not at all true. the 289 in my 65 mercury would hit 9k no problem. and that car was from 1965. the biggest problem at that rpm is valve float. the biggest benefit ohc engines is better variable valve timing. its easier to independently control intake and exhaust timing with ohc engines. and it couldve been done with no cam if freevalve had taken off earlier. that couldve pushed ICE engines another decade if they got it out the door sooner.