My review of the Raptor 90 G4 is going to need a powerful and reliable engine to perform at its best, and what could be better than the OS 91HZ-R 3D with Powerboost Pipe. This basic engine is actually available in several different forms. First as mentioned here with a black head designed for 3D flying, and as an F3C version with a red head, and finally as a combination engine and powerboost pipe.
This engine is designed to be used with 10-30% nitro, with the Demand Regulator System supplying a constant supply of fuel throughout the most demanding maneuvers. OS has a long history of both power and reliability, which is exactly the combination we need with our helicopters. And how about that pipe??? Is that a thing of beauty or what???? I can just see myself polishing it every time I fly.
This OS 91, and their complete line of engines, can be viewed with more technical information at the TowerHobbies web site http://www3.towerhobbies.com/cgi-bin/wti0091p?&P=SM&C=GCR&V=OSM