Here is the info from Clevite with the attached Graphs
Various forms of main bearing grooving have been used over the years. We are frequently asked what difference grooving makes. First, it’s essential to understand that bearings depend on a film of oil to keep them separated from the shaft surface. This oil film is developed by shaft rotation. As the shaft rotates it pulls oil into the loaded area of the bearing and rides up on this film much like a tire hydroplaning on wet pavement. Grooving in a bearing acts like tread in a tire to break up the oil film. While you want your tires to grip the road, you don’t want your bearings to grip the shaft. The primary reason for having any grooving in a main bearing is to provide oil to the connecting rods. Without rod bearings to feed, a simple oil hole would be sufficient to lubricate a main bearing. Many early engines used full grooved bearings and some even used multiple grooves. As engine and bearing technology developed, bearing grooving was removed from modern lower main bearings. The result is in a thicker film of oil for the shaft to ride on. This provides a greater safety margin and improved bearing life. Upper main shells, which see lower loads than the lowers, have retained a groove to supply the connecting rods with oil. In an effort to develop the best possible main bearing designs for performance engines, we’ve investigated the effects of main bearing grooving on bearing performance. The graphs illustrate that a simple 180-degree groove in the upper main shell is still the best overall design. While a slightly shorter groove of 140 degrees provides a marginal gain, most of the benefit is to the upper shell, which doesn’t need improvement. On the other hand, extending the groove into the lower half, even as little as 20 degrees at each parting line (220 degrees in total), takes away from upper bearing performance without providing any benefit to the lower half. It’s also interesting to note that as groove length increases so do horsepower loss and peak oil film pressure which is transmitted directly to the bearing.
