I suspect the APC 15x4 would work better than the APC 14x6, but I don't have a 15x4 to find out for sure.

Based on my previous experiences with mousse can pipes on .25 to .46 size engines, I would have sworn a ST 90 would have needed a larger mousse can. Surprisingly, I've had the best results with a 1.75" x 6.5" can....which is the same size can that work best on my .40 to .46 engines. So far the ST 90 w/MCP is turning an APC 14x6 at 11,700......which has resulted in a major increase in vertical performance on the UCD.

The ST 75 with a stock muffler should easily hover a UCD. Just don't expect it to leap out of vertical hover. However if the MCP experiment on the ST 90 is any indication, the ST 75 should respond well to a 1.75" x 6.5" mousse can and give you pretty darned good acceleration out of a vertical hover.

But rather than duplicate information that is already in the Fun Fly forum, I'd suggest going to this thread for the latest info about the project to boost the power output of the ST 90:
http://www.rcuniverse.com/showthread...448#post451112]