Hi everybody!Firdt gas engine!!What's the best prop for a mvvs 2.15 (35cc) gas engine in a 80" funtana?Maybe 20 x 6?How many liters for the brake in?Thanx.

It may be a bit late for this response, but I just got this forum to work for me.
18x10 will spin up to 9000 rpm which is over the top, 20x6 is fine and so is 22x6

I fly mine with a 20X10 and it is still not enough, it will fly a 22X8 well and still not be under 7k. I do a lot of low speed stuff so I keep the 20x10 because it spools well and the motor stays cool, I have about 25 gallons through it with this combo.

For normal flying, the larger props are OK up to even 22x10. For 3D the story has a different rationale:
Power is needed for two things: First to hover the plane, and secondly there should be spare power to accelerate the prop so you can exit vertically from the hover.
So for 3D, you need the prop spinning above the max rpm when at full power, and close below full power rpm at the hover. Now the engine can unfold full power to accelerate the prop, and thus accelerate well out of the hover.