Well contrary to what I thought the ST51 handled the 11X7 quite well at 11500+ on 15% nitro and 20% oil. I felt it would be a little sluggish but it wasn't. It had good transition and idle with the 11X7 and to be honest it pulled the Ultra Stick 40 a little too fast, especially considering it has seen a few crashes. The muffler is too quite for me but I will stick with it for now. Now after about 30+ tanks I am wondering how it will run after 30+ gallons. If you have ever tacked a OS 55AX with 11X7 please chime in.