Rather than "80 mph" he meant, above your usual cruising speed. You don't want the end carbs opening up until well past cruise, that way they can be jetted for power at WOT without worrying about economy. The only time the end ones are needed is wide open. Some guys used to put a Chev starter solenoid working them and a switch under the gas pedal, step on it hard and CLICK! Everything open but the windows!