16-6 or 17-6 seems to be a good choice for 3D. Like Dave shadel says in the second link below, keep the engine in the 9000-9500 RPM range. The 16-6 might transition a little faster which is what you want for 3D, but watch that peak RPM. Throttle management might be needed if it goes near 10,000 on the 16-6.

http://www.rcuniverse.com/forum/m_23...tm.htm#2394156

http://www.rcuniverse.com/forum/m_18...tm.htm#1896208

http://www.rcuniverse.com/forum/m_18...tm.htm#1875425