I measured the width of the grooves and multiplied by the total length. A grooved plate has 20% less surface contact area. A four plate smooth has the same contact area of a five plate grooved.
Perhaps that is why I see a common recommendation of 4 smooth clutches in the rear drum and 5 grooved clutches in the front, to get the same surface area but with more aggressive shift qualities from 2-3 gear change.
