Actual Power

I somewhere picked up that not using the factory airbox on a 100hp Rotax 912 reduces the max power by 3-4% (using recommended filters instead)

I too have heard of this reduction in power (although not quantified) when not using the Rotax Airbox.

I can't actually see why this would be, unless it somehow relates to elevated air temperature within the engine compartment. This is assuming that the Airbox has cool ducted air going to it and the non airbox carbs are sucking on heated air from within the cowling.

The power output measurement on the engines is done with the airbox I can confirm that.  Power without the airbox is not anything that they will validate.  The testing without airbox has not been done (that anyone will confirm) at the factory.  So is it actually less?  I doubt that.  The carbs will vibrate a bit more and have other issues perhaps, but power drop I do not believe will be seen.

Cheers

My guess is that in real world conditions there would be more power made using the airbox (with outside air) vs using two air cleaners drawing air from inside the cowl. The difference in the intake air temperature would be substantial, especially at altitude, which creates a denser charge. Carburetors drawing air from inside the cowl would be similar to running carb heat all the time. 

Hi Jeff,

"Carburetors drawing air from inside the cowl would be similar to running carb heat all the time"

I agree however its not all bad - my last 912ULS didn't have carburettor heat, in the 11 years I had the aircraft, never a hint of ice. Undercowl temps routinely 10C above ambient. 

My new 912ULS, has a home produced airbox, delivering outside air (has water heated anti ice) Diffrent aircraft/propeller so hard to make performance comparison.