Hey Guy, I too have a Hellfire and am looking for some hop-up parts. HPI has indicated that there are none. By that time I had already bought the 3 sp trani. However, it cannot work on the Hellfire. Also bought the RBInnovations supercharger. It too couldn't fit. Got some encouraging results with the 16T bell gear, though. Blew the hell out of the competition for top end speed. Didn't lose much on take off either. Imagine what would happen with that 3sp. box!