true - and I do the same -
TheZDZ160 . engine, has been plotted by someone in Germany and the results show on Amelung's site.
The figures show what was already known (and is really "enough info" for most), that the engine has a flat torque band from 6000-6500.
But what is not shown, is that a good pipe setup raises the torque and the rpm increase happens using the same LOAD- Even your Zenoah, responds well to a pipe. raising the torque.
My only point here is that an open exhaust reading may not tell enough of the story -especially on engines designed specifically to use tuned systems .