Friction modifier keeps the clutches in the differential from prematurely wearing out. You do want then to slip; that's the whole point of a differential. It differentiates the speed of the inside and outside wheel in a corner. If it didn't do that, you'd break parts. Then when it's under load, it locks pretty much solid for maximum traction.
Here is a good article on how limited slip differntials work:
http://www.auburngear.com/oemdiffere...tedslip/about/
Bottom line is that you need to use the modifier or you'll be replacing your diff. shortly.
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93 Coupe, long tubes, pro chamber, flowmasters, 3.73, auburn, alum driveshft, subframes, tri-ax, pulley, 73mm MAF, cobra Rs.