First, I will assume you have a YS 91SR and not an "SR 91".
Because, well, it makes a difference.
The YS 91SR is a hell of a good motor.
It tunes easily and makes good power.
I fly around a guy that uses this motor in a Synergy N7 and he does hardcore 3D / smack at 1950 RPM headspeed.
The next thing you may want to figure out is the color of the head.
That will tell you if it is:
Each one has a different personality and use.
I flew the YS 91SR 3DS for a few years in Raptor 91 and Trex 700N.
And now I have the YS 96SR-X and YS 120SR-X
The YS 120SR is a good motor as long as you never exceed its operating RPM (max = 16,000). But, the crankshaft is not well balanced. Above 16k RPM, the vibrations have cause Hatori pipes to break off.
The YS 120SR-X is the biggest and most powerful heli motor on the market today. The torque is awesome
It has a crankshaft whose balance is fine-tuned by computer in the factory so all the vibration problems are gone.
But, it still need to stay in the 16k RPM range.
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