"these potted motor machines are very low power ,, they do not do well on heavy fabric's, Both of these have one steel gear and one fiber gear ,,, silver is steel, red is a fiber"
I'm not sure where you're getting this part about them not doing well--the potted motor machines are plenty strong, and the 15-91's are my go-to machine for sewing through heavier stuff. The fiber gear has nothing to do with the machine's performance.
Bottom line is if you have a 15-91 that runs properly with lightweight fabric, there's no reason it won't do as well with heavier fabric unless you're using the wrong type/size of needle or needle point, you're using the wrong thread, you've got it misthreaded, or something on your particular machine is not set up correctly.