I have heard that they are about 80% efficient. But that's anecdotal, so who knows. I believe it - a V-belt will rob power turning all those corners.
Have you thought of using a small transmission from a two stroke dirt bike? I have heard of guys removing the crankshaft and machining a through shaft to replace it (supported with sealed bearings), driven on the stator side with a short serpentine belt (more efficient than a v-belt). The whole unit can be mounted on end to create space, as these transmissions are not pressure lubed anyway. That's what I would like to do when I get around to making my diesel motorcycle.
