my new black model D and the one year old grey model D at the gym with two damper settings. They are as follows:
Damper / Grey / Black
5 / 126 / 147
10 / 198 / 231
I basically have to adjust about 2.5 damper settings to get the same drag. Granted, the grey is a year old and has some dust in the fan, but not too much since it is cleaned regularly. I read on the C2 website that a brand new model D should have a drag factor of 210-220 at damper setting 10, and my black is 231 after about 300k meters. http://www.concept2.com/service/monitor
If the 210-220 range is true, its possible there is something about the black models that makes them harder to row. But the weird thing is that C2 says a new rowers should have a drag factor of 90-100 at damper setting 1, and mine is only 83 (That is when my damper is all the way to the bottom. Is that setting 1 or zero?)
Drag factor is a numerical representation of the rate of deceleration of the flywheel, ie, how quickly the flywheel is slowing down. A bigger drag factor number indicates a faster rate of deceleration, and a higher energy requirement to accelerate the flywheel from one given rpm up to another. Air coming in contact with the fan blades is the source of resistance- let more air in (higher damper setting) and the drag factor increases. Close the damper down (or block the air flow with rabbit hair) and the drag factor drops. At a given damper setting on a given machine, the drag factor will increase as the temperature drops (cold air is denser than warm air). DF will decrease as elevation increases and as barometric pressure decreases (less dense air). Air which already has some velocity imparted to it before it enters the flywheel cover will also effect the DF.
From a manufacturing viewpoint, The perforated metal that surrounds the flywheel cover has a tolerance range for the hole size, and this will have an effect on drag factor range. Given all those possible variables, I think the numbers you're reporting are all withing range.
Personally, whenerver I'm setting up an erg I go to the DF screen on the monitor and set the damper for my preferred DF (115-120) and don't pay much attention to the damper number.....C2JonW