"Each unit is methodically re-engineered to overcome many of the inherent factory weaknesses by adding strength and durability. Some of the issues addressed include boosting line pressure from an OEM level of between 140-150 psi to 220 psi, and keeping the unit balanced to avoid "shift shock" (banging shifts) by tightening down clearances; this results in firm, quick shifts as driver input becomes more aggressive. From front to rear, upgrades include a BorgWarner 29-element dual-cage sprag assembly, Sonnax Smart Shell to eliminate the stripping of splines and breakage, a set of Raybestos carbon composite performance clutches with new Raybestos steels, and two Sonnax Super Hold billet servos for Second and Fourth gears for maximum hold to name but a few. Follow along and check out just what it takes to bring a stock 4L60E to the highest level of performance."

https://www.hotrod.com/articles/performabuilt-shows-how-they-upgrade-a-4l60e-transmission/