A .46 is basically a .40 size engine or case thats been bored out for larger piston. When it comes to Airplanes, weight is everything. So it makes sense to try and get as much power out of a particular size engine without having to create much additional weight! For example: If you set a O.S .40 sf next to a O.S .46 sf, the physical appearance and size are the same. Its whats inside that makes them different. Same goes for the GMS .40 and GMS .46 etc. etc. etc. etc.