Interesting - after looking at that article briefly, where it said check rpm, I was about to say why on earth would rpm be reduced, because lift and drag are both proportional to air density as well. That may be true, but then I realized that a contrubting factor that does NOT change with elevation is mechanical friction in the engine. Therefore, so it seems to me, the prop load at a given rpm would be reduced by the same factor as the horsepower, but the friction at a given rpm is the same, therefore the rpm would indeed drop due to the reduced horsepower.

As to the effect on the engine, that is a complex question. Ummm..

- returning rpm to the same level as at lower elevation is still a lower horsepower situation.. due to reduced prop load but the original engine friction level
- as you go above that point in rpm, of course prop load increases as - I believe - rpm squared, but internal friction also increases. So to spin the prop faster on the ground to the point where it is absorbing the same amount of horsepower as before, which I think would be the right way to return original flight performance, the total horsepower must be higher to account for the increased enginge friction. So then it stands to reason that the engine would have a higher thermal load to handle, and internal temp could be higher due to original cooling rate but increased horsepower output.

So my first guess is that reproducing the original horsepower might be a hair harder on plugs than it was at the original altitude. Whether or not the difference is enough to care about is beyond me.

Okay, now my head is spinning. I'm not trying to be a smarty-pants here, just thinking through the problem out loud, and I could be all wet on the whole bag.

MJD