Unless the air conditioning is turned on seperately, the compressor is never turned when you hit the defrost/defog button [at least that is true for abou 80% of 4-cylinder vehicles on the road today, V-6 and V-8 cars are a little different], although you are right, dry air does the job quicker, in which case you can turn the air temperature up in winter click on the A/C and clear your windows. The absolute simplest trick, however, is directed airflow, when you hit the defog button, the vent positions change under the dash to direct more air on the window than into the passenger compartment, and in the winter time when you usually want to defog the windows the most, NOTHING happens until the engine is warmed up, the thermostat opens and the heater core is at the proper temperature. Now, if you're driving along and hit a rainstorm with the A/C on, you'll never notice the difference, as you said, the air is dry; in the same conditions if your heater is on, you need to re-direct the airflow from your feet to the glass to correct for the sudden change in temperature between the outside and inside of the glass. The bottom line is that as long as air is being directed over the windows in the necessary volume and at a consistant temperature, the windows are going to clear, otherwise how would automakers deal with the liability resulting from allowing cars to be sold without air conditioning in northern areas where A/C is pointless even during the 2-day summer?

BTW, I'm glad you mentioned the eddy current dynos. I haven't had the chance to use one, they're still very much high $$ equipment, but I do understand that they also have a few problems. Namely, since you're measureing the current draw of [effectively] and electric engine, you run the risk of having a bush bounce on the commutator, causing a carbon arc, which will cause irregular current draw, and skew the results :a: Oh well, I guess. No perfect dyno's yet, the search continues, even the inertial dynos have to deal with the error factor of warn and dragging bearings on the support shaft, but so far, it is still the most fool-proof way to dyno the output of a car and/or engine.

FYI: It's nice to talk with someone that can go back and forth without throwing around credentials and getting all bent outta shape