It will be 5S. I have run this setup in the Extreme Flight Extra 300 on 14X8.5 and a 15X8 APCE props. My Extra had plenty of vertical at 6 lbs -5 ozs and was capable of having all the vertical performance and speed needed to fly the AMA Masters Sequence. I am using the Duralite F3A 5350ma packs that I am using on my 10S pattern plane(Beryll).