Yet they failed miserably trying to do a duplicate treatise to a B/RB head,..... & I thought Mullin used 5 & 9 inches. I'd have to go back to those 'old' books & check again tho'. Smokey did so much extensive testing with His shop partner, & discovered that once You achieved the velocities created by 28"H2O, it always correlated to track performance........more flow @28", more power/speed on the track, and once He declared this 'rule', almost everybody followed it....'cause He was Smokey. We know more now for sure, but it's not a bad rule, just an incomplete one.