You wouldn't want to hold max power on a Classic for 20 seconds- it is a very good 'hoover-upper' and that is not a recommended procedure with 747s for surge reasons. If you are desperate, just clock max power and let go the brakes as soon as poss otherwise it is unnecessarily alaming for the occupants. Because of slow thrust lever response, 747s now wind up to 1.20 EPR before advancing to T/O power (which is most likely to be reduced thrust). Unless the runway is limiting or weight almost maximum, there is no need to hold on the brakes at any time- rolling takeoffs are recommended and use much less runway time for ATC reasons.