Well it is hard to tell now with details now gone from the web site. But typically the non 57 series used metallic ducting and was horrible at temperature isolation. They typically fell behind in performance behind the 57's. I think it was to appeal more to visual aspects while still giving some improvement but at a slight loss. I am not sure as to why the others did not get certified CARB. i typically don't recommend anyone that does not use high density polyethylene for its ducting. so much better at thermo isolation.