The YS91 has gobs of power. I would see no need for a pipe.

I ran a YS91AC turning a 15x7 Bollie prop and pulled a CG Ultimate (9.5LBS) with tons of authority. A 15x7 Bollie prop with turn around 9700 rpm on that engine which is right where it should be.

If you want more power run 30% Cool Power Heli fuel (has 22% low viscosity synthentic oil) or at a minimum of 20/20. The oil should be all low viscosity synthentic. The more nitro the better for YS engines, they love it. Castor oil can plug the fuel injector over time. Wildcat and Powermaster make a special YS blend called YS 20/20 and I prefer the Wildcat myself. I keep a bottle of the Cool Power 30% on hand for those times I am looking to have more power for the 3D stuff with the Ultimate.

Check out the Ys Eningine support thread, tons of great info on these engines.

http://www.rcuniverse.com/forum/YS_E...mid_197/tt.htm

My Ultimate with a new YS110 (replacement for the YS91AC that it had and YS no longer makes)