'89 - '93 were not as efficient as the '94 - '95. The big difference is a larger air intake to the sc and newer more effecient gears that allowed for teflon (or something like that) coating on the roaters for a "closer shave" to the housing. End result, more air flow with less revs. The big difference between a roots and centrifigal is how the air is forced out of the sc. My understanding of a roots style is that is displaces a set amount of air per revolution. I'm not up on how centrifigals work, but I do know that they are much more efficient in that they produce less heat at the same lvl of boost that a roots does. I know there are some experts out there who can explain it much better than me, but I think that is it, brolen down to its simplest form. And there are other types out there as well that are more efficient than our roots, but I really can't get into the specifics as to how they are different. I just don't know that much....yet.