Just received my GP 300s. I've read the forums, and it has been said that the .55ax will deliver outstanding performance? I am looking for 3-D and sport flying power. I have a .55ax, and one brand new in the box. Great engine, as with any O.S. that I own. My own opinion of course. Anyway I also have a newer .70 FS. The box/instructions say for a .46 to .81 . It also says the plane to weigh in at a heavy side of 6lbs. or @. I "HAD" an old 300s from GP in the red and white scheme. My first nitro arf actually. And it was a great plane to me. But as with many others in the sport, I'm progressing in skill and want to push the envelope. I would really like to have the power to do 3-D. Well hope someone can give me some ideas. Thanks for reading. Blue Skies...........Vince .