Bill Tindall
Advocates of closed mouths have been open in their objection to the fiddly adjustment of the cap iron to mitigate tear-out. By revisiting the Kato-Kawai video one can determine what range of mouth opening would be efficacious for affecting tear-out.

The video experiment involved against the grain planing with the grain running -14 degrees. The shaving thickness is about 0.004". The splits propagate no more than twice the thickness of the shaving ahead of the blade tip in this experiment, 0.008". There is no upward movement of the wood beyond the tip of the split, 0.008". So, any downward force beyond this point does nothing useful for preventing levering up of the wood fibers and crack propagation.

Thus, it would seem for this experimental situation that the leading edge of the plane mouth would need to be greater than 0.004" to clear a shaving and less than 0.008" to provide any downward force in an effective location to prevent splitting ahead of the blade tip.

Steeper grain angles would require an even more narrow range of mouth opening to be effective. Weaker wood where the crack propagated farther ahead of the blade tip would increase the effective range of mouth opening.

One could wonder why Kato and Kawai didn't use this wonderful capability to study the effect of a pressure bar ahead of the blade. A possible answer is that the most of their work involved studying how to prolong blade life, not how to prevent tear-out. The cap iron studies focused on a self sharpening effect that can occur when the cap iron is in certain positions, a topic we have yet to discuss. (Some of their planing experiments continued for kilometers!!, a distance Wilbur would fine exhausting.)