IM6 vs. IM7

There is little to no different between 2 and 4 pc rods for most manufacturers in terms of feel or ability of the rod.  It takes a very trained caster to tell the difference in most cases and most of the time it is a difference in taper felt, but that the 4 pc rod has more ferrules or a worse feel, just different.  You should have no problems with a 4 pc rod.
 
IM6 and IM7 were originally the trade monikers of the prepreg graphite sheet as sold to the blank manufacturers.  There is debate about what IM stands for and no one has a definitive answer.  At first, the IM numbers were used to indicate the 'quality' of a rod.  The higher the number the better the rod was suppossed to be.  It was marketing hype though, since rod performance is a combination of prepreg sheets, scrim, taper, and processing.  Now IM are used in the same way, but with a lot less emphasis by the major manufacturers.  The private label and store brand rods still use it to an extent to sell rods.
 
Typically when you see IM6 you are talking about graphite scrim with a young's modulus of 36-41 million PSI.  IM7 typically is 41-48 million depending who you ask.  Again, IM doesn't really mean much.  Given the same scrim, taper, and processing an IM7 rod will be lighter for the same stiffness or stiffer for the same weight.  But considering how close they are in properties and the differences in all other considerations the graphite itself will not be the cause of any differences you see.
 
thanks,
rick

casts_by_fly, i enjoy reading the technical part of fly fishing, such as your input about im6 and im7. i read quite a lot before i buy or just for the knowledge, most people wont and it's guys like you that they can get info on without searching the books and just asking questions. i applaud you in taking the time to inform the general fishermen. t.t. most of the owners of the fly shops i've visited in the past have and are going to stay buying 4pce. rods. except for maybe a little more weight there is little if any different feel in casting each. technology has come a long way and a multi 4 pce. is the way to go in many respects. if your rod comes apart while casting often don't be affraid to take it back. i had one do so and they took care of it. you'll know if a section is loosening by the feel of an extra wobble. let this be the warning to refit the sections. ~dt