I’ve recently gotten a few new fishing rods, two of which are graphite. One is listed as being constructed from “Im-6” and the other “Im-8”. Both rods filled gaps in my fishing tackle needs, (and were on sale 
), so I bought them, however I’m curious as to what these designations mean. Does it have to do with the quality of the materials used in the manufacture? Is one preferable over the other?
It’s basically the rod’s “springiness” rating. The technical term is modulus of elasticity, and the higher the number is the stiffer, more snappy and sensitive the rod will be. You’ll find that this isn’t always a terribly useful metric, however, since an IM6 from one manufacturer might respond very differently than an IM6 from another. Also note that while higher numbers are generally more desirable, it also means that the rod is more prone to be brittle. Lower IM numbers have more “give” and are less likely to break under stress. Which rod is best is largely a matter of the type of fishing you want to do and how “snappy” you like your rods to be. In the end, it’s more personal preference than anything else, IMO.
Thanks for the answer Q . Strange thing is that the IM6 rod was in a heavier action than the IM8 rod, so the stiffer graphite in my case actually ends up being “whippier” than the softer rod.