I own both a MVVS 77 and a 91. Haven't used the 91 yet, but it is an ounce lighter than the 77 and has a Prettner style carb. It is a work of art. The 77 will spin a Zinger 14x6 10,500 rpms. Using the mini pipe and 5% fuel. Easily hovers my 7 1/2 pound plane at about 50% throttle opening with unlimited vertical. The 91 has been upgraded with a bigger prop shaft, it no longer uses a replaceable stud. It is supposed to turn a 15x 7 prop at the same rpm as the 77 with the same set up. Really looking forward to getting the 91 in the air.