If I replace a Cobra 2820/12 970 KVA with a Cobra 2826/10 930 KVA will it draw more or less current (or the same).  Assuming the same prop and same RPMs.  Prop is a APC 12x6 EP marked F2B cut down to a 11-1/2".  RPMs are set at 10k with a KR timer governor.

 

I made that exact swap in my Vector 40 ARF. Unfortunately I don’t keep meticulous notes, but I think the end of flight battery percent is up slightly with the bigger motor. The difference is negligible, maybe 2% more.

Brushless motors are pretty efficient things, and so are ESCs.  So Jim's 2% change isn't surprising.

Curare's experience with a smaller motor also isn't surprising -- even if the efficiency doesn't change much, you're wasting the same amount of power in a smaller motor.  Wasting = burning, and smaller things can't dump heat as well as bigger things.  Smaller package, same heat flow -- sizzle.

That's what's this is all about.  I smoked the smaller motor (my fault) and the bigger one is all I had on hand.  Sounds like added weight is the only down side for the bigger motor.  Hope to try it tomorrow. 

Flew the Cobra 2826/10 930 KVA today,  The put back was in the spread of the put back for the Cobra 2820/12 970 KVA.  If anything a little less.  So changing motor size does not affect current draw every thing else being equal.  The added noise weight was not noticeable by me.  But then at my level of flying skill (low) I am not a good judge of small changes in trim.  All is GOOD!!!         

John changing the size doesn't seem to be dramatic enough. Might want to try the AXI F2B that has same motor mounts and same weight. Do that test and see the results. Moreover change to a XOAR 12x6 https://www.ebay.com/p/Xoar-PJN-12x6-RC-Electric-Model-Airplane-Propeller-12-Inch-Prop-Wood-Beechwood/1249428526?iid=161899725901&chn=ps  the electric type one and see the results. XOAR makes in my opinion the best wood props out there.

The AXI F2B at over $100 is out of my price range.  Also the chart shows  a 6S  battery required.  All of my setups are 4S (more$$$$).  The prop looks good except it doesn't come in a LH rotation.   Anyway some day if I get beyond the beginners pattern and go to full body stunters  I will look into a high end setup.  I have no doubt that it works well.  As with any competitive sport the three most famous words are "If I only had -----)"         

The 2826/10 engine has lower MR resistance than the 2820/12, so probably greater current consumption.

Nope.  Exactly the opposite, in fact.  The system as a whole converts electrical energy to mechanical energy.  So for identical mechanical power outputs, the electrical power input must be the mechanical power output plus all the losses in the system.  All else being the same, a higher winding resistance means a higher electrical loss in the motor (which is part of why smaller motors get hotter), which means more current needs to come from the batteries.

As noted in earlier posts, people's actual experience shows that the difference isn't much, which shows that the optimal motor vs. efficiency tradeoff is pretty broad.  This makes all of our lives easier, because we don't have to obsess on getting just exactly the right motor.  (Well, it makes your lives easier -- I'm still flying slime).